Why is short-circuit current rating important?
There are several problems with electrically powered equipment. One of them is a short circuit. However, prior to 2008 there was no definition of a short-circuit current rating in the NEC. Since this rating is now part of the NEC guidelines, it is important to verify and improve the short-circuit rating in order to remain in compliance.
Article 100 of the NEC defines a short-circuit 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."
In essence, a short-circuit current rating gives a baseline for the fault current that a piece of equipment or individual component within the equipment can withstand for a specific amount of time, or until it clears the circuit with a fuse or the opening of a circuit breaker. In newer equipment with higher short-circuit current ratings, this baseline is not much of an issue. However, it can be a problem in older equipment when the equipment is moved from one location to another.
If a short circuit exists and is an ongoing problem, there are a number of different approaches that may help reduce the issue.
One potential solutions is to replace the existing circuit breakers with circuit breakers that have a higher interrupting capability. It may also help to reduce the current to the breakers by installing fault current limiters or series reactors. Isolating any issue that is leading to the short circuit may help but sometimes it may be necessary to rebuild the system.
How to improve the short-circuit current rating
There are a number of ways in which the current rating can be improved. The first step in the process is to have the equipment or component tested to ensure that it is in compliance. Determine the available short-circuit current at the part or point of installation to test compliance. The available fault current must be lower than the current rating associated with the equipment or component.
Not meeting those factors means the equipment or component is in violation of NEC 110.10. Do not use or install the equipment unless the current rating is improved or the fault current is reduced through an acceptable method.
One way to improve the short-circuit current rating is through the installation of current limiting fuses. When considering the equipment options, there may be additional solutions.
Test NEC compliance whenever installing or moving new equipment from one area to another. Also, it is recommended to perform testing on an ongoing basis as part of a preventive maintenance program.