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.
- National
Fire Protection Association (NFPA).
National Electric Code,
NFPA 70-2011, Quincy, MA USA
- 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.
- Underwriters
Laboratories (UL). Standard for Safety Industrial Control
Panels, UL 508A, Northbrook, IL USA