What's My Short-Circuit Value?

Feb 1, 1999 12:00 PM, By Jerry R. Borland, P.E., Technical Editor

Here's why your electric utility won't and sometimes can't give you an available fault current value.

To adequately protect the distribution equipment in your plant, you must know its available short-circuit value. To getthis value, you call your local utility:but what you get instead is the size in kVA of its transformer, its percent impedance, and the lengths and types of distribution lines that feed your facility. Why won't the electric utility give a definite value for the short-circuit current? Having worked as a distribution engineer for an electric utility, this author offers the following comments.

It's a matter of time. Doing short-circuit current value calculations manually or with software programs takes time. Even a small distribution feeder could take weeks, months, or possibly longer. Because time is such an issue, most utilities perform short-circuit calculations only from the generation station through the transmission system, ending at the distribution system feeder terminals at the substation. You end up with information for protecting loads on the distribution feeder and short-circuit values at the substation.

It's a matter of knowledge. Most technicians, whom the utility employs to perform distribution level design work, are usually not sufficiently trained to solve these complex calculations. And though the utility usually has some type of nomograph depicting what type of protection will coordinate with an upstream protective device, it won't know the exact value of the short-circuit current at any point along its distribution system. Instead, it bases selection of protective devices on line-loading characteristics.

It's a matter of liability. Your utility uses assumed information that may change at any given moment. Most of these changes are a result of its distribution system reconfiguring.

You may wonder how your utility reconfigures its distribution system. (It reconducts a line to a different size, changes the line length, or upgrades a substation transformer.) These might happen due to line loading, emergency conditions, etc., and the changes can be temporary or long-term.

Let's look at an example of an emergency condition reconfiguration that becomes long-term. As shown in the diagram, the normal feed to your plant comes from Utility Substation No. 1. An animal invades the substation and causes a short circuit, taking this substation out of service for six months. To restore service to everyone fed from Substation No. 1, the utility reconfigures the line by closing Switch No. 2 and opening Switch No. 1. Now, the short-circuit current available at Location B changes.