- Prospective short circuit current
The prospective short circuit current (PSCC) is the highest electric current which can exist in a particular electrical system under short-circuit conditions. It is determined by the voltage and impedance of the supply system. It is of the order of a few thousand amperes for a standard domestic mains electrical installation, but may be as low as a few milliamperes in a SELV (Safety Extra Low Voltage or, occasionally, Separated Extra Low Voltage) system or as high as hundreds of thousands of amps in large industrial power systems.
When designing an electrical installation fuses and circuit breakers must be chosen so that they can safely break the current of a short circuit. When a large electric current is interrupted an arc forms, and if the breaking capacity of a fuse or circuit breaker is exceeded, it will not extinguish the arc. Current will continue, resulting in damage to equipment, fire or explosion.
In designing domestic power installations the short-circuit current available on the electrical outlets should not be too high or too low. The effect of too high short circuit current is discussed in the previous paragraph. The short circuit current should be around 20 times the rating of the circuit to ensure the branch circuit protection clears a fault quickly. Quick disconnecting is needed, because in live-ground short circuit the grounding pin potential on the power outlet can rise compared to the local earth (concrete floor, water pipe etc.) during short circuit to a dangerous voltage, which needs to be shut down quickly for safety. If the short circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual current device (sometimes called a ground fault interrupter) for extra protection.
The short circuit current available on the electrical outlets is often tested when inspecting new electrical installations to make sure that the short circuit current is within reasonable limits. A high short circuit current on the outlet also shows that the resistance from the electrical panel to the outlet is low, so there won't be unacceptably high voltage drop on the wires under normal load.Categories:
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