Earthing and Bonding Techniques for Electrical Installations

611

m

The IEE Buildings Electrical Technology Professional Network

Earthing and Bonding Bec U@$ for Electrical Installations

Peter L. Smith, Trans-Euro Engineering Services Ltd.

0 Trans-Euro Engineering Services Ltd. Printed and published by the IEE, Michael Faraday House, Six Hills Way, Stevenage, Herts SGI 2AY, UK .

612

Earthing and Bonding Techniques for Electrical Installations BiographicaI Notes Peter L. Smith is a director of Trans Euro Engineering Services Ltd. An independent engineering company based in Northamptonshire. He is an electrical engineer of considerable experience, particularly in design, power quality diagnostics and maintenance within the modern high technology environment, and uses this experience in providing comprehensive electrical power safety training within the telecommunications industry. His experience was gained initially in the merchant navy as a chief electrical officer, and subsequently as a senior technical officer with HV and LV responsibilities within government establishments. Peter was the electrical engineering manager for a London based bank where he was involved in the transition to complex technologies and the associated power requirement. Peter leads the training programmes at Trans Euro Engineering Services Ltd, lecturing both nationally and internationally on subjects covering electrical safety, competence training, Part P and earthing problems.

The Clean Earth Demystified

Ever since the publication of the IEE Wiring Regulations in the format first seen in 1981 (15th Edition), there have been problems with the terms earthing and bonding. I n fact the misunderstanding of the application of earthing and bonding that developed during the early years of the 15thedition is still with us today in the form of bonding mania. Imagine the problems multiplied when during the later days of the 15thedition regulations had begun to appear in different sections of the book referring to Functional Earthing, there was also a note where the term Clean Earth was also mentioned and a statement that the manufacturer of a piece of equipment should be consulted. Some of the regulations, which at the time were in Part 5, were eventually incorporated into Section 607 in later years. So what is a clean earth?

Well that's easy, it's a functional earth!

6i3

So now you know.

Well not quite, because there are other terms:

Functional earth Ctean earth Dedicated earth Quiet earth Low noise earth Low volt earth

Low 2 earth Switching earth Signal earth Telecom earth Ground Drain

This list is by no means all of the jargon used but the terms do illustrate the complexity of earthing when it comes to equipment and function. The manufacturers will often only quote that they require a clean earth. This led to a hunt for clean earth resulting in the following:

i

Another term that is associated with the clean earth is the dirty earth, more confusion!

6 I4

To try and explain the clean earth we should begin with separating protective earthing and functional earthing. Protective earthing illustrated in the next picture shows the path down which an earth fault current flows eventually causing a protective device to operate.

Source of Supply

. i

Phase

i

Installation Protective Device

. -

Extraneousnductive-Part

Star Connected Transformer

B

I

-

This diagram of a TN-S'system shows a single phase system for simplicity. During the earth fault, current flows in the CPC and the earthing conductor, (impedance), according to Ohms Law a voltage will develop along the CPC, (touch voltage). I n a normal installation this touch voltage should not exceed 50 V and should be disconnected within five seconds. Where an installation is considered a high risk environment the touch voltage may be reduced to as near zero as practicable and the disconnection time would be reduced to less than 0.2 seconds. This simple explanation illustrates the operation of an earthing system and shows that when current flows in a wire there will be a voltage created from end to end. This illustration of ohms law may be regarded as a dirty earth, well, only when a current flows, which is during a fault condition. At other times the earthing system is referenced to 0 V i.e. at the neutral point in this TN-S system. When we reconsider the term clean earth, and the other names shown in an earlier illustration it becomes evident that and they all referred to voltage.

615

I n fact and they all referred to zero volts or as near as practicable.

So, a clean earth is a volt free earth. -

If an installation is functioning normally there should be no current in the circuit protective conductor within the installation and so if there is no current there is no voltage Le. volt free! '

Unfortunately, we now have to connect electronic equipment to this earthing system, which has a property of leaking current to earth.

Signal

n

I

The above diagram shows part of a filter .that is important to the IT equipment as it is intended to prevent voltage disturbances from causing internal problems within the equipmentand possibly affecting the output signal. However, the penalty for preventing the disturbance is that a current is now present in the CPC, which has now become a combined earth in which the protective requirement prevaiIs. Manufacturers sometimes appear to forget this.

Returning to ohms law, the current flowing in the cable (impedance), will create a voltage at the point of connection of the CPC to the chassis a t the equipment.

616

Therefore the equipment is creating its own problem i.e. a voltage The amount of the current leakage flowing within the protective conductor is dependent upon the type of equipment. This is illustrated in the following diagram.

BS EN 60950:2000

Type of Equipment

All Hand-held Movable (not hand-held) Stationary pluggable type A Stationary pluggable type B or permanent (small) Stationary pluggable type B or permanent (large)

Max Leakage Current mA 0.25 0.75 3.5 3.5 3.5 5% of Input

current

If we consider a typical desktop computer and assume that the leakage current is a maximum and add up the number of computers within a building then the leakage current can be quite high giving rise to a voltage on the earthing system earth. When we talk about high leakage currents (high protective conductor currents) in an earthing system what do we actually mean? There is little manufacturers information available however, one international manufacturer of IT equipment has stated that any voltage above 15 mV could cause disruption. So any current in the earthing will cause a voltage to be created that may be high enough to cause failure of the equipment. It would seem that the answer to our earthing requirement, both protective and functional is to ensure that the impedance is very low.

Consider:

617

Prokctiwe and Functional requirements

In protective mode the designer requires a suitable magnitude of current to trip the relevant protective device whereas the manufacturer of equipment requires a volt free earth. Both requirements are linked by the impedance, in particularthe earth path.

I f the earth path was a low impedance then both of the requirements would be met. However, impedance is complex. The following 'formula reminds us of the calculation for impedance.

Impedance is therefore frequency dependent; generalty, many electrical people assume that when leakage current is considered it is at 50 Hz. However, the switch mode power supply within the IT equipment will back feed through the filter at higher frequencies than SO Hz and will be superimposed on the 50Hz waveform. This means that a cable will now react differently than it would a t 50 Hz and the impedance may be increased.

To illustrate this effect consider a typical method of wiring commonly found in false floor instaltations, Steel Wire Armoured Cable (SWA). Assuming the wire armour is the circuit protective conductor (CPC) a worse case scenario, the following diagram illustrates what may now happen when the leakage current flows along the steel wire armour.

6/8

Protective conductor current leakage

V noise

z high? I 1 I

c

,

Multicore Steel Wire Armoured Cable a coil maybe?

As the armour is part of the earthing system and the equipment is connected to the

earthing system, then there will be a voltage, (noise) along the length of the cable instead of the 0 V reference preferred. This alone would indicate that the steel wire armour of the cable is unsuitable as an earth path in an IT installation, particularly when length of cable is significant. Use of armoured cables is not an ideal design. The use of bus-bar type power distribution would be much better as the cross section of the bar provides a better surface area when there are high frequencies involved i.e. skin effect. But returning to our original concept of a 0 V environment, i.e. the bathroom. This was achieved by bonding both the extraneous-conductive-parts and the exposed-conductive-parts toget her. We may be able to redesign the IT environment to achieve a volt free earthing system using the same concept. I n other words volt free, clean.

I n a standard installation class I equipment is connected to the protective system by the CPC and under fault conditions current would flow around the earth fault loop thus triggering response of the protective device.

619

DistributionBoard

Earth Bar

WOCf

i

I

IT equlp

I

IT equip

The diagram shown above illustrates this application of the CPC.

Any Leakage Current created within the I T equipment will flow along the cables and unless the sizes and lengths are the same there will be different potentials each earth reference within the equipment. Any attempt to link this

IT equipment together may cause electrical disturbances.

However, consider our earlier example of a no volt environment (EEBADS wimin a

bathroom). The designer can attempt to create a similar situation within the IT environment by the use of a more complex application of EEBADS e.g. an SRPP (single Reference Potential Plane) This concept is illustrated in the next diagram

Earth Bar

I

!

i 1-lf

-

I J

IT equip

I

I I

Ai

False f l o o r

6/10

The diagram illustrates the use of a grid (YESH-BN) mesh bonded network, beneath the false floor, which is connected to the distribution board that is associated with the equipment in the room. The effect of this arrangement is to lower the impedance so that any voltage created by leakage currents will be minimised. Taking a alternative earth path to the main building earthing as illustrated in the next diagram will create parallel paths that have different impedances depending again on frequency and hence voltages will be created. Any interconnection between equipment may now be affected by electrical disturbances.

6/11

Building

Equipment CPC in Power Cord

- Equipment Rack GN wire to Clean Ea

,

Clean Earth Bar

Case Histories:

1. Video switching in a banking environment was affected to such a degree that hum bars appeared on all VDU's in the building. Video switch racks

Distribution

board

r

'H 1 1

IIIII

I

CPC'S

P-

t

I

Bonding

I

6/12

When the leakage currents from each piece of equipment combine along the length of bonding there are different voltages being applied to the casings of the racks, which are the earth reference points hence disturbance.

1

I

Earth bat

*

0 4

i

Bonding

By using a piece of copper lightning conductor tape 25" x 5mm as a bus-bar between the earth bar in the distribution board and the short bonding conductors supplied with the racks the overall impedance is reduced and so the voltage on the earth reference is reduced to a level where it had no effect on the equipment. 2. A banking dealing room (600 people) was affected by LCD screens going into sleep mode at random many times a day within.one area of the room.

Measurements indicated that the was approximately 300mV of noise on the earthing beneath the floor. The installation had been powered by the use of bus-bar track methods but each set of bus bars were supplied by an armoured cable which ran a significant length back to a distribution board. The solution was to try to find out what the threshold was for the sleep mode to be activated but at this time the manufacturer (Japanese) was not responding to enquiry. I t was decided that the solution was to convert the casings of the bus bar system

into a grid as earlier illustrated. This had the effect of lowering the voltage on the earthing /bonding arrangement.

6/13

Measurement of the leakage current for accuracy, should not be carried out by clamping the CPC or Earthing conductor, because the current may have found an alternative path. The measurement should be taken where practical by using the core balance method i.e. phase and neutral. The message is that any electrical installation where there is to be electronic equipment installed must be designed with an earthing arrangement that will not only provide the necessary protective requirements of BS 7671 but also take into account the functional requirement i.e. leakage current at complex frequencies,