Method C IEC Short Circuit Calculations- 20 Hz?
Method C IEC Short Circuit Calculations- 20 Hz?
(OP)
A co-worker stumped me with an IEC question.
ETAP software provides three methods for calculating short circuit currents. Method A gives approximate results, Method B provides more detailed results and requires more information. Method C appears to calculate the impedances at 20 Hz.
Our client is claiming that we should use Method C instead of B.
Is anyone familiar with Method C? Can someone shed light on the advantages of running a short circuit study calculation at 20 Hz?
Or do I not understand what Method C is since I am ANSI and not IEC?
Thanks,
R C Wilson
ETAP software provides three methods for calculating short circuit currents. Method A gives approximate results, Method B provides more detailed results and requires more information. Method C appears to calculate the impedances at 20 Hz.
Our client is claiming that we should use Method C instead of B.
Is anyone familiar with Method C? Can someone shed light on the advantages of running a short circuit study calculation at 20 Hz?
Or do I not understand what Method C is since I am ANSI and not IEC?
Thanks,
R C Wilson






RE: Method C IEC Short Circuit Calculations- 20 Hz?
If you have Conrad St. Pierre's book, A Practical Guide to Short Circuit Calculations, he has a pretty good description of the various IEC methods. He also proposes an alternative approach that he feels is acceptable under IEC-60909.
Method C does use 40% of the actual reactance to compute the X/R ratio, but it also uses the base frequency X/R and uses the two together to come up with an adjusted X/R. This is supposedly a more accurate representation of real-world situation. But I'm not burdened by a lot of knowledge of IEC calculations, so I could be wrong.
If you're doing any IEC calcs and come from the ANSI world, I'd highly recommend getting his book.
RE: Method C IEC Short Circuit Calculations- 20 Hz?
The initial symmetrical current (the "proper" short circuit current), the breaking current and the steady state current are calculated at 50Hz or 60Hz.