Short-circuit ratings

Modern commercial buildings, particularly high-rise buildings for banks, building societies and insurance companies, require higher short-circuit ratings for the main switchboards, sub-distribution boards and final distribution boards. There are two reasons for this – firstly the trend to locate substations within these buildings, close to the load centre, and secondly the use of modern low impedance busbar trunking for power distribution.

The short-circuit rating of equipment must be higher than the prospective fault current at the position where the equipment is installed. The prospective fault current is the maximum overcurrent that could be expected to flow in the event of a fault of negligible impedance between live conductors, or between a live conductor and earth. This current will depend on the impedance in the conductors between the point of supply (normally a substation transformer) and the equipment.

If substations are located within a building – possibly in the basement or even at intermediate levels or in roof-top plant rooms, the distance between the transformer and switchboard will be relatively short. Furthermore the use of low impedance busbar risers and lateral distribution systems means that impedances are even lower than with traditional cable distribution.

Prospective fault-current levels at the main low voltage switchboard in a major office or factory used to be of the order of 50kA but fault-current levels up to 100kA have to be considered today. This is reflected down through the installation to panelboards and even final distribution boards. For example a 6-10kA short-circuit rating for miniature circuit-breakers and RCBOs may no longer be adequate; 15kA is sometimes specified.

All this means that systems should be designed to withstand the stresses associated with higher prospective fault currents. This applies to the busbar distribution systems and tap-off units, switchboards, panelboards, distribution boards and all the current-carrying devices installed in them. The system design must take account of the MV supply and main LV distribution, and the downstream systems.

The specifier is advised to consult the switchgear manufacturer to ensure that equipment is appropriate to the needs of the installation.

Other requirements of the modern marketplace also impact upon equipment design. For example the cost of space means that more compact designs are demanded. The need for maximum safety of users is leading to the specification of higher Forms of Separation. Meanwhile modular construction can speed up design, manufacture and installation, ensuring consistent electrical performance and, frequently, more compact panels.

Cubicle switchboards to BS EN 60439-1 are now available with a busbar rating as high as 10,000A and short-circuit withstand of 100kA for one second (220kA peak) where previously the maximum was 4000A and 80kA respectively. Cubicles are available with Form 4, Type 6 or 7 separation.

Compatible air circuit-breakers and fused combination switches are readily available and even MCCBs are now available with short-circuit ratings of 100kA or higher.

Modular panelboard systems, capable of being assembled on site by a competent contractor, are available in ratings up to 2000A with a short-circuit rating of 50kA for one second while distribution boards are being developed with increasing levels of short-circuit rating.

Meanwhile, miniature circuit-breakers are sometimes specified instead of MCCBs in order to save space. MCCBs with ratings of 16-63A typically occupy 25-30mm per pole. In contrast, MCBs are available up to 63A occupying only 18mm per pole. Consequently specifiers are tending to call for MCBs rather than MCCBs in this band; but this in turn means that the MCBs must have higher short-circuit ratings. MCBs and RCBOs with a 15kA short-circuit rating are now available

Table 1. Prospective fault levels in a modern high-rise building

Ref

Ip

fault level

at main switchboard

Icu

Fault level

at tap-off

Icn

Fault level

at distribution board

Busbar 1

Busbar 2

Busbar 3

54kA

54kA

54kA

46.91kA

46.91kA

45.62kA

9.77kA

14.94kA

13.88kA

Maintenance

Historically the maintenance of switchgear and other electrical power equipment has been time-based. In the modern cost-sensitive environment, new approaches based on the predicted performance of the equipment may be more reliable and cost-effective.

Cost is a major factor in these decisions. Too-frequent maintenance, based on pre-determined time intervals, can be very costly but if equipment is left until it fails, the results could be catastrophic and the cost in disruption of business could far outweigh the savings in maintenance costs. A more scientific approach is required.

Another factor that has come to the fore in recent years is the effect of maintenance downtime on business. With many commercial businesses operating 24 hours a day, seven days a week, even planned maintenance can prove extremely costly.

Time-based maintenance (TBM)

The principles of time-based (scheduled) maintenance are well-established. It is particularly appropriate to low-voltage distribution systems involving fused switches, moulded-case circuit-breakers, miniature-circuit-breakers etc.

Some devices may remain unused for many years because they are there for emergency use; however, when they are called upon, correct operation will be crucial. Contacts that have been unused for extended periods of time may become dirty or undergo chemical changes that could ultimately lead to arcing. Grease can migrate or harden as a result of heating and so affect the correct functioning of the device.

The fundamental requirements for switchgear maintenance are:

• Operate all switches and circuit-breakers annually

• Lubricate switching mechanisms annually

• Check all terminals annually

• Operate all residual current devices quarterly

Annual operation of switches and circuit-breakers not only confirms that they are still functioning but also exercises the mechanism and cleans and lubricates the contacts and mechanisms. Moulded-case circuit-breakers should be tested by switching off and on manually several times; the trip to test facility should be tried with the main switch ON; shunt trips, undervoltage releases, auxiliary/alarm contacts and command devices such as time switches and contactors should all be tested for correct operation at least annually.

Annual maintenance should also include a visual check of equipment for mechanical damage. Any scratches on equipment enclosures should be touched up to prevent corrosion. Seals and gaskets should be checked, and care taken that they are undamaged and seated correctly, especially when closing enclosure doors. Any build-up of dust should be removed and, if possible, steps should be taken to prevent further accumulation.

Condition-based maintenance

Condition-based maintenance relies on monitoring parameters that indicate the condition of the equipment and using this to determine the appropriate time to carry out maintenance.

For many years non-destructive test equipment has been used to predict plant failure but rarely to assist in determining plant maintenance periods. However, with improvements in reliability of non-destructive apparatus, and the accumulation of data over the years, there has been a shift towards the use of non-destructive testing procedures as a basis for condition-based maintenance.

Oil sampling is a well-established technique for testing for evidence of corrosion or wear in older oil-filled switchgear or oil-filled transformers. A small sample of the oil is drawn off and taken for analysis, rather than changing the oil completely. Some manufacturers have developed their products to facilitate sampling.

Infra-red detection, or thermographic measuring, is another non-destructive procedure that is now well established for identifying ‘hot spots’ caused by terminations that have worked loose or components that are overheating. The switchgear is scanned by means of a hand-held thermographic camera. Some technical skill is required to interpret the results correctly.

Partial discharge testing is a relatively recent technique that can be used to predict possible switchgear and bushing failures in medium voltage equipment by sensing partial discharges. The approach is suited to medium voltage equipment in the range 4-38kV. The latest techniques involve the permanent installation of sensing equipment within the plant to alert the operator to any change in the operating parameters. Discharge activity is recorded and analysed to indicate the most appropriate action to take. The technique is especially appropriate to expensive or mission-critical equipment.

Condition-based maintenance has enabled some operators to reduce the cost of switchgear maintenance by 80 percent by extending the intervals between servicing.

Reliability-centred maintenance

A third maintenance regime relies on statistical sampling of equipment where the user has a high population of similar units. Typically, something like 10 percent of units are subjected to detailed testing according to a time-based programme. The results of this examination are used to determine if and when the other 90 percent of units are serviced.

In many cases the operator’s FM staff may not have the necessary skills, or the time, to pursue a systematic maintenance policy. It is here that the skills and experience of a specialist maintenance company may be an advantage.

The maintenance company should have the experience to determine the most appropriate maintenance regime for the equipment in question. It should be capable of maintaining a wide range of equipment from different manufacturers and will have an accumulated knowledge of operational characteristics for each product.

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