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Short Circuit Current Rating

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Written by Steve Hudgik

Why is determining the Short Circuit Current Rating (SSCR) important?

NEC 409 covers industrial control panels intended for general use and operating at 600 volts or less.

NEC 409.22 states: “Short-Circuit Rating. An industrial control panel shall not be installed where the available fault current exceeds its short-circuit rating as marked in accordance with 409.110(4).”

NEC 409.110 covers labeling of industrial control panels.  Paragraph (4) states that the short-circuit current rating must be marked on the control panel. This means there must be a label on every industrial control panel, that gives the short-circuit current rating of the components within that panel.  It states that the SSCR can be based on either:

a. Short-circuit current rating of a listed and labeled assembly

b. Short-circuit rating established utilizing an approved method.

So what is the short-circuit current rating (SSCR)?

NEC defines the short-circuit current rating as: “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.”

What this means is that the short-circuit current rating is the maximum level of short-circuit current that a device or assembly can withstand without damage.

When there is a short-circuit in a device an electrical current that is many times higher than normal may exist. Having components that can withstand this high current is important for safety. For example, when a breaker opens to interrupt a short circuit current, an arc may form as the contacts open. This may allow the current to continue flowing. If the current results in vaporized metal or ionized gases, the arc will continue and potentially result in an arc flash.  To prevent this devices such as breakers, fuses and MCCs must include design features that extinguish the arc.  However, there are limits, and these devices must be selected to have a higher short-circuit current rating than can be produced by the circuit.

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NEC 110.9 states:

“Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.”

What this means is that the maximum possible 3-phase fault current in the circuit protected by a breaker, fuse or MCC must be known in order to select the correct device, and to label the panel containing that device.

There are several methods for calculating the short-circuit current rating. The two most common are the ANSI/IEEE method described in IEEE Std. C37.010-1979/1999, and calculating the Thevenin equivalent impedance as seen by the system at the point of the fault.

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NEC 409 requires a variety of information to be marked on electrical panels. Some of these labels will be applied by the manufacturer.  But other labels, such as those with the short-circuit current rating, can not be made until the specific circuit in which the panel is used is known.  In addition, when changes to electrical circuits are made the short-circuit current rating must be recalculated and a new label made.

DuraLabel brand printers are the ideal label printers for making short-circuit current rating labels, as well as other electrical labels.  From wire markers to “No Admittance” signs for areas enclosed by chain-link fences, DuraLabel printers make the electrical safety signs and labels you need.  Call 1-888-326-9244 for more information about making electrical safety labels and signs using a DuraLabel printer.

 

The information presented in this document was obtained from sources that we deem reliable; Graphic Products does not guarantee accuracy or completeness. Graphic Products, Inc. makes no representations or warranties of any kind, express or implied. Users of this document should consult municipal, state, and federal code and/or verify all information with the appropriate regulatory agency.

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