Fault Ratings

Electrical Enclosures containing busbars, of the type manufactured by Rolla, are normally assigned a fault rating or short circuit rating. This is a brief and simple introduction to the concepts involved.

All our products are designed, tested and manufactured in accordance with international standards, either BS EN 60439-1:1999 or the new version BS EN 61439-2:2011 (and general rules BS EN 61439-1:2011). These standards define a short circuit current as "over current resulting from a short circuit due to a fault or an incorrect connection in an electric circuit". In the case of a typical switchboard or motor control centre, the resulting short circuit current can be very large therefore the busbar system and supporting structure must be strong enough to withstand the effects without itself failing.

 

 

Most of our products are connected to a 3 phase grid system, fed via a step-down transformer. In the UK this will often be the 11kV local network (11,000V), down to 400V nominal. The size of the transformer will be determined by the required load, which will in turn determine the prospective fault magnitude. The table below shows some typical transformer sizes, associated loads and prospective 3 phase (symmetrical) fault levels. (Approximate values only)

3 Phase Power Transformer

 

Transformer size (kVA) Typical Load (A) Typical Fault Level (A)
315 400 8000
500 630 14000
800 1000 22000
1000 1250 28000
1250 1600 35000
1500 2000 41000
2000 2500 55000
2500 3200 69000
3000 4000 83000

As can be seen from the table, an MCC with a load of up to 1000A, has to be able to withstand up to 22,000A in the event of a short circuit fault.

Under fault conditions, there are 2 major effects which the system has to withstand :- The magnetic effects which translate into mechanical forces, and the thermal effects due to the high current causing a rise in busbar temperature.

Magnetic effects act on the busbars, and between the busbars and the surrounding steelwork. There is an initial "hit", at the start of the fault, called a peak, and then successive hits at the system fequency (50Hz in the UK). All this happens very fast, and the forces involved are often large enough to break welds or shear bolts, hence the need to test.

In normal circumstances, it is expected that the protective device (fuse or circuit breaker) will act to disconnect the supply in the event of such a fault. The switchgear, however, still has to carry the fault for a short time in order to allow the protective device to operate. Modern protective systems will normally operate very quickly and an Air Circuit Breaker, for instance, should clear a large fault in around 50mS (0.05 Seconds). Historically this was not always the case, and even today we assume a large safety margin by testing the switchboard for a fault rating withstand of 1 second.

 

 

Graph of current against time under short circuit conditions.

 

 

Rolla standard values for short circuit ratings tests are:-

25kA for 1 second (UCC product, 400A to 1000A)

26kA for 1 second

50kA for 1 second

50kA for 3 seconds

80kA for 1 second

100kA for 1 second

Board under test

The effects of such large currents are proportional to the square of the current, meaning that for a doubling of current, the effect is quadroupled. This is why you cannot extrapolate test results outside the values which have been tested. For example, it is often believed that if a board is tested with 1000A bars at 22kA, fitting 2000A bars should increase the capability to 44kA. This is not the case and it is always necessary to carry out a test for increased fault rating as the effects are multiplied by 4. Put another way, the hammer-blow effect which tries to break the system is now 4 times the magnitude and has not been proven unless tested.

One of the most difficult faults to deal with is a short circuit to earth (single phase fault). In this case the fault current will return through the earth bar, and also the steelwork of the cubicle. If the structure is not strong enough or the fixings are not substantial, the earth bar will become detached and the fault may not be cleared, leaving a very dangerous situation. All Rolla earth bar systems (main and dropper bars) are tested at the same time as the phases for a single phase fault condition thus ensuring total system integrity.

 

 

 

 

Test Station connections for short circuit tests.