TABLE OF CONTENTS This section analyzes the protection of electrical system components from fault currents. It gives the specifier the necessary information regarding the withstand rating of electrical circuit components, such as wire, bus, motor starters, and so on. Proper protection of circuits will improve reliability and reduce the possibility of injury Electrical systems can be destroyed if the overcurrent devices do not limit the short-circuit current to within the withstand rating of the system's components. Merely matching the ampere rating of a protective device will not assure component protection under short-circuit conditions.
The NEC covers component protection in several sections. The first section to note is NEC Section 110-10.
The overcurrent-protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be so selected and coordinated as to permit the circuit-protective devices used to clear a fault without the occurrence of extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.
This requires that overcurrent-protective devices such as fuses and circuit breakers be selected in such a manner that the short-circuit withstand ratings of the system components will not be exceeded should a short circuit occur.
The short-circuit withstand rating is the maximum short-circuit current that a component can safely withstand. Failure to...
