Protective devices

A short circuit is the result of a fault caused by a direct connection between two wires with different voltage. Usually this is happened when a live is directly connected to the neutral or another phase. The low impedance allows a very high current to circulate. In result cables and wires are overheating and will cause fire.

It has been define that there is a short circuit when the current is more than 10 times the rating current of the circuit or load.

The detection and the protection against short-circuit is done by Fuses, Magnetic relay or Circuit Breaker.

Overload is the result of a current exceeding the nominal value of the driver or the load. This current is enough to cause overheating and its consequences. Over the duration of the heating, the greater the risk of fire is high. To avoid nuisance tripping of protective devices, detecting an overload causes the opening of the circuit after a while.

It has been define that there is an overload when the current is between 1.1 to 10 times the rating current of the circuit or load.

The detection and the protection against overload is done by Thermal relay or Circuit Breaker

The natural overvoltage is often cause by atmospheric phenomenon (thunderstorm). The high peak voltage for few milliseconds is very destructive and also can provoke fire. Specials devices have been design: Lightning arrester.

The man-made overvoltage is created by equipment that is not working properly. An inductive or capacitive load can create overvoltage up to twice the supplied voltage. This can cause insulation damages and overcurrent.

An Earth Leakage is cause by a fault made by a live conductor touching the metallic part of equipment connected to the ground. It detects small current and it is used to protect people against indirect contact in TT earthing system.

The device used to protect against earthing leakages is the Earth Leakage Circuit Breaker (ELCB) or the Residual-Current Device (RCD)