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Available Fault Current?

Mar 1, 1999 12:00 PM, By EC&M Staff

The Article “ What’s My Short-Circuit Value,” on page 48 of the EC&M February ’99 issue, infers the Utility Company doesn’t have to answer inquiries of the maximum Available Short-Circuit Current (ASCC).

It’s a matter of time, knowledge, and liability. However, is an engineer supposed to pick a value to make the project price attractive or a value that is extra high to protect the engineer? If an inspector can’t obtain a maximum ASCC value, then the inspector should do the same as the utility. The inspector should claim: “It’s a matter of liability,” and reject the installation. This would not allow the utility company to supply power.

The utility company could give a “worst-case value.” This would entail the highest value of the ASCC.

Looking at the article’s accompanying diagram, I think the utility company would be obligated to provide a maximum ASCC value when the circuit is supplied from SUB No. 2.

Yes, the first paragraph of the article notes the utility company will give the transformer kVA, percent of impedance, and types of distribution lines feeding your facility. However, the utility also decides what source will supply the facility, under emergency or normal maintenance.

I’ve had problems with the utility company upgrading distribution facilities to small factories and residential properties and not notifying the inspector. Sites approved a few years ago are no longer NEC compliant. Who will know this if an ASCC equipment-related catastrophe occurs in a few more years on customer’s property? I doubt many inspection records indicate the ASCC value at a site.

Glenn W. Zieseniss
Crown Point, Ind.

Author's Response

I agree with Mr. Zieseniss: The electric utility company (EUC) should be knowledgeable of its facilities. However, I stated the EUC is understaffed, under-trained, and/or constrained (due to liability potential) from responding to questions regarding short-circuit current availabilities.

The first two reasons for a utility company’s noncompliance (time and knowledge) are relatively self-explanatory, but the matter of liability deserves more attention. Calculating short-circuit current values is not an exact science. The calculations include various assumptions, but the most important is deciding whether or not to include contributions from outside sources.

Of course, with article-length requirements, it’s sometimes impossible to include every detail. I understand how a vague answer—or even no answer—can bring many problems to an engineer or inspector. However, in today’s litigation-prone society, the utility does not want the responsibility for this design consideration.

Jerry R. Borland, P.E.
EC&M Technical Editor

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