Short Circuit Calculations are to be performed for all projects utilizing one of the computer programs on the PC computers (Bussman or SKM)
Short circuits Value are necessary to determine the rating of equipment and bracing of busbars.
Provide short calculation at following critical point:
1) Service Entrance Switchboard/Switchgear
2) Distribution board/ Panel Board/ Load Center
3) Motor Control Center
4) Motor Starter
5) Automatic/ Manual Transfer switch
6) Dimming Panel, Electrical Distribution Equipment
Source of short circuit Contribution:
1) Utility generation
2) Local Generation (Emergency/Co- gen generator)
3) Synchronous Motor/ Induction Motor
4) Capacitor Bank Discharge current
Component required for short Circuit calculation:
1) Impedance and length of feeders
2) Impedance of transformer
3) Contribution Source
Type of fault:
1) Bolted three phase fault- Maximum worst case calculated to determine all AIC rating of equipment.
2) Bolted line to line fault- approximately 87% of 3ph fault.
3) Line to ground fault- approximately 25-125% of 3 ph fault. Very rare that it is higher than 3 ph fault which depending on system parameter and load on phases.
Conductors’ short circuit limitation
All conductors have limitation of short circuit current withstand. Provide proper current limiting devices to limit AIC thru conductors- Reference NEC 110-10
Fuse let-thru
Fuse application will limit instantaneous peak short circuit thru to protect equipment (decrease the AIC rating equipment).
