Advances in LV Swbd

Issued by Switchgear Contracts Division LARSEN & TOUBRO LIMITED Powai Works, Mumbai 400 072

April-June 1994

Prospect / Retrospect

Advances in LV Switchboards It is almost decade ago that the sheet steel-enclosed, freestanding LV power and motor control centers from indigenous manufacturers appeared on the Indian industrial scene. For the first ten years, these switchboards were simple, robust and available only in fixed version. However, in a few cases, switchboards received as a part of a package from foreign suppliers used to be drawout type. Around 1970, the first indigenous design of switchboards with withdrawable modules appeared in the market. Industry was moving rapidly towards radical technological developments. Various process industries had realised the essence of minimum downtime during maintenance and switchboards with withdrawable modules fulfilled the need. Since then boards with withdrawable modules have emerged as the preferred version for many continuous process industries. However, the technological advances in LV switchboards were not restricted to the drawout system

alone. New processes, new materials, precision machines for manufacture and availability of computers for design and development have resulted in revolutionary concepts in LV switchboards. With past experience and the pressures for running the plants with their maximum capacity utilisation, the customers have started demanding: • • •

increased reliability in operation; increased safety of personnel and equipment; ease of maintenance, installation and commissioning.

While standards have become more stringent in many cases, customers are not satisfied with meeting the standards alone. Some of them have formulated their own specifications based on past experience. In this issue of L&T Current Trends, we are highlighting some of the good engineering practices that have been adopted in LV switchboards in the design and assembly stages.

Some of these may not be put on paper as essential by the customer (with the purpose of getting competitive prices from the supplier). Once exposed to such practices and features, the customer is unwilling to sacrifice these in future and in fact looks for more such improvements in practices.

L&T's Corporate Leadership L&T has been ranked first in terms of high quality of services/products and second in corporate leadership by Asia's leading economic journal. The Hong Kong-based Far Eastern Economic Review has introduced an annual survey of Asian corporate leadership, Review 200: Asia's leading companies. This survey was published in the journal's issue of 30 December - 6 January 1994. More than 4000 leading professionals throughout Asia participated in the survey. The Review 200 comprises 90 multinational companies from outside Asia who do business in the region and 110 companies based in Asia. Multinationals were ranked in all 11 countries in the survey. In India, The Review noted "Industrial engineers Larson & Toubro, which wins orders in the Gulf and Southeast Asia, was ahead in the poll in quality of products".

FEATURE

Good Engineering Practices For Delighting The Customer Indian tropical conditions demand innovative and cost effective measures in designing and manufacturing LV switchboards. The conditions prevailing in India are:

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Heat, dust and humidity in coastal areas;

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Presence of vermin (rats, lizards, etc.);

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Predominant use of aluminium cables and busbars;

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Opportunities for maintenance of equipment are few and far between.

The switchboard design optimized for relatively cold, dust-free and vermin-free conditions prevalent in advanced countries, turn out to be inadequate for Indian subcontinent. Some of the engineering practices which may be adopted in LV switchboards to counter these conditions are explained below:

A. Busbar Zone Maintenance of clearances as per standards is an essential condition but may not be entirely sufficient. An analysis of faults which occurred in the seventies indicates that 85% of the faults are due to vermin or falling tools or hardware, shorting the phases momentarily and initiating arcing fault or tracking of supporting surfaces. Though the busbar systems are designed to withstand 50 kA fault level, the 5-10 kA arcing faults, with temperatures reaching 3000 degree centigrade, can cause major damages in the switchboards.

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R. S. Mahajan, Asst. Sales Manager, SWT-C

Therefore the busbar and link arrangement must eliminate arcing faults. Following measures may be considered: 1. Increase the clearance between phases and Ph-E. Maintain at least 25 mm clearance upto the incoming device terminal. 2. Provide secondary insulation on busbars and vertical dropper system. This can be easily achieved by a heat shrunk PVC sleeve. Alternatively, electrical grade epoxy powder paint can be used. Special grade heat shrunk type sleeving on the busbars/links requires an established process of heating the sleeved busbar/ links in oven for a stipulated temperature for a specific time (See photo 1).

with insulating materials. This was achieved earlier by using a bituminous compound or adhesive tapes. But use of aluminium busbars necessitates checking tightness of the joints periodically. For the ease of maintenance, click fit, easily removable joint shroud are more suitable. They do not require any tool for opening or assembling (see photo 1). 6. Busbar joints are to be properly made. Improperly made joints create

3. Ensure that the protruding bolts of different phases are staggered and do not see each other directly. 4. Round edges of busbars and links do not support arcing as much as nonround edges. 5. To achieve a fully insulated busbar system, the joints are also to be shrouded

Photo 1 L&T Current Trends, April-June 1994

8. All the contact surfaces need to be machine-polished and greased before joining. It is essential that the greasing operation be carried out on the polished surfaces within a few seconds of polishing. This prevents the surface from getting oxidised. Techniques are available for simultaneous polishing and greasing of aluminium conductors.

considerable problems for the maintenance persons. Correct sized bolts for optimum contact pressure and optimum configuration of bolts for uniform pressure distribution over the entire joint surface are essential for a good contact. The joints should be made in accordance with DIN 43673 (see chart 1). 7. Fish plates are required to connect busbars of two transport units. A thin slit on the fish plates improves the joint efficiency by

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9. For all joints on busbars and droppers, during maintenance checks, access is required to both

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Chart 1 utilising a greater portion of the bolt torque on the joint pressures. Also a 6 mm thick busbar system, with a provision for 6/12 mm thick fish plates sandwiched between the flats, provides a more flexible joint as compared to 10 mm or 12 mm thick busbar flats.

The very purpose of a drawout switchboard is to minimise the downtime during the repair of faulty modules. Emphasis can be laid on the following points: 1. The guidance system for a withdrawable module plays an important role in ensuring that all the power and control contacts meet accurately every time the module is inserted.

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Photo 3

nuts and bolts. However, if the concept of a nut plate or a nut retainer plate is used, then access to the bolts from the front is good enough. This saves considerable trouble for maintenance personnel and the tightening of bolts is completed in less than half the time required otherwise. 10. Strong need has been felt for separate insulating supports for separate phases. This has brought down the chances of arcing faults. Surface profile for these supports has come in for increased attention. Built in creepage barriers add to the integrity of insulators whenever the insulator is common for all phases (see photo 2).

A channel or angle provided for this guidance serves this purpose to a limited extent. It does not support the module while withdrawing and the module needs careful handling at the time of withdrawal. A telescopic rail which is equivalent to a linear ball bearing is a proven solution inspite of incremental increase in cost. It is robust, smooth and precise. It supports the module even in the withdrawn position. Insertion and withdrawal of the module can thus be smooth and independent of the operator's skill. 2. Drawout contacts always undergo wear and tear due to frequent withdrawal and insertion of modules. To prevent oxidation these can be tin plated or silver plated. Tin plated contacts offer higher mV drop and also have less resistance to wear and tear. Comparatively silver plated contacts provide low mV drop and better abrasion resistance. Specialised greases improve contact performance in highly corrosive atmosphere (see photo 3).

Photo 2 L&T Current Trends, April-June 1994

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3. For testing of a circuit, test facility created by switching off the power switch may not, by itself, prove to be a fully satisfying system. Signals going to field, PLCs, DDC systems are necessarily to be blocked. Standby feeders/motors should not get wrong commands. This can be achieved by a distinct test position and suitably programmed control terminals. A positive test position ensures that the power circuit is isolated and all the field signals are suitably blocked without any extra device (like a Normal/Test switch) or any extra operation. 4. It should be possible to carry out routine maintenance checks or small changes in a module without physically taking the module away from the switchboard. Telescopic rails facilitate maintenance position of a drawout module on the switchboard itself. It saves considerable time and fatigue of maintenance personnel and also eliminates the danger of mixing of identical size modules after routine maintenance. 5. For power connections in modules, cables, bare copper flexibles and solid links are used.

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Cables are normally upto 35 mm copper conductor and upto 100A/160A feeder rating for higher ratings, if cables are used, larger sections are required and these large conductors exert substantial tension on the equipments terminals, sometimes resulting in cracking of insulators. Bare copper flexibles or braids give more flexibility but may lead to inadequate clearances once the module is in service. Solid links, made with precision, are the best for carrying large currents. With solid links, precision manufacturer, desired clearances can be maintained and the termination on equipment terminal can be free from any tension (see photo 4) 6. Lugs are normally used for all power and control wire terminations. Lug selection should be done with prior study of equipment on which termination is to be done. Fork type lugs are preferred for control wiring while ring type lugs are preferred for power and CT circuits.

7. Increasingly, AC and DC wiring is being done with different colours .All power wires and control wires tapping voltage from power circuit should be provided with a phase indicating colour sleeve at each termination.

8. Component mounting with screws upto M4/M5 size can be done on base plate directly. For larger components requiring bigger screws, either flaring of holes or nut welding is essential, thereby simplifying the component mounting in modules. Providing only a nut behind the base plate is inadequate since it falls down once the component is removed for replacement/checking. Photo 4

C. General Apart from busbar zone and modules, there are other areas which also need engineering considerations: 1. Gland plates should be easily removable for punching holes at site. For this purpose, the gland plate should be a plate and not in the profile of a tray. Since glands mounted on the gland plates project below gland plate, gland plates should be at least 40/50 mm above the bottom of base frame of the switchboard. 2. Cabling area above the gland plate should be free of any earth bus, neutral bus, etc., so that total gland plate area is usable for cabling. If a neutral bus or earth bus runs just above the gland plate, it can hinder cabling. 3. Since a large number of aluminium cables are used, suitable adopter links with adequate mechanical support are essential to eliminate stress on the drawout terminals of modules and equipment. 4. Whenever more than one incomer source exists on a switchboard and external control supply is not available, a reliable control supply should be derived from incoming sources. This point is normally overlooked during initial engineering. 5. When a common frame size is used for multiple ratings of circuit breakers, positive interlock should be provided to ensure that circuit breakers of one rating cannot be inserted on the cradle of other ratings of the same frame size. Specifications and standards alone do not endure customer delight. It is essential that switchboard manufacturers contribute their experience to devise and implement good engineering practices which lead to switchboards which are more reliable, safe and easy to maintain. This alone will give the customer a pleasant surprise when he starts using the switchboard.

Edited, printed and published by Cerena de Souza for Larsen & Toubro Limited. Published from L&T House, Ballard Estate, Bombay 400 038 and printed at Vakil & Sons Ltd., 18 Ballard Estate, Bombay 400 038