Calculating Interrupt Cycles

The utility provides the available fault current at the service point on the building. It will vary depending on the utility system. You then must choose equipment that will match this fault current. For the sub panels, you must calculate the fault current for each panel using the fault current at the main panel as the source.

Dear Sir,

Fault duty i.e. 50 kA, 65 kA, 35 kA and 25 kA are symmetrical fault current issued by switchgear manufacturer that their equipment already passed the test (type test) by independent laboratory and able to withstand against fault on those values (maximum).

During equipment sizing we conduct a short-circuit study. Simple system may be calculated by hand, but for complicated system which are consist several bus sometimes more the 100 buses we have to calculate by using software i.e. ETAP, EDSA, and etc.

Prior to conduct calculation we have to provide single line diagram of impedance, which consist of:

1). Current sources such as: Synchronous generators, synchronous motor or condensers if any, induction machine (motor and generators), electric utility system if any and adjustable speed induction or DC motors.

2). Single line diagram indicate the impedance of current sources, cables, transformer and motors.

3). To determine the operation scenarios for normal operation and abnormal condition (the worst operating scenarios).

4). To operate with minimum and maximum generation (Numbers generator required).

5). To recognize and implement subtransient reactance (determine current during first cycle after fault occurs) and transient reactance (to determine current after several cycles).

The calculation result on each bus shall be less than kA switchgear, let say. We selected 31.5 kA if the calculation result is less than 31.5 kA. Some company stated that the calculation results shall be 31.5 kA – 10% x 31.5 kA = 25.2 kA, other company takes 5% only.

If the calculation results slightly higher than 31.5 kA, we have to select switchgear 45 kA (if any).

If you want more clear about s.c calculation please read IEEE-Std 141-1993, Chapter.4.

Regards,