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Short Circuit Data – Per Unit, Amps and Symmetrical Components

Making Sense of the Numbers
Utility Company Short-circuit Data For Arc Flash Studies

Electrical Contractor Magazine – November 2012
Jim Phillips, P.E.

One of the first steps in performing an arc flash hazard calculation study is to request the short-circuit data from the electric utility company. This information is critical because it defines the magnitude of current that could flow from the utility and is used as a starting point for arc flash calculations.

In addition to requesting this data for normal operating conditions, it should also be requested based on minimum short-circuit current conditions, if available. The minimum condition could be for a utility transformer or transmission line out of service or similar scenario. The minimum value can then be used to determine if the lower current could result in a protective device operating more slowly, which may increase the total incident energy during an arc flash.

Too many numbers—now what?

Unfortunately, a single standardized format for short-circuit data does not exist. Instead, depending on the individual utility, data may be provided in one of several different formats such as the following:

  • Short-circuit amperes (A)
  • Short-circuit megavolt-amperes (MVA)
  • Per-unit and symmetrical components

Of course, with multiple formats, confusion could (and often does) result. I will compare the different formats using a three-phase short-circuit current of 6,000A at the 23-kilovolt (kV) level. Since arc flash calculations are based on a three-phase model, only the three-phase short-circuit calculations are used. Some of the values are slightly rounded.

Short-circuit ampere format

This is the simplest format because it defines the short-circuit current in terms of amperes at a specified location. As an example, the utility has provided the following information:

Short-circuit amperes three-phase = 6,000A
Voltage = 23 kV line-to-line

Since the data is already in terms of amperes, no additional calculations are necessary.

Short-circuit MVA format

Utility companies often provide short- circuit data in terms of short-circuit MVA. This format combines the short-circuit current with the voltage and the square root of 3 (for a three-phase representation) to provide the data in terms of short-circuit power. Below is an example of the MVA format.

Three-phase short-circuit MVA = 240 MVA
Voltage = 23 kV line-to-line

To convert three-phase short-circuit MVA to short-circuit current in amperes, use the following equations:

Short-circuit amperes = [MVA x 1,000] / [kV line-to-line x the square root of 3]

where 1,000 is the conversion from MVA to kVA

Short-circuit amperes = [240 MVA x 1,000] / [23 kV line-to-line x 1.732]
Short-circuit amperes = 6,000A