Cable and Circuit Types
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Cable and Circuit Types Learner Work Book
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LEARNER WORK BOOK
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Table of Contents Foreword ........................................................................................................5 Cables and Circuit Types Unit Overview .....................................................6 Practical Skills .................................................................................................... 6 Knowledge Requirements .................................................................................. 6
Cable Types....................................................................................................7 Light duty cables ................................................................................................ 8 Medium duty cables ......................................................................................... 13 Heavy duty cables............................................................................................ 15
Circuits and BS7671 ....................................................................................17 Division of the installation ................................................................................. 17 Accessories...................................................................................................... 18 Electrical equipment ......................................................................................... 19 Isolation and switching ..................................................................................... 20
Circuit Categories ........................................................................................24 Category 1 Circuits........................................................................................... 25 Category 2 Circuits........................................................................................... 26 Category 3 Circuits........................................................................................... 28
Standard Circuit Ratings .............................................................................30 Lighting Circuits ..........................................................................................31 Lighting points .................................................................................................. 32 Lamp types ...................................................................................................... 33 Lighting design current calculations.................................................................. 37 Lighting control................................................................................................. 39 Lighting circuit wiring ........................................................................................ 41 Lighting circuits exercise .................................................................................. 44
Power circuits ..............................................................................................52 Cooking appliances .....................................................................................52 Cooker types .................................................................................................... 53 Cooker control.................................................................................................. 54 Cooker design current calculations................................................................... 55
Motors ...........................................................................................................57 Motor control .................................................................................................... 57 Motor design current calculations ..................................................................... 58
Water Heating...............................................................................................60 Water heating control ....................................................................................... 61 Water heater design current calculations.......................................................... 62
Electrical Heating.........................................................................................63 Direct acting heaters ........................................................................................ 63 Convection heaters .......................................................................................... 64 Thermal storage heaters .................................................................................. 64
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LEARNER WORK BOOK Electric heating control ..................................................................................... 64 Electrical heating design current calculations ................................................... 65
Standard Socket circuits .............................................................................66 BS1363 Plug top and socket ............................................................................ 67 The Fused Connection Unit.............................................................................. 68 The Ring Final Circuit....................................................................................... 69 The Radial Circuit............................................................................................. 70
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Foreword In this section we will look deeper into the different circuits used within everyday installations. Different types of mains voltage circuits use the same type of cable but will most likely be a different size to safely cope with the demand of that circuit. Some circuits need specific cable types to function correctly. Circuits operate and function in dependence upon what information the electrician knows about the circuit and how he connects them up. Circuit diagrams, if used correctly, ensure that circuits will function as they are designed to do. BS7671 dictates how all our everyday lighting and power circuits are installed but the emergency systems that we come into contact with everyday in the training centre, in a public place are governed by their own British Standard.
This workbook is to be accompanied by PowerPoint “Cable and Circuit Types”
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Cables and Circuit Types Unit Overview Practical Skills To achieve the learning outcome the candidate must be able to: Complete the practical tasks in the electrical unit ensuring correct cables are selected Select correct sizes conductors and use correct installation methods Complete wiring diagrams for various types of lighting circuit Complete design current calculations for various types of lighting and power circuit Select the correct circuit components for lighting and power circuits
Knowledge Requirements To achieve the learning outcome the candidate must know: The different cable types used within industry for the common types of circuit What BS7671 says about circuits and their parts How circuits are categorised with regard to their duty The standard ratings of circuit types About lighting types and how lighting circuits operate The different types of power circuit and how they are typically controlled
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Cable Types Insulation properties are one of the most important things to consider when selecting a cable for a circuit. If the insulation is not rated for the environment the circuit is installed in it can have a number of detrimental effects on its operation and safety. What can happen to a circuit if the insulation is incorrectly rated? List the factors below
Mechanical protection for a cable is probably as equally as important as its insulation. If a circuit with limited mechanical strength is installed in an environment where there is a risk of it coming into contact with heavy objects that circuit will most likely become unsafe through being damaged. Name the type of cables you have heard of in the box below.
So you can improve your understanding of cables as we progress through the power point note down the common names of the cables next to their picture below
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Light duty cables 6242Y Flat twin and earth
Available sizes: 1.0mm²; 1.5mm²; 2.5mm²; 4.0mm²; 6.0mm²; 10.0mm²; 16.0mm² (CPC is one size less) Description: PVC insulated and Grey or white PVC sheathed. Solid copper conductor. Approved to BS 6004 Uses: Domestic fixed wiring for any type of circuit; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking 6242YH Flat twin brown and earth
Available sizes: 1.0mm²; 1.5mm² Description: PVC insulated and Grey or white PVC sheathed. Solid copper conductor. Approved to BS 6004 Uses: Domestic fixed wiring for switch wires only; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking 6243Y Flat three core and earth
Available sizes: 1.0mm²; 1.5mm² Description: PVC insulated and Grey or white PVC sheathed. Solid copper conductor. Approved to BS 6004 Uses: Domestic fixed wiring for two way strapping wires or circuits that require permanent live and switched live conductors such as extractor fans or automatic security lighting; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking
With all flat twin and three core cables the circuit protective conductor is smaller than the line and neutral conductors. A 1.5mm² cable has a 1.0mm² cpc. A 2.5mm² has a 1.5mm² cpc and so on. This bare copper conductor needs to be covered with green and yellow sleeving to protect it from live parts within an enclosure.
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LEARNER WORK BOOK 3182Y Two core round flex
Available sizes: 0.5mm²; 0.75mm²; 1.0mm²; 1.5mm² Description: PVC insulated and White or Black sheathed. Stranded copper conductor. Approved to BS 6004 Uses: Double insulated appliances that do not require earthing; domestic lighting cabinet display etc. push switches, table and standard lamps, radios, and lighting pendants etc
3183Y Three core round flex
Available sizes: 0.5mm²; 0.75mm²; 1.0mm²; 1.5mm²; 2.5mm² Description: PVC insulated and White or Black sheathed. Stranded copper conductor. Approved to BS 6004 Uses: Domestic lighting cabinet display etc; kettles, toasters etc; earthed equipment; extension leads.
3184Y Four core round flex
Available sizes: 0.75mm²; 1.0mm²; 1.5mm² Description: PVC insulated and White or Black sheathed. Stranded copper conductor. Approved to BS 6004 Uses: Domestic security lighting where permanent and switch live conductors are needed; bathroom fans.
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LEARNER WORK BOOK 3185Y Five core round flex
Available sizes: 0.5mm²; 0.75mm²; 1.0mm² Description: PVC insulated and White or Black sheathed. Stranded copper conductor. Approved to BS 6004 Uses: Domestic and commercial heating and ventilation controllers; timers.
3183TQ Three core rubber flex
Available sizes: 0.75mm²; 1.0mm²; 1.5mm² Description: Heat resistant, rubber insulated and White or Black sheathed. Stranded copper conductor. Approved to BS 6500 Uses: Domestic and commercial heating supplies; equipment in high temperature zones up to 85°C.
6242BH Flat twin and earth
Available sizes: 1.0mm²; 1.5mm²; 2.5mm²; 4.0mm²; 6.0mm²; 10.0mm²; 16.0mm² Description: Low smoke and fume twin and three core & earth. To BS 7211. Solid copper conductor Uses: Domestic fixed wiring for any type of circuit where reduced risk of smoke hazard is required such as in schools; buried in building fabric or clipped direct; Commercial wiring if above a ceiling or installed on tray or in trunking
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6181Y Double insulated meter tails
Available sizes: 10.0mm²; 16.0mm², 25.0mm²; 35.0mm² Description: PVC insulated Grey PVC sheathed. Stranded copper conductor. Approved to BS 6004 Uses: Double insulated surface wiring cable for meter tails for connection to consumer units and switchfuses.
6491X Single core PVC insulated and 6491B Single core LSF insulated
Available sizes: 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², 25.0mm² and higher Description: PVC insulated single core. Approved to BS 6004 or LSF insulated single core. Approved to BS 7211; Solid or Stranded copper conductor Uses: Fixed wiring within conduit and trunking; earthing conductors; main equipotential and supplementary bonding Bell wire / speaker wire
Available sizes: Solid 0.2mm² / 13, 42, 79 strands Description: Twin stranded plain annealed copper cores laid side by side in a flat figure 8 construction. Uses: Low voltage circuits (up to 60v); bells and chimes; door pushes.
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CAT5E Network cable
Available sizes: 4TP (four twisted pairs) 0.2mm² Description: Solid plain annealed copper. Category 5 cables offer extended transmission distance over frequency ranges up to 350Mhz and data speeds up to 1000 Mbps Uses: Computer networking. Standard co-axial / Satellite cable
Available sizes: 75 Ohm (6.7mm OD) Description: Solid plain annealed copper. Air spaced polythene insulation with copper wire braiding Uses: Television and radio down leads for aerials; Linking satellite dishes to receivers Security alarm cable
Available sizes: 0.5mm² Description: 4, 6 or 8 core tinned annealed copper conductors, PVC insulated and sheathed Uses: Flexible cables normally used for the wiring of burglar alarm and other low voltage circuits
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Medium duty cables Medium duty cables are selected due to their mechanical strength. Typical environments where they would be used are construction sites, business premises, warehouses and small factories. Any of the light duty cables seen previously can be used in these installations as long as they are sufficiently protected. What extra risks might exist where medium duty protection is required?
3183AG Three core arctic flex
Available sizes: 1.5mm²; 2.5mm² Description: Exterior use with tools, lighting and equipment. Stays flexible at -30ºC. Generally to BS 6500. Stranded copper conductor Uses: Construction site temporary lighting; power tools; outdoor equipment; extension leads (Yellow for 110v, Blue for 230v).
YY Control flex
Available sizes: 0.5mm², 0.75mm², 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², Description: Three, four, five core up to 25 core PVC / PVC insulated multi core. Approved to BS 6500. Stranded, number coded copper conductor Uses: Used as connecting cable, as measuring, checking and control cable in machine tool manufacturing, plant engineering and on assembly lines and production lines. Suitable for fixed installation (tray work) or flexible applications without exposure to tensile load.
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SY Control flex
Available sizes: 0.5mm², 0.75mm², 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², Description: Three, four, five core up to 25 core PVC / PVC insulated, galvanised steel wire braided multi core. Approved to BS 6500. Stranded, number coded copper conductor Uses: Used as connecting cable, as measuring, checking and control cable in machine tool manufacturing, plant engineering and on assembly lines and production lines. Suitable for fixed installation (tray work) or flexible applications without exposure to tensile load. Due to the galvanised steel wire braiding, these cables can even be used under adverse operating conditions or when exposed to high mechanical strain.
CY Control flex
Available sizes: 0.5mm², 0.75mm², 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², Description: PVC / PVC insulated, copper wire braided multi core. Approved to BS 6500. Stranded, number coded copper conductor Uses: Used as connecting cable, as measuring, checking and control cable in machine tool manufacturing, plant engineering and on assembly lines and production lines. Suitable for fixed installation (tray work) or flexible applications without exposure to tensile load. These cables with copper screening are ideally suitable for interference free data and signal transmission in measuring and control technology
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Heavy duty cables 6943X, 6944X, 694*X - XLPE Steel wire armoured
Available sizes: 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², 25.0mm² and larger Description: 3-core (6943X) and 4-core (6944X); *up to 48 core galvanised Steel Wire Armoured Cable. Black XLPE sheathed, PVC insulated. Approved to BS 5467. Available in copper or aluminium (above 16mm² only Uses: General circuit wiring (on tray work or ladder or clipped direct) where there is an increased risk of mechanical damage; underground cable routes
Mineral insulated copper conductor
Available sizes: 1.0mm², 1.5mm², 2.5mm², 4.0mm², 6.0mm², 10.0mm², 16.0mm², 25.0mm² and larger Description: Single core, two, three, four, seven, twelve and nineteen core. Orange (general use), red (fire alarms) or white (emergency lighting) LSF sheathed, mineral (magnesium oxide) insulated. Solid copper conductor. Approved to BS 6207 Uses: General circuit wiring (on tray work or ladder or clipped direct) where there is an increased risk of mechanical damage; underground cable routes
What sort of environment might this cable be used for in lighting circuits?
BS5308 Instrumentation cable
Available sizes: 0.5mm², 0.75mm², 1.0mm², 1.5mm²;. Description: 1, 2, 5, 10, 20, 30 and 50 pair. XLPE or PVC Blue sheath for I.S; Black sheath for general purpose. Stranded copper conductor. Approved to BS 5308. Each cable combines one part and one type. For example: Pt1 = XLPE sheathed Pt2 = PVC sheathed; Ty1 = Un-armoured Ty2 = Armoured A Pt1 Ty2 cable is Armoured Collectively Screened or individually & collectively screened with XLPE sheathing Uses: Used to carry voice and data services, they also serve as the interconnection between electrical equipment and instruments, particularly in and around process plants, where signals are transmitted through to panels, controllers and other devices
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In pairs complete the exercise below. You are asked to select a cable type and wiring system and state the reasons why for some different situations. You will need to state the installation method and number given in BS7671. You will need your Tables from BS7671.
1. A supply to a garage, 10 metres from a house where an overhead route is not an option.
2. An office building heating system is controlled via thermostats carrying low voltage digital signals. All wiring is above the ceiling. 3. A supply is required for a lift shaft lighting circuit. The supply will be used at various points to ensure illumination levels are sufficient on all floors. This is a high risk mechanical protection environment 4. An extra socket is required in a kitchen. It will not be surface mounted so the cable will have to be buried in the building fabric. 5. A new classroom is being built in a school and there needs to be a reduced risk of hazardous fumes in the event of a fire. The installation needs to be easily rewireable for future alterations. 6. An evacuation / fire alarm speaker needs to be installed in a boiler house where there is very loud machinery and an ambient temperature of 50ºC. 7. A new industrial plant requires data to be transmitted from instrument to panel on tray work
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Circuits and BS7671 Division of the installation Section 314 of BS7671 requires an installation to be divided into circuits, as necessary. Dividing an installation into circuits helps to avoid hazards, facilitates safe inspection, testing and maintenance and minimises the inconvenience in the event of a fault. It is imperative that careful consideration is given to how many circuits are required for a given electrical installation. It is also important to select the most appropriate overcurrent and fault protection devices and to position them to ensure, as far as possible, that the only circuits to be disconnected are the ones where a fault has occurred. Separate final circuits provide separately controlled parts of an installation so each final circuit must connect to a separate way in the consumer unit. BS7671 states: Every installation shall be divided into circuits, as necessary, to: i. Avoid hazards and minimise inconvenience in the event of a fault ii. Facilitate safe inspection, testing and maintenance (see also 537) iii. Take account of danger that may arise from the failure of a single circuit such as a lighting circuit iv. Reduce the possibility of nuisance tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation v. Mitigate the effects of electromagnetic interferences (EMI) vi. Prevent the indirect energising of a circuit intended to be isolated
Typical domestic installation
Typical commercial installation
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Accessories BS7671 defines an accessory as: “A device, other than current-using equipment, associated with such equipment or with the wiring of an installation.” They are the items that are used to control or utilise current using equipment on an installation. Before any work is undertaken or regarding the installation of any new accessories time needs to be spent reading any instructions for the new parts. If the work involves the replacement of accessories in an existing installation you must make sure that the new parts are fully compatible with those on the existing electrical installation. Name five different types of accessories
BS7671 makes the following reference to accessories: • • •
• •
All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. (Reg:132.12) An accessory is an item of electrical equipment that does not use any current e.g. a switch or a socket-outlet. (Definitions) Every termination, connection or joint between live conductors including the neutral conductor must be made in a suitable accessory or enclosure. (Reg: 421.7 and 526.5) All accessories must be fitted to an appropriate mounting box. (Reg: 530.4.2) A wall mounted socket outlet must be mounted high enough above a floor or work surface to prevent damage to the flexible cord of a plug top.(Reg: 553.1.6)
BS 7671 does not give specific heights for accessories but the Building Regulations 2002 require the following: Accessories must be mounted between 450mm and 1200mm from the finished floor level in habitable rooms in new dwellings The centre of a socket outlet should be a minimum of 150mm above the kitchen work surface Accessories must be installed a minimum of 300mm from the edge of cooker spaces, kitchen sinks and draining boards Accessories should not be mounted so that it is necessary to lean or reach over a cooker to operate them Accessories should be mounted on the building fabric and not on kitchen furniture Socket-outlets supplying washing machines and dishwashers should be installed so that water that may drip from plumbing equipment is unlikely to affect the socket-outlet or plug top
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Electrical equipment BS7671 defines electrical equipment as: “Any item for such purposes as generation, conversion, transmission, distribution or utilisation of electrical energy, such as machines, transformers, apparatus, measuring instruments, protective devices, wiring systems, appliances and luminaires” Class 1, 2 & 3 equipment Class I Equipment has exposed conductive parts connected to a protective conductor Class II Equipment has both basic and supplementary insulation with no exposed metalwork connected to a protective conductor Class III Equipment is SELV equipment Give three examples of equipment for the three main classifications of duty
Connections of such equipment are very important. The cable sheath should enter the enclosure, the c.p.c should be sleeved and no more insulation removed from the conductors than is necessary to make a proper connection.
BS7671 makes the following reference to electrical equipment: • • • • •
•
All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. (Reg: 132.12) All items of current using equipment must be provided with a functional switching device. (Reg: 537.5.1.3) All electrical equipment must conform to all relevant standards and be suitable for its location and use. (Reg: 133.1) Electrical equipment must not create a fire hazard to nearby materials. (Reg: 421.1) A ring final circuit must not supply an immersion heater, storage heaters or a cooker rated more than 2kw. (Reg: 433.1.5 and App.15) For all relevant BS or EN numbers see Appendix 1.
Any alterations or additions to an electrical installation may have an effect circuit’s maximum demand. If the circuit length is increased this will have an effect on the disconnection time for the circuit. These changes could mean that the overcurrent protective device and the circuit conductors are no longer suitable and would need to be re-assessed in terms of voltage drop, shock protection and thermal constraints.
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Isolation and switching Isolation and switching serves the following purposes as stated in BS7671: Isolation of electrical energy Isolation for mechanical maintenance Functional switching Emergency switching
BS7671 defines isolation as: “A function intended to cut off for reasons of safety the supply from all, or a discrete section, of the installation by separating the installation or section from every source of electrical energy.”
BS7671 makes the following reference to isolation: • •
• •
• • •
Isolation is used to remove the supply to all or part of an installation for safety reasons. (Definitions) When an isolating device is installed remotely from the equipment it is meant to isolate, the device must be able to be locked in the OFF position. (Reg: 537.2.1.5) A main switch must be provided to cut off the voltage to an installation. (Reg: 132.15.1) A double pole main switch or linked circuit breaker must be installed as close as possible to the incoming supply at the origin of the installation. (Reg: 537.1.4) Isolation devices should preferably be double pole. (Reg: 537.2.2.5) An isolation device should be labelled if the equipment it isolates is not obvious by its position. (Reg: 537.2.2.6) For more detailed information on devices suitable for Isolation see Table 53.2 p117
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LEARNER WORK BOOK BS7671 defines mechanical maintenance as: “The replacement, refurbishment or cleaning of lamps and nonelectrical parts of equipment, plant and machinery.” Equipment is not always isolated for electrical purposes. Mechanical maintenance tasks such as pump repairs or machinery guard’s replacements all require electrical isolations to ensure that it is safe to carry out the work.
BS7671 makes the following reference to mechanical maintenance: •
•
•
•
•
• •
• • • • •
Mechanical maintenance is the repair, replacement or cleaning of non electrical parts of electrical equipment. (definitions) Electrical equipment that may cause injury during mechanical maintenance i.e. fans, must be provided with a means to switch off the supply. (Reg: 537.3.1.1) A double pole switch, circuit breaker and plug top & socket outlet may be used to switch off the supply during mechanical maintenance. (Reg: 537.3.2.1) A double pole main switch or linked circuit breaker must be installed as close as possible to the incoming supply at the origin of the installation. (Reg: 537.1.4) In a TN-S or TN-C-S system the neutral conductor is not required to be switched or isolated when it is known that the supply neutral and earth conductors are connected. (Reg: 537.1.2 and Reg: 537.2.1.1) A single pole switch must only be used with a line conductor. (Reg: 132.14.1) Only a linked switch that breaks all related line conductors can be used with an earthed neutral conductor. (Reg: 132.14.2) A neutral conductor must not be independently fused or switched. (Reg: 530.3.2) A main switch must be provided to cut off the voltage to an installation. (Reg: 132.15.1) Extractor fans must be provided with an easily accessible means of switching off the supply.(Reg: 132.15.2) Unless its purpose is obvious, all items of switchgear must be labelled. (Reg: 514.1.1) All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. (Reg: 132.12)
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LEARNER WORK BOOK BS7671 defines functional switching as: “An operation intended to switch ‘on’ or ‘off’ or vary the supply of electrical energy to all or part of an installation for normal operating purposes” Functional switching is not isolation or emergency switching or switching for mechanical maintenance
BS7671 makes the following reference to functional switching: •
• •
Functional Switching is used to switch on or off the electrical supply to an installation or any part of an installation under normal operating conditions. (Definitions) All items of current using equipment must be provided with a functional switching device. (Reg: 537.5.1.3) Fuses and unswitched fused connection units (unswitched spurs) must NOT be used for Functional Switching. For more detailed information on devices suitable for Functional Switching (see Table 53.2 p117)
BS7671 defines emergency switching as: “An operation intended to remove, as quickly as possible, danger, which may have occurred unexpectedly.” Emergency switching is usually part of an under-voltage protection circuit such as motor control BS7671 makes the following reference to emergency switching: • •
Emergency Switching is intended to remove any unexpected danger as rapidly as possible. (Definitions) Fuses, unswitched fused connection units (unswitched spurs) and plug top & socket outlets must NOT be used for Emergency Switching. For more detailed information on devices suitable for Emergency Switching (see Table 53.2 p117)
As you can see by the definitions above there are very distinct differences between isolations and switching. It is very important to know the difference between them so there is no confusion over whether a circuit is isolated or not. Other circuit information A durable notice giving details of all the circuits fed is required to be posted in or near each distribution board. The information required is the equipment served by each circuit, its rating, its design current and its overcurrent device breaking capacity. When the occupancy of the premises changes the new occupier must be provided with full details of the installation. This data must always be kept up to date.
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Now answer the questions below 1. Explain why we need to divide an installation into circuits
2. In your own words, define what an accessories is
3. State three references to accessories in BS7671
4. Explain why you think it is not permissible to mount accessories onto kitchen units?
5. In your own words define what electrical equipment is
6. State three references to electrical equipment in BS7671
7. In your own words define isolation and switching
8. What needs to be displayed at each consumer unit in an installation?
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Circuit Categories The regulations state that circuits from different categories cannot be mixed in an installation. This is due to the detrimental interference between each circuit type. Magnetic fields generated by mains cables can interfere with the transmission of data signals and produce harmful or incorrect values on a data network. When we refer to different categories of circuit we mean:
Category 1 – Circuits operating at low voltages (50 to 600 volts AC) and supplied from the electrical mains. Category 2 – Any data, telecommunication, intruder alarm systems and circuits operating at extra low voltage. (not exceeding 50 volts AC and 120 volts DC)
Category 3 – Any fire detection system, emergency lighting or alarm system. One exception to this rule is that as long as each circuit’s insulation is rated to the highest circuit voltage they can be installed together without any segregation. On site a typical installation will have several routes for carrying the different circuit types. There may be three cable trays, side by side, each designated to carry only the circuits form each category. In an office installation there may be multi compartment trunking or a single tray work with sufficient space between them (approx 300mm) so no interference takes place.
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Category 1 Circuits Circuits operating at low voltages (50 to 600 volts AC) and supplied from the electrical mains. Lighting circuits All lighting circuits and their control wiring are included in category 1. One way; two way; intermediate; contactor controlled, timer controlled, automatically controlled are all examples of Category 1 circuits. Lighting that operates at less than 50 volts from a direct supply is NOT included. Power circuits All power circuits and their control wiring are included in category 1. Ring main and radial socket circuits; cooking appliances; fixed equipment; motors are all examples of Category 1 circuits. Control circuits that operate at voltages less than 50 volts are not included. Heating circuits All heating circuits and their control wiring are included in category 1. Dual-element immersion heaters; cistern-type water heaters; non-pressure water heaters; Instantaneous water heater; controllers; timers; thermostats; pumps; boilers are all examples of Category 1 circuits. Environmental control circuits All circuits that provide cooled/heated air to a building are included in category 1. Air conditioning units; controlled; pumps; electronic shutters; valves; heating elements are all examples of Category 1 circuits. Standby power supplies All circuits that provide back up supplies to an electrical installation are included in category 1. Generators; temporary power leads; fixed back up battery units are all examples of Category 1 circuits.
The electrical supply in this country is very reliable and secure. However, as with all systems there are occasional interruptions that for some installations would be dangerous as well as inconvenient. Hospitals, air-traffic control and the petrochemical industry are just a few installations that could not tolerate an interruption to the mains supply, so a standby system needs to be available. Large installations need a standby generating system, whereby a large combustion engine cuts in automatically and drives a generator capable of supplying the load needed to continue working safely Smaller establishments such as small offices cannot afford complex standby generation systems, but nevertheless they may have computer systems that cannot afford to be off or, worse still, risk losing data. In this situation standby power systems known as Uninterruptible Power Supplies (UPS) are used, which consist of a battery supply that is charged up via the mains when not in use. When the mains supply is lost the UPS automatically cuts in and, via the electronics contained in it, converts the D.C battery supply to a mains supply capable of powering several computers.
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Category 2 Circuits Any data, telecommunication, intruder alarm systems and circuits operating at extra low voltage. (Not exceeding 50 volts AC and 120 volts DC) Data Transmission Fibre-optic cables This cable is used for digital transmissions used by equipment such as telephones and computers. They are made from optical-quality plastic (the same as spectacles) where digital pulses of laser light are passed along the cable from one end to another with no loss or interference from mains cables (assuming the insulation is sufficiently rated). They look like SWA cables but of course they are much lighter and contain either one core or many dozens of cores. Tight radius bends in this type of cable should be avoided, as should ‘kinks’, as the cable will break. Jointing of these cables requires specialist tools and equipment. Never look into the ends of the cable as the laser light could damage your eyes. The applications of optical fibre communications have increased at a rapid rate since the first commercial installation of a fibreoptic system in 1977. Telephone companies began early on replacing their old copper-wire systems with optical-fibre lines. Today’s telephone companies use optical fibre throughout their system as the backbone architecture and as the long-distance connection between city phone systems. Fibre-optic cables are also used in Local Area Networks (LAN). These collective groups of computers, or computer systems, connected to each other, allow for shared program software or databases. Colleges, universities, office buildings and industrial plants, just to name a few, all make use of fibreoptic cables within their LAN systems. Power companies are emerging as big users of fibre optics in their communication systems. Most power utilities already have fibre-optic communication systems in use for monitoring their power grid systems.
If you wanted to see down a dark corridor, you might shine a torch down it. But what if the corridor had a bend in it? You could probably put a mirror in just the right place at just the right angle and shine the light round the corner. But what if the corner had lots of bends? Well, what if I made the entire corridor walls out of mirrors, then I wouldn’t need to put them in just the right place or angle. The light would be able to bounce around all the mirrors along the walls. Believe it or not, that’s the theory behind fibre-optics, as the glass core is essentially a mirror wound into a thin tube. Some 10 billion digital bits can be transmitted per second along an optical fibre link in a commercial network, enough to carry tens of thousands of telephone calls. The hair-thin fibres consist of two concentric layers of high-purity silica glass, the core and the cladding, which are enclosed by a protective sheath.
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Cat (Category) 5 cable This cable is used extensively for data transfer in computer networks and telephone systems. It has four pairs of wires that transmit and receive data along them at very high frequencies; typically 350 MHz. Special termination ends are required for these cables.
There are three basic types of cabling used in data systems: coaxial, fibre-optic and Unshielded Twisted Pair (UTP). Coaxial is widely installed in older networks but is not recommended for new network installations. Fibre is used for high-speed networks and to connect networking devices separated by large distances. But UTP is currently the most common and recommended cabling type. UTP is inexpensive, flexible and can transmit data at high speeds. Most new installations are currently installed with Cat-5 UTP cabling and components.
UTP (CAT5) cable
Intruder / Security Alarms Alarm systems are classified as Category 2. All cabling that connects to keypads, remote sensors, sounders, beacons or door contacts all need to be kept separate.
Closed Circuit Television (CCTV) Camera equipment uses a co-axial cable that transmits (and sometimes powers) cameras on an installation.
Co-axial cable
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Category 3 Circuits Any fire detection system, emergency lighting or alarm system Fire Alarms A correctly installed fire-alarm system installation is of paramount importance and can be compared to any other electrical undertaking, as life could be lost and property damaged as a result of carelessly or incorrectly connected fire-detection and alarm equipment. Fire alarms typically use red FP200 (or equivalent) or MICC cable. MICC Cable
Emergency Lighting Emergency lighting is not required in private homes because the occupants are familiar with their surroundings. However, in public buildings, people are in unfamiliar surroundings and in an emergency they will require a well-illuminated and easily identified exit route. Emergency lighting should be planned, installed and maintained to the highest standards of reliability and integrity, so that it will operate satisfactorily when called into action. Emergency lighting typically use white FP200 (or equivalent) or MICC cable. When they are installed as an integral part of a lighting circuit they use the general circuit wiring (i.e. PVC cable). Also included in category 3 are any circuits that are used as an evacuation system. This could be a system that is a person-operated loudspeaker system or a computer or panel operated one.
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LEARNER WORK BOOK Check your learning by answering the questions below
1. State the voltage range of a category 1 circuit
2. What type of lighting circuits are not included in category 1?
3. Give two examples of a power circuit.
4. What is a UPS and how does it function?
5. What type of circuits are included in category 2?
6. Explain the basic theory behind fibre optics.
7. Explain why you think category 3 circuits have their own classification.
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Standard Circuit Ratings The table below shows most of the standard circuits found in domestic premises and the ratings of the overcurrent devices that may be used to protect them. TYPE OF CIRCUIT Lighting Socket outlets (BS1363) Socket outlets (BSEN60309) Immersion Heater (3-4kW) Cooker, electric shower (6-10kW)
RATING (A) 5, 6, 10 20, 30, 32 20 15, 16, 20 30, 32, 40,45
TYPICAL CONDUCTOR SIZES (mm²) 1.0, 1.5 2.5, 4 2.5 2.5, 4 4, 6, 10
There may seem little difference between a 5A and 6A protective device and indeed, in terms of performance, there is little difference. This reflects the changes in standards over time. Rewirable fuses to BS3036 and cartridge fuses to BS1361 are made in ratings 5, 15 20, 30 A. Miniature circuit breakers and (mcb’s) to BS EN60898 are made in sizes 6, 10, 16, 20 and 32A. The third column on the table shows the cable sizes that are typically used in these circuits. The list is not an inclusive list as unique installation conditions may require a larger cable. The list is for thermoplastic (p.v.c) cable. If m.i.c.c were being used then it would be found that a cable one size smaller would usually suffice. This is because the mineral insulation can withstand higher temperatures than p.v.c and so the current ratings of m.i.c.c are higher as a result. RCDs are required to protect users of appliances and equipment where sockets rated up to 20 amps are used. The highest risk is outside of the equipotential zone. This is due to the fault return path not being sufficient
Class discussion Dave, an electrical apprentice needs to isolate a lighting circuit so he can fit a dimmer switch to an upstairs bed room for his parents. Explain the step by step process he will go through to ensure he does the isolation correctly.
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Lighting Circuits Lighting is a vast and varied subject and beyond the scope of these notes. However we will see some of the basic requirements, the different lighting circuits used and the types of lamps associated with standard installations.
BS7671 makes the following reference to lighting circuits: • •
• • • •
• • • •
• •
All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. (Reg: 132.12) Where an installation has only one lighting circuit, the circuit may need to be divided into two circuits to minimize the danger that may arise in the event of a fault.(Reg: 314.1 (iii) ) Light fittings must be installed so that any radiated heat does not cause any damage. (Reg: 559.5.1) The fixing supporting a light fitting must be able to support a weight of at least 5kg. (Reg: 559.6.1.5) Domestic lighting circuits should not be rated at more than 16A. (Reg: 559.6.1.6) Edison screw lampholders (excluding types E14, E27) should have the outer contact connected to the neutral conductor. (Reg: 559.6.1.8) Lighting circuits must be controlled by the appropriate number of switches. (Reg: 559.6.1.9) Through wiring is only permitted in a light fitting where the light fitting is designed for such wiring. (Reg: 559.6.2.1) Through wiring in a light fitting must be suitable for the temperature generated inside the fitting. (Reg: 559.6.2.2) The lamp inside an outdoor light fitting mounted less than 2.8m above the ground must only be accessible after removing an enclosure or barrier with the use of a tool. (Reg: 559.10.3.1) Electrical equipment outdoors must be rated at least IP33. (Reg: 559.10.5.2) Switchlines must be marked brown at their terminations. (Reg: 514.3.1 and App.7 chapter 3
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Lighting points For each fixed lighting point one of the following must be used. 1. Ceiling rose
2. Device for connecting a luminaire (DCL)
3. Batten lamp holder
4. Luminaire designed to be connected directly to the circuit wiring
BS7671 states that the maximum rating of overcurrent protective devices of circuits must not exceed the lamp holder rating. These are: Small Bayonet Cap Bayonet Cap Small Edison Screw Edison Screw Giant Edison Screw
B15 B22 E14 E22 E40
Max Rating 6 Amps Max. Rating 16 Amps Max. Rating 6 Amps Max. Rating 16 Amps Max. Rating 16 Amps
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Lamp types Incandescent filament lamps (GLS and reflector) Filament lamps are used mainly for domestic and display lighting. There are many types of filament lamp, the most common being general lighting service (gls) and decorative. Their finish – clear, diffuse/pearl or coloured – is a significant factor in their application. Reflector lamps are similar but have an envelope with an internal reflective coating. Advantages of filament lamps include low initial cost, simple operation (no control gear required) and good colour rendering. Disadvantages of filament lamps are low efficacy (measure of the energy efficiency of a light source, ie lumens per watt) and a relatively short life. Certain extended life filament lamps have only about half the efficacy of standard lamps. The light output of filament lamps is particularly sensitive to voltage variations.
Halogen filled filament lamps (tungsten halogen) The main reason for filling a tungsten filament lamp with a halogen gas is to prevent evaporated tungsten from blackening the envelope. Tungsten halogen lamps also have an increased light output and/or an extended life compared with standard filament lamps. The envelope is of small dimensions and made of quartz or hard glass. Some mains voltage lamps have an outer protective envelope. Lamps that are suitable for use in luminaries without a safety screen should be so marked. Otherwise, tungsten halogen lamps should only be used in suitably enclosed luminaries. Extra low voltage (elv) lamps are, in general, more compact than their mains voltage counterparts and the small filament size can improve the optical efficiency of integral or external reflectors (generally dichroic). Elv reflector lamps make it possible to use compact luminaires for display lighting.
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Low pressure mercury fluorescent tubes The light output from a tubular fluorescent lamp comes from phosphors that convert energy from a low pressure gas discharge into visible light. The colour temperature and colour rendering are determined by the phosphor mix coated on the inside of the tube. The argon-filled t12 (38mm diameter) tubes are being discontinued. The modern range of krypton-filled triphosphor t8 (26mm) diameter tubes should be the first choice for switchstart, quick start and high frequency luminaries. Such lamps have a higher efficacy, longer life, improved lumen maintenance and better colour rendering than earlier types of tube. Triphosphor (or multi-phosphor) tubes offer a wide range of colour temperatures from very warm (2700k), warm (3000k) and intermediate (3500k) through to cold white (4000k), daylight (5000-5500k) and northlight (6000-6500k). The main tube lengths and their respective power ratings are: 600mm = 18w 1200mm = 36w 1500mm = 58w 1800mm = 70w
Compact fluorescent lamps A compact fluorescent lamp (cfl) has the characteristics and advantages of linear fluorescent lamps but its compact size is achieved by folding the discharge path, retaining high efficacy. The two main groups of cfls are those with external control gear and those with internal control gear. High frequency control gear is now available integrated into the cfl lampholder, making lamp conversion from gls to cfl very simple. Many modern fluorescent lamps are operated at high frequency (typically at or above 30 khz) which results in a reduction of energy losses both in the lamp and the control gear. The control gear size and weight are often less, the efficacy higher, dimming where required is easier, and operation is silent. Incandescent CFL Wattage Wattage 25 5 50 9 60 15 75 20 100 25 120 28 150 39 Wattage comparison chart
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High pressure sodium lamps Light is generated by an electrical discharge in a gas containing sodium and mercury (sodium amalgam) contained in a sintered alumina arc-tube. High pressure sodium lamps are used for road lighting, floodlighting and industrial interior lighting.
Low pressure sodium lamps Low pressure sodium lamps consist of a u tube containing the discharge, and an outer heat reflecting glass jacket. The monochromatic light is concentrated in the yellow part of the visible spectrum which is close to the maximum sensitivity of the human eye at normal lighting levels. The efficacy is the highest of all lamp types, but with very poor colour rendering. Low pressure sodium lamps are used mainly for exterior applications such as road and security lighting (but are not suitable for repeated on/off (operation).
Metal halide discharge lamps Metal halide lamps have quartz or sintered alumina (ceramic) arc tubes, generally with an outer glass envelope. Light output is from mercury and other metallic elements introduced in the form of halides. Metal halide lamps of the ‘protected’ type are now available for operation in luminaries without safety screens. According to the mix of elements, there is a wide range of efficacy and/or colour appearance, but colour rendering is generally good. Metal halide lamps are generally used in commercial interiors, industry and floodlighting, and (in smaller ratings) for retail lighting.
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High pressure mercury discharge lamps The high pressure mercury discharge operates in a quartz envelope. Mercury lamps were used for illuminating road signs and industrial lighting but have largely been replaced by the more efficient lamps now available. Such lamps offer low cost discharge lighting where high efficacy is not important. They often incorporate a third electrode for starting and in such cases the control gear required generally consists only of a ballast and a power-factor corrected capacitor.
Induction lamps Induction is a process whereby electrical power is passed from one circuit to another without the use of physical electrical conductors. It enables lamps to be constructed without the need for wire connections to pass through the glass or quartz envelope. Induction lamps are available as low pressure mercury lamps, using the same triphosphor coating of the inner envelope surface as the familiar fluorescent tubes. The commercially available range of induction lamps is limited.
Light emitting diodes (leds) Light emitting diodes have been used for indicating purposes for several decades and recent developments have created larger diodes and extended the range of colours including white. A dramatic increase in efficacy is predicted in the near future. Leds have an extremely long life and are likely to be built into the luminaire and will not be a consumable item as far as the end user is concerned.
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Lighting design current calculations Assumptions 1. A lighting outlet shall be considered to have a connected load of minimum 100 W. (Standard lighting points with lamps) 2. Discharge lighting calculations needs to take into to account harmonic currents and control gear losses. Where the exact manufacturer’s information for gear losses is not available the maximum demand shall be assumed to be 1.8 x the lamp rating Example 1 A house with ten rooms requires a light in each room. Calculate the design current. i)
Select the correct formula (basic resistive lighting)
P I= V ii)
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
10 × 100 = 4.35 A 230
Example 2 A warehouse office has two rooms with two, four tube light fittings rated at 36 watts per tube. Calculate the assumed design current for the 230v circuit. i)
Select the correct formula (discharge lighting). Three line circuits use a similar formula but the V (230v) is replaced by √3 x 400
I= ii)
P × 1 .8 V
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
2 × 2 × (4 × 36 ) × 1.8 = 4.51 A I= 230 Note: If the manufacturer’s discharge lamp information is obtainable from a catalogue this would be used to gain a more accurate value of design current. Standard lamp ratings are approximately increased by 10%.
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Complete the exercise below. You are asked to calculate the design current
1. A 230v lighting circuit supplying twelve rooms with one standard light point in each.
2. A 230v lighting circuit feeding twelve rooms has one centre light in each room and two wall lights in two rooms. The wall lights are twin 40w maximum type
3. A 230v discharge lighting circuit of four flood lights with a total load per fitting of 250 watts. Manufacturers lamp detail not known
4. An open plan office has twelve 4x36w light fittings all one line. Manufacturer’s lamp information reveals each lamp to be 39 watts (accounting for control gear losses).
5. A car show requires display lighting using 21 x 250 watt metal halide (discharge) lamps. The supply will be triple pole and neutral (TP&N).
6. How many lighting points are permissible to be supplied on one single line 10 amp circuit breaker?
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Lighting control Lighting accessories or luminaries must be controlled by a switch or switches to BS 3676 or BS 5518 and must be suitable, where necessary, for the control of discharge lighting circuits. Light switches can be one way, two way or intermediate. The number of switches per accessory are called 'gangs'. They are generally rated to carry 6 amps but some are available that can safely switch 20 amp circuits. The type of light switch finish depends on the installation (bathroom, kitchen, outdoor etc), the number of points of switching (one way, two way, intermediate) and the number of lamps to be switched from that location.
1 Gang or 1G White Plastic Switch
2 Gang or 2G White Plastic Switch
3 Gang or 3G White Plastic Switch
Label the switch terminals and draw their internal switching mechanisms
One-way switch control This indicates the user has the ability to switch the lighting circuit from one point only. The switch wire, which is usually blue, should be identified at both ends as a line conductor either by line colour or the letter 'L'. Two-way switch control This indicates the user has the ability to switch the lighting circuit from two points. Often used in bedrooms, on stairs, halls and rooms that may contain more than one point of entry. The extra wires that are used to connect the two light switches together are known as strapping wires or ‘strappers’. All strappers should be identified at both ends as line conductors either by colour or the letter 'L' More than two switching points Commonly known as 'intermediate switching', the circuit contains a twoway switching position at each extremity and extra “intermediate” switching positions within the circuit (as many as required). These are used in long rooms, stairs or corridors that require many switching points, where all it may be necessary to avoid the user walking long distances in the dark to operate the circuit.
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LEARNER WORK BOOK Centre switch terminals
Other information • A minimum of one fixed lighting point is required in each room, on hallways and on stairways/ landings • Each light has to be controlled by a switch • When determining the position of the lighting point and switch positions how each room is to be used must be taken into consideration • Additional lighting that may be required in certain rooms, such as lounges, kitchens, bedrooms and bathrooms • Switching from more than one point should be considered for a room with 2 or more points of entry, in bedrooms, long passages and halls as well as landings and stairs • The height of the light switches is set out in Regulation 553-1-6
Three gang switch terminals
Contactor control A contactor is a switching device that contains an electromagnet. When a voltage is applied to the electromagnet (or coil) it generates a magnetic field around it and forces contacts within the contactor to close. The full load current circuit connections are electrically separate from the coil so it can be operated with a different voltage. Contactors can be employed in lighting control circuits to switch loads that exceed the rating of switching devices. This means that standard 6 Amp switches would be used to control the supply to the coil and the entire lighting load would be taken by the contactor. Single or three line banks of lighting, power loads including motors can be controlled in this way. Due to the current that energises the contactor being negligible, the associated overcurrent device and circuit wiring can be sized accordingly. This means that smaller rating devices and conductors can be used for the control circuit. Below is a typical example of how a contactor might be used.
Typical contactor controlled lighting circuit
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Lighting circuit wiring Loop in Method (or Three Plate Method) The loop-in method is the most common wiring system employed in lighting installations using PVC twin and earth, steel wire armoured or MICC cables. No joints or connections are to be made anywhere except at the recognised termination points of the circuit, i.e. at switch terminals, ceiling roses or lamp holders. Each ceiling rose or lighting point requires three banks of terminals or plates. They are labelled Live, Loop and Neutral (earth is taken as always being present). The main supply cable runs from the circuit protective device at the fuse board to the first loop in ceiling rose then on to the next ceiling rose in the next room and so on. From each ceiling rose a cable is run down to the light switch, this acts as the supply conductor and the switched return conductor. The normally blue, switched conductor is required to be marked as a line conductor; both at the light switch and at the ceiling rose.
Pendant connections
Three plate ceiling rose connections
Conventional Method (or Two Plate Method) The conventional method was the only way to wire lighting circuits many years ago as manufacturers had not created a three plate accessory. Junction boxes would have been used to save on cable costs and lighting points would only have the facility for live and neutral terminations. Today, the same accessories can be used in this method but the loop terminal is not used. To achieve correct switching principles the feed from the fuse board must terminate at the switch. This means that the supply neutral conductor must terminate into a separate connector block inside the switch box. A switched live and neutral would then carry the supply on to the lighting point. Mixing the two methods is totally acceptable and in some cases may be the most practical. Especially where cable routes are considered. The disadvantages of the two plate method: • There is only ever a switched live at any one lighting point making alterations or additions to lighting circuits more difficult • Where junction boxes are used this presents “hidden” fault locations and laborious fault finding by moving furniture and carpets to lift floor boards to access the jb’s
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LEARNER WORK BOOK One way switch connections using single core or multi-core
Or 1G1W Switch connections wired in conduit (Single core)
1G1W Switch connections wired in 6242Y (Multi core)
Two way switch connections
+ 1G2W Switch connections wired in conduit (Single core)
1G2W Switch connections wired in conduit (Single core)
Or Two way switch connections
+ 1G2W Switch connections (first switch) wired in multicore
1G2W Switch connections (second switch) wired in multi-core
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LEARNER WORK BOOK Intermediate switch connections
+
+ Intermediate switch connections wired in conduit
1G2W Switch connections wired in conduit
1G2W Switch connections wired in conduit
Intermediate switch connections
+ 1G2W Switch connections (first switch) wired in multicore
+ Intermediate switch connections wired in multicore
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1G2W Switch connections (second switch) wired in multi-core
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Lighting circuits exercise Complete the exercises on the next pages once you understand the completed diagram below. Follow the supply to the loop, to the switch and then back to the light. This is the path the current takes when it operates the light.
L
LOOP
N
6A LIVE NEUTRAL Switch wire Circuit feed
C
L1
1 Gang 1 Way, 1 Light, 3 Plate method using multi-core
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L
LOOP
N
L
LOOP
N
C
L
1 Gang 1 Way, 2 Lights, 3 Plate method using multi-core
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LEARNER WORK BOOK
L
LOOP
N
C L 1
L 2
L 1
L 2 C
1 Gang 2 Way, 1 Light, 2 Plate Conduit method using single core
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LEARNER WORK BOOK
L
LOOP
N
C L 1
L 2
L 1
L 2 C
1 Gang 2 Way, 1 Light, 3 Plate Method using multi-core
Truth tables A truth table can be very useful if you forget how to connect up a lighting circuit. Draw the circuit and try it out before you wire it up by “operating” one switch at a time. If it doesn’t work on paper then it won’t work in real life! Example to be completed with the trainer Switch A LEFT RIGHT Switch B
LEFT
LEFT
Light – on / off
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L
LOOP
N
C L 1
L 2
L1
L1
L2
L2
L 1
L 2 C
1 Gang Intermediate, 1 Light, 2 Plate Conduit Method using single core
Complete the truth chart for the lighting circuit on this page by switching one switch at a time. Switch A Switch B Switch C
L S L
L S
S
S
X
X
R
Light – on / off
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L
LOOP
N
C L 1
L 2
L1
L1
L2
L2
L 1
L 2 C
1 Gang Intermediate, 1 Light, 3 Plate Method using multi-core
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LEARNER WORK BOOK Now complete the next two circuits in the space below ensuring safe and effective switching. 1. A lighting circuit contains two lights. One light is switched from one location; the other is switched from two locations. The circuit is to be wired in multi-core cable using the three plate method.
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LEARNER WORK BOOK 2. You are required to wire and terminate a lighting circuit on part of an office block. There are four rooms and a long corridor. Three of the offices have one door and the fourth has two. The long corridor has an entrance at both ends with the office entrances along its length. The corridor length is 30 metres long. The circuit is to be wired in PVC singles in conduit. Assume one light per circuit part for simplicity.
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Power circuits A power circuit is generally anything that is not a dedicated lighting circuit.
These include: • Cooking appliances • Motors • Water-heaters (instantaneous type) • Water-heaters (thermostatically controlled) • Space storage and floor heating installations • Standard arrangements of final circuits o Final circuits using socket-outlets complying with BS13632 (Standard household sockets) o Fused connection units complying with BS 1363-4 o Final radial circuits using socket-outlets complying with BS EN 60309-2 • Stationary equipment
Cooking appliances A cooker is regarded as a piece of fixed equipment unless it is a small table-mounted type fed from a plug by a flexible cord. Such equipment must be under the control of a local switch, usually in the form of a cooker control unit. This switch may control two cookers, provided both are within 2 m of it. In many cases this control unit incorporates a socket outlet, although often such a socket is not in the safest position for use to supply portable appliances, whose flexible cords may be burned by the hotplates. It is often considered safer to control the cooker with a switch and to provide a separate socket circuit. The diversity applicable to the current demand for a cooker is 10 A plus 30% of the remainder of the total connected load, plus 5A if the control unit includes a socket outlet. A little thought will show that whilst this calculation will give satisfactory results under most circumstances, there is a danger of triggering the protective device under some circumstances. For example, at Christmas it is quite likely that grill and oven, all four hotplates and a 3 kW kettle could he simultaneously connected. Just imagine the chaos which a blown fuse would cause! This alone is a very good reason for being generous with cable and protective ratings.
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BS7671 makes the following reference to cooker circuits: • • • •
•
All 13A socket outlets must be 30ma Rcd protected. Reg: 411.3.3 All final circuits must be wired separately from all other final circuits. Reg: 314.4 All items of current using equipment must be provided with a functional switching device. Reg: 537.5.1.3 The Building Regulations require that accessories should not be mounted so that it is necessary to lean or reach over a cooker to operate them All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. Reg: 132.12
Cooker types Hobs On electric cookers there are two main types of hob: the hotplate and the ceramic hob. Hotplates can be broken down into two main types; Radiant rings and sealed plated. Radiant rings is coiled metal, often the cheapest due to the fact they take the longest amount of time to heat up and cool down and are often hard to clean. Sealed plate hobs are thin iron discs covering heating elements and sometimes have thermostats to prevent overheating. They are also quite slow to heat up and cool down but are easier to clean and are very durable. Ceramic hobs have halogen or semi-halogen heating elements under a heat resistant glass. Halogen is a bulb with a tungsten element and halogen gas. Halogen hobs are much the same as radiant the main difference being it has a faster response time offering better heat control. Semi-halogen is a halogen bulb surrounded by a radiant element Ovens Ovens type are normally broken down into two types; coventional and fan assisted. In a conventional oven the thermostat controls the heat in the middle of the oven; the oven will be a slightly hotter in the middle. Fan ovens work differently by using a fan to circulate heat around the oven. This creates a temperature throughout that is even also meaning the oven heats up very quickly, reducing cooking times and saving energy. Double ovens Double ovens mean you can set each oven differently, as well as offering more capacity. Often the main oven will be fan assisted with the smaller second oven being conventional. The second oven generally has a primary function to act as a grill.
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Cooker control Cookers must have a functional switching device for convenience. This can take a number of forms but is generally achieved via a double pole switch. Units with power ratings of less than 3kW can be supplied by a dedicated socket outlet circuit or via a switched fused spur unit. A double pole switch makes and breaks both the live and the neutral conductors
32A or 40A over current protection (with RCD if supplying a switch with a socket outlet)
2
2
6.0mm or 10.0mm Supply cable 32A or 45 Amp double pole switch with or without neon indicator and socket
Or
Or
45A connection unit for easy removal and reconnection of the oven (usually mounted directly behind the oven)
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Cooker design current calculations Assumptions 1. Domestic ovens and hobs are to be calculated upon their maximum loading value 2. Domestic cookers that contain an oven, grill and four rings can have diversity applied to them. We assume 10 amps, then 30% of the remainder plus 5 amps if the functional switch has a 13A socket on it Example 1 An oven has a rating of 2kW. Calculate the design current. i)
Select the correct formula
I= ii)
P V
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
2000 = 8.70 A 230
Example 2 A radiant ring oven has four rings (2 x 1kW and 2 x 1.5kW); a grill (2kW) and an oven (3kW) and is controlled via a cooker switch with a socket outlet. As it is unexpected that all these parts will be in use at once we can apply what is known as diversity. The diversity allowance to be applied to the full load current for cooking appliances is: The first 10 A of the rated current plus 30% of the remainder plus 5A if the control unit incorporates a socket i)
Work out the total power rating and then calculate the full load current
Power = (2 × 1) + (2 × 1.5) + (2 + 3) = 10 kW
I= ii)
10000 = 43 .48 A 230
Using the diversity allowance stated above work out the design current
a) Bank the first 10 amps of the full load current = 10A b) Then add 30% of the remainder (43.48A – 10A = 33.48A)
I = 33 .48 ×
30 = 10 .04 A 100
c) Then add 5 amps to a) and b) for socket outlet
I = 10 + 10 .04 + 5 = 25 .04 A Cables and Circuit types REV5.1
This is the expected current demand
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Complete the exercise below. You are asked to calculate the design current
1. A built in oven unit has a rating of 3kW and is controlled via a BS1363 socket outlet.
2. A combined oven and grill has ratings of 3kW and 3kW respectively and is supplied by a 45A double pole switch. No diversity is allowable.
3. A 2 ring sealed plate hob fitted with a BS1363 plug has ring ratings of 1kw and 1.5kW. No diversity allowable
4. A four ring halogen hob has ratings of 2 x 2kW and 2 x 3kW. The unit is supplied by a double pole switch. Apply diversity as necessary.
5. A stand alone unit has 2 x 1kW and 2 x 1.5kW rings and an oven compartment with a 3kW rating. The unit is controlled by a switch incorporating a socket outlet. Apply diversity.
6. A stand alone halogen unit has 2 x 2kW and 2 x 3kW rings. A 2kW grill and an oven compartment with a 3kW rating. There is no socket on the cooker switch. Apply diversity.
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Motors A motor is a machine that converts electrical energy into mechanical energy. This means that using a motor we can turn a machine or conveyor system or pump liquid through a pipeline. The power needed to drive the motor is dictated by the motor’s size and construction and are many different types of motor and many different ways to control them. The scope of this subject is humongous and warrants its own course so here we will learn to appreciate the simple motor circuit construction and calculating motor current.
Motor control As mentioned above there are many ways to control motors. The most common is the “direct on line” control method. Basically put, you press green the motor starts. You press red the motor stops. It’s that simple. Other ways include variable speed drives, forward / reverse and star / delta starting which makes use of the motors construction and principles of operation to get the most out of the motors torque and starting currents. The basic circuit is shown below.
Fuse board either single or three lines. Overcurrent protection depends upon motor rating Supply cables rated to suit design current Remote stop / start
DOL Starter (contains contactor and overload)
Motor
Isolator local to motor for maintenance
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Motor design current calculations Assumptions: 1. Calculations here are based upon single and three line motors 2. Each motor is subject to two types of losses that affect the total power the motor demands. They are: a. Power factor Cos Φ (which is the difference in the balance between the voltage and current due to inductance created in the copper windings of the motor) b. Efficiency η (which is the difference in the power output over the power input due to mechanical losses such as air and bearing friction Example 1 A 5.5kW single line motor is coupled to a pipeline producing fabric softener. The power factor for the motor is 0.9 and its efficiency is 90%. Calculate the current. i)
Select the correct formula (single line formula)
I= ii)
P V × cos Φ × η
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
5500 = 29.52 A 230 × 0.9 × 0.9
Example 2 A conveyor system uses a number of three line motors to drive the rollers on the system. The motors are rated at 3kW each with 85% efficiency and a power factor of 0.85. Calculate the current demand of one motor. i)
Select the correct formula (three line formula)
I= ii)
P 3 × V × cos Φ × η
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
3000 = 5.99 A 3 × 400 × 0.85 × 0.85
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Complete the exercise below. You are asked to calculate the design current
1. A three line (400v) conveyor motor with a total power of 15kW watts, a power factor of 0.90 and an efficiency of 0.90
2. A single line (230v) pump with a total power of 0.75kW watts, a power factor of 0.95 and an efficiency of 0.95
3. A three line drive motor with a total power of 8.5kW watts, a power factor of 0.80 and an efficiency of 0.75
4. A three line lift motor with a total power of 30kW watts, a power factor of 0.87 and an efficiency of 0.85
5. A single line motor with a total power of 1.5kW watts, a power factor of 0.95 and an efficiency of 0.90
6. A conveyor system fitted with 10 three line motors with a total power of 100kW watts, a power factor of 0.8 and an efficiency of 0.85 are to be used. State the maximum demand.
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Water Heating There are two main methods of heating water electrically: either heating a large quantity stored in a tank or heating only what is required when it is needed. Immersion heater Heating large tanks of stored water (typically 137+ litres) is done using a 3kW immersion heater fitted into a large water tank and then controlling via either a timer switch or an on/off switch.
Dual element immersion heater
Cistern-type Where larger volumes of hot water are needed, for example in a large guest house, then a cisterntype water heater (9 kW+) is used which is capable of supplying enough hot water to several outlets at the same time.
Cistern type water heater
Non-pressure Non-pressure water heaters, which are typically rated at less than 3 kW and contain less than 15 litres of water, heat the water ready for use and are usually situated directly over the sink, such as in a small shop or hairdresser’s salon.
Non-pressure type water heater
Instantaneous type water heater
Instantaneous Instantaneous water heaters heat only the water that is needed. This is done by controlling the flow of water through a small internal water tank which has heating elements inside it; the more restricted the flow of water then the hotter the water becomes. The temperature of the water can therefore be continuously altered or stabilised locally at whatever temperature is selected. This is how an electric shower works, and showers in excess of 10 kW are currently available. The shower-type water heater must be supplied via its own fuse/MCB in the consumer unit and have a double pole isolator located near the shower.
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BS7671 makes the following reference to water heaters: • •
• • • • • •
An immersion heater must not be supplied by a ring final circuit. Reg:433.1.5 An immersion heater must be connected to the supply by a double pole linked switch only. The use of a plug top and socket outlet is not permitted. Reg:554.3.3 An immersion heater must be provided with a thermal cut-out to prevent the water from boiling if the thermostat fails. Reg:554.2.1 Electric showers are not to be installed in zone 0. Reg:701.55 All circuits in a bath or shower room must be protected by a 30ma RCD. Reg:411.3.3 All final circuits must be wired separately from all other final circuits. Reg:314.4 All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. Reg:132.12 All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. Reg: Reg:132.12
Water heating control Water heaters must have a functional switching device for convenience. This is generally achieved via a double pole (sometimes fused) switch. Units with power ratings of more than 3kW must be supplied from a suitably rated double pole switched.
16A - 40A over current protection (with RCD if supplying within a special location) 13 (sometimes fused), 20, 32 or 45Amp double pole switch with or without neon indicator
Supply cable
Or
Or
Flexible cord
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Water heater design current calculations Assumptions 1. Electric water heating is resistive power so is not affected by any power factor or efficiency losses Example An over sink water heater has a rating of 2kW. Calculate the design current. i)
Select the correct formula
I= ii)
P V
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
2000 = 8.70 A 230
Complete the exercise below. You are asked to calculate the design current
1. An instantaneous water heater has a power rating of 2.75kW.
2. A cistern water heater has a rating of 12kW
3. A 10kW shower is to be installed in a domestic property
4. A football changing room is fitting 4 electric showers. Each shower is rated at 8.5kW. State the maximum demand.
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Electrical Heating The type of electric heating available falls into two main categories: direct acting heaters and thermal storage devices.
Direct acting heaters Direct acting heaters are usually just switched on and off when needed; some of them can be thermostatically controlled. Direct heaters fall into two categories: radiant and convection. Radiant heaters The radiant-type heaters reflect heat and come in a variety of shapes, sizes and construction as follows. Traditional electric fire: has a heating element supported on insulated blocks with a highly polished reflective surface behind it; these range in size from about 750W to 3kW.
Tubular heater: lowtemperature unit designed to supplement the main heating in the building. Consists of a mild steel or aluminium tube of about 50 mm diameter in which is mounted a heater element. The elements themselves are rated at 200 W to 260 W per metre length and can range in length from about 300 mm up to 4.5 m. The surface temperature is approximately 88°C.
Infrared heater: consists of an iconel-sheathed element or a nickelchrome spiral element housed in a glass silica tube which is mounted in front of a highly polished surface. Sizes vary from about 500 W to 3 kW; the smaller versions are usually suitable for use in bathrooms and may be incorporated with a bulb to form a combined heating and lighting unit.
Oil-filled radiator: consists of a pressed steel casing in which are housed heating elements; the whole unit is filled with oil. Oil is used because it has a lower specific gravity than water and so heats up and cools down more quickly. Surface temperature reaches about 70°C, and power sizes range from about 500 W to 2 kW.
Under-floor heater: consists of heating elements embedded under the floor which heat up the tiles attached to the floor surface. The floor then becomes a large low-temperature radiant heater, and a room thermostat controls the temperature within the room. The floor temperature does not normally exceed 24°C. The elements have conductors made from a variety of materials such as chromium, copper, aluminium, silicon or manganese alloys. The insulating materials used are also made from a variety of materials such as asbestos, PVC, silicon rubber and nylon.
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Convection heaters Convection heaters consist of a heating element housed inside a metal cabinet that is insulated both thermally and electrically from the case so that the heat produced warms the surrounding air inside the cabinet. Cool air enters the bottom of the cabinet and warm air is passed out at the top of the unit at a temperature of between 80 and 90°C. A thermostatic control is usually fitted to this type of heater. Fan heater: operates in the same way as a convector heater but uses a fan for expelling the warm air into the room. Fan heaters usually have a twospeed fan incorporated into the casing and up to 3 kW of heating elements.
Thermal storage heaters Thermal storage heaters use coiled heating elements surrounded by thermal block that store heat. They are generally rated at 3kW each so to save energy they are heated “off peak” and release heat during the day at peak times
Electric heating control Electric heaters must have a functional switching device for convenience. This is generally achieved via a double pole (sometimes fused) switch. Units with power ratings of less than 3kW generally come supplied from a suitably rated BS1363 plug top.
16A - 40A over current protection (with RCD if supplying within a special location)
13 (sometimes fused), 20, 32 or 45Amp double pole switch with or without neon indicator or socket outlet
Supply cable
Or
Or
Flexible cord
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Electrical heating design current calculations Assumptions 1. Electric water heating is resistive power so is not affected by any power factor or efficiency losses Example A convection heater has a rating of 2kW. Calculate the design current. i)
Select the correct formula
I= ii)
P V
Input the data into the formula and work it out to two decimal places and be sure to add the unit (A)
I=
2000 = 8.70 A 230
Complete the exercise below. You are asked to calculate the design current
1. A radial circuit is to supply 3 tubular heaters within a substation switch room. Each heater has a power rating of 400W.
2. A room requires under floor heating. The total power rating of the system is 8kW.
3. Ten oil filled radiators each rated at 2kW each are to be used as a heating method within a construction site office block. What is the maximum demand?
4. A pub garden requires heating. Assessment of the maximum demand permits a further of 45A only. If each heater is rated at 2kW how many heaters can they use within the limit? Cables and Circuit types REV5.1
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Standard Socket circuits Do you have enough sockets for all of your appliances at home? In many situations there is a need for several socket outlets to be close together so that they are available to feed appliances and equipment without the need to use long and potentially dangerous leads. For example, the domestic kitchen worktop should be provided with ample sockets to feed the many appliances that are likely to be used (deep fat fryer, kettle, sandwich toaster, carving knife, toaster, microwave oven, coffee maker, and so on). Similarly, in the living room we need to supply a television, DVD player, entertainment system, table lamps, room heaters, etc. A computer station alone may require six socket outlets! If there are plenty of sockets this will allow for occasional rearrangement of furniture, which may well obstruct access to some outlets. Do you know how many sockets outlets do you have in your house? List them below in each room and compare the number with somebody else. Why do you think there are different numbers of outlets for each person? Kitchen Dining Room Lounge Garage Conservatory Garden Bedroom 1 Bedroom 2 Bedroom 3 Bedroom 4 Hall Landing Other TOTAL If each one of these socket outlets were wired back to the mains position or to a local distribution board, large numbers of circuits and cables would be necessary, with subsequent high costs. There are options available which cuts cable and installation costs dramatically. The creation of the radial and ring power circuits using the BS1363 plug top. The creation of the ring main brought forward a method of feeding many sockets from one circuit breaker or fuse. A ring circuit protected by a 30 A or 32 A device may well feed twenty, twin 13A socket outlets. However, there are restrictions in place so that ring mains are not overloaded. They are: 1. Do not feed heavy and steady loads from the ring circuit (the domestic 3kW immersion heater is the most obvious example), but make special provision for them on separate circuits. 2. Make sure that the ring circuit does not feed too great an area. This is usually ensured by limiting a single ring circuit to sockets within an area not greater than one hundred square metres.
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BS1363 Plug top and socket We have already learned that a 30A or 32A fuse or circuit breaker is likely to protect a large number of outlets. If this were the only method of protection, there could be a dangerous situation if, for example, a flexible cord with a rating of, say, 5 A developed a fault between cores. Appendix 3 shows that a 30A semi-enclosed fuse will take 5 seconds to operate when carrying a current of almost 90 A, so the damage to the cord would be extreme. Because of this a further fuse is introduced to protect the appliance and its cord.
BS1363 Plug Top
The fuse is inside the BS 1363 plug is a BS1362 cartridge fuse. They are generally rated at 13A or 3A, although many other ratings are available. A plug to BS 1363 without a fuse is not available. The circuit protection in the distribution board or consumer's unit covers the circuit wiring only, whilst the fuse in the plug protects the appliance and its cord. In this way, each appliance can be protected by a suitable fuse, for example, a 3A fuse for a table lamp or a 13A fuse for a 3 kW fan heater. The minimum cross-sectional area for flexible cords should be: 0.5mm² where the radial circuit is protected by a 16A fuse, 0.75mm² for a 20A fuse, or 1.0mm² for a 30A or 32A fuse. Where the cord length must he 10 metres or greater, the minimum size should be 0.75 mm² and rubber-insulated cords are preferred to those that are PVC insulated due to their durability.
Twin Switched Socket Outlet
Single Socket Terminations
The British fused plug system is probably the biggest stumbling block to the introduction of a common plug for the whole of Europe (the 'euro plug'). The euro plug is a reversible two-pin type, so would not comply with the Regulations in terms of correct polarity. If we were to adopt it, every plug would need adjacent fuse protection, or would need to be rewired back to its own protective device. In either case, the cost would be very high.
The euro-plug
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The Fused Connection Unit
Load Side
Switched Fused Spur with Neon Indicator
Feed side
Switched Fused Spur without Neon Indicator
A fused connection unit can be connected to a ring or radial circuit with 2.5 or 4.0mm² conductors. It is used where large loads (generally 3kW) need a supply or for fixed equipment such as boilers or kitchen appliances. Fused connection units can also be used to supply sub-circuits such as lighting or a number of sockets off a radial or ring circuit. The amount of fused spurs permitted on a ring main is unlimited (providing the over current protective device is not exceeded). They can be switched, unswitched, with or without neon and with or without flex outlet The BS1362 fuse incorporated should be sized according to the current carrying capacity of the cable used for the load side of the spur. The one shown above is a switched version with neon indicator. Each spur contains a feed side (where the ring circuit connects) and the load side (where the sub circuit connects) BS1362 plug top fuses
When a socket outlet or sub circuit is wired from a fused spur the minimum size of conductor is 1.5mm² for PVC insulated cables with copper conductors or 1.0mm² for MICC cables 13Amp fuse Ring or radial circuit
2
2.5mm Cable 2
2
1.5mm or 2.5mm Cable
Fused connection unit application
Where sockets and fused connection units form a ring main or radial circuit an unlimited amount can be installed. However, it is essential that consideration has been given to the expected maximum demand on the circuit/s. Unfused spurs are limited to one per socket on the circuit. Cables and Circuit types REV5.1
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The Ring Final Circuit
Un-fused spur via 30A JB
30-32A over current protection (with RCD if not sufficiently mechanically protected)
Un-fused spur via socket
2
2.5mm Cable Maximum floor space 100M
2
BS7671 makes the following reference to ring final circuits: • • • • • • • • •
• • •
All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. Reg:132.12 Unless specifically labelled or suitably identified, all 13A socket outlets must be 30mA Rcd protected. Reg:411.3.3 Unless a ring final circuit is run in rigid steel conduit using the conduit as a protective conductor, the cpc must also be installed as a ring. Reg:543.2.9 A ring final circuit must not supply an immersion heater, storage heaters or a cooker rated more than 2kw. Reg:433.1.5 and p362 A ring final circuit must not serve a floor area greater than 100m2. p362 A ring final circuit must start and finish in the consumer unit and be connected to a 30/32A fuse or mcb. Reg:433.1.5 and p362 The minimum csa of the live and neutral conductors is 2.5mm2. The minimum csa of the cpc is 1.5mm2. Reg:433.1.5 and p362 An unfused spur can feed either one single or one double socket outlet only and can be taken from the fuse or mcb in the consumer unit. Reg:433.1.5 and p362 The number of spurs supplied from a fused connection unit (a switched fused spur) and the size of the cable used to supply the spurs depends on the size of the fuse in the fused connection unit. Reg:433.1.5 and p362 All final circuits must be wired separately from all other final circuits. Reg:314.4 Socket outlets must be spaced at least 150mm away from gas pipes unless there is a pane of non combustible insulating material separating them. OSG p18 Lengths of unfused spurs off a ring final circuit should not generally exceed 1/8 the cable length from the spur to the furthest part of the ring. OSG p54
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The Radial Circuit
Un-fused spur via 30A JB
20A over current protection
30/32A over current protection
2
2.5mm Cable Maximum floor 2 space 50M
Un-fused spur via socket
2
4.0mm Cable Maximum floor 2 space 75M
Two types of radial circuit are permitted for socket outlets. In both cases the number of sockets permitted to be supplied is not specified, so the number will be subject to load and diversity. Basically, an unlimited amount of sockets are permissible providing the 20A or 32A fuse size is not exceeded Radial circuits can be especially economic in a long building where the completion of a ring to the far end could effectively double the length of cable used.
BS7671 makes the following reference to radial power circuits supplying sockets: • •
•
•
•
• • •
Unless specifically labelled or suitably identified, all 13A socket outlets must be 30ma Rcd protected. Reg:411.3.3 A radial final circuit supplied by a 20A fuse or mcb must have live conductors with a minimum csa of 2.5mm2, a cpc with a minimum csa of 1.5mm2 and cover a floor area not greater than 50m2. Reg:433.1 and p363 A radial final circuit supplied by a 30A/32A fuse or mcb must have live conductors with a minimum csa of 4.0mm2, a cpc with a minimum csa of 1.5mm2 and cover a floor area not greater than 75m2. An unfused spur supplying one single or double socket outlet can be run in 2.5mm2 cable and may be connected to the fuse / mcb in the consumer unit. Reg:433.1 and p363 The number of spurs supplied from a fused connection unit (a switched fused spur) and the size of the cable used to supply the spurs depends on the size of the fuse in the fused connection unit. Reg:433.1 and p363 All final circuits must be wired separately from all other final circuits. Reg:314.4 All electrical equipment must be accessible for operation, inspection & testing, maintenance and repair. Reg:132.12 Socket outlets must be spaced at least 150mm away from gas pipes unless there is a pane of non combustible insulating material separating them. OSG p18
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Now complete the questions below.
1. Why can’t the UK adopt the euro plug?
2. Where would it be wise to install a dedicated ring main in a domestic installation? State your reasons why.
3. What is the minimum size of flexible cord permitted to be installed on a ring main?
4. Name the protective device type and BS number that protects the flexible cord on an appliance.
5. When or where would a fused connection unit be most likely used?
6. How many sockets and fused connection units are permitted to be installed on either type of socket circuit and are there any limitations?
7. Name two recommendations for ring mains to ensure they do not over load.
8. State the device ratings, minimum cable sizes and maximum floor space limits of the ring main and both types of radial circuit.
9. Where or when might you decide to install a radial circuit as opposed to a ring main?
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LEARNER WORK BOOK Reserved for future use Environmental control Security systems Fire detection Emergency lighting Data and communications
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