Ordinary current ratings of equipment are most commonly in the range of tens or hundreds of amperes. Fault currents may be in the range of thousands, tens of thousands, or even hundreds of thousands of amperes. These very high currents cause extremely strong magnetic fields that can tear equipment apart when the currents pass through the equipment or cause overcurrent devices to explode when the current is interrupted. so electrical equipment must be adequate for the fault current that may possibly flow through or be interrupted by that equipment or device.

The maximum available fault current is a calculated value that is also called the available bolted fault current or the available short circuit current. It is calculated by considering the capacity of the source and the reduction in capacity caused by the impedance of all circuit elements, such as conductors and transformers. up to the point of the potential fault. Some overcurrent devices may also further reduce the level of some fault currents.

Current ratings include interrupting and short circuit current ratings.

Interrupting Rating

The interrupting rating of a device is defined as the highest current at rated voltage that a device is identified to interrupt under standard test conditions. It is generally applied to overcurrent devices such as fuses and circuit breakers

Short Circuit Current Rating

The short circuit current rating is defined as the prospective symmetrical fault current at a nominal voltage to which an apparatus or system can be connected without sustaining damage exceeding defined acceptance criteria.