Calculating Short Circuit Currents (SCC)

In my last blog post (which was much too long ago), I wrote that the National Electric Code (NEC) requires the equipment be “selected and coordinated to permit the circuit protective devices used to clear a fault to do so without excessive damage to the electrical equipment of the circuit” [1].  To achieve a safe condition requires that equipment connected to the electrical system have short circuit current rating (SCCR).

To ensure that the equipment is being installed such that the calculated or tested SCCR is sufficient, a short circuit current (fault current) study must be performed.  Short circuit current (SCC) studies can be conducted through hand calculations, or software simulations.  Software simulations are the main method for conducting short circuit current studies.  While there are a number of vendors of this software, EasyPower, SKM, and ETAP seem to be the most used.

To aid engineers or installers when an SCC is not readily available, one can approximate the SCC to determine whether or not the SCCR of the equipment is suitable for the point of application.  The approximation of the SCC can be conducted by referencing the parameters of the closest upstream transformer and conducting some simplistic calculations.

The SCC calculation at the secondary of the transformer can be determined by the power rating of the transformer (P), the full-load secondary current of the transformer (I), the line-line voltage at the secondary of the transformer (V) and the transformer impedance (Z) [2].  The power rating is in kVA, the current is in amperes (A), the voltage is in volts (V), and the transformer impedance is in percent.

The full-load secondary current (I) of the transformer can be calculated by EQ1.

                                                     I = P / (V * SQRT(3))       EQ1

The SCC can be calculated by EQ2

                                                           SCC = I / Z    EQ2

 

As an example, suppose that we were going to install an industrial control panel on circuit that was supplied from a 500 kVA transformer with a voltage of 480 V, 3W+G, and an impedance of 3 %.  Using EQ1 and EQ2 yields a short circuit current of 20.05 kA.  To properly apply the industrial control panel, the equipment is required to have an SCCR that is greater than 20.05 kA. 

 

Common values of equipment SCCR are 5 kA, 10 kA, 14 kA, 22 kA, 25 kA, 30 kA, 42 kA, 50 kA, 65 kA, 85 kA, 100 kA, 125 kA, 150 kA, and 200 kA [3].   

 

An industrial control panel for this application would need to have an SCCR of at least 25 kA.  The SCCR can be calculated using UL508A Supplement SB.

 

This is a simplistic calculation that does not take into account the parameters associated with electrical conductors.  Also, as the power rating (P) of the transformer increases, the available SCC increases.  As the impedance (Z) transformer increases, the available SCC decreases.

Ensuring the equipment is properly applied is the responsibility of engineers and installers.  The NEC requires that equipment have an SCCR that is equal to or greater than the SCC at the point of installation.  Appling an industrial control panel or any other electrical equipment with an SCCR that is less the SCC at the point of installation will compromise the safety of nearby personnel.

REFERNCES.

1. National Fire Protection Association (NFPA).  National Electric Code, NFPA 70-2011, Quincy, MA USA
2. McKeown, D., Simple Methods for Calculating Short Circuit Current without a Computer, Retrieved 2012 August 10, Available [on-line] at http://www.geindustrial.com/publibrary/checkout/Short%20Circuit?TNR=White%20Papers|Short%20Circuit|generic.
3. Underwriters Laboratories (UL).  Standard for Safety Industrial Control Panels, UL 508A, Northbrook, IL USA