Short circuit can produce cough thermal and electromagnetic forces to destroy any protective device.
When selecting a protective device, it is very important to consider the available short circuit amperage, or SCA, which is the potential amperage at any site in the system.
The SCA will be measured at the equipment terminals, the utility transformer and the distribution panel. The highest value will be the power transformer
The conductivity of the material, its size and its length will reduce the SCA down the line from the transformer. The correct size over current protection device can be chosen when the system’s SCA is known. SCAs are measured in amperes.
Fuses and circuit breakers are assigned an amperage interrupting capacity, or AIC, which indicate the SCA that be can sustained before tripping. Unless otherwise designated, fuses are rated 10,000 AIC and circuit breakers are rated 5,000 AIC.
Protection devices are rated to manage both the normal maximum load and the potential short circuit amperage at any given part of the system.
Equipment controls should have a short circuit rating that enables them to absorb current while the protective device clear the circuit. If the rating of the controller is lower, a fast-clearing fuse with a lower rating than the controller should be used
