Overcurrent Protective Devices
As well as providing protection against electric shock, electrical installations must be protected from excess current ( overcurrent ) which may damage cables and equipment or cause a fire. The term ‘overcurrent’ may be subdivided into two categories:-
· overload current
· short circuit current
Overload Currents
Electrical circuits must be designed to suit the intended load. This is known as the design current of the circuit. Overload currents are usually no more than twice or three times this design current. They occur for one of the following reasons.
· Equipment is overloaded
· Installation is abused
· Installation has been badly designed
· Installation has been modified by an incompetent person.
The danger in all such cases is that the temperature of the conductors will increase to such an extent that the effectiveness of insulating materials will be impaired.
The devices used to detect such overload currents, and to disconnect the circuit when they occur, are the fuse and the circuit breaker.
In order to protect against overload current:
The rating of the protective device must be greater than or at least equal to the design current of the circuit.
The current carrying capacity of the cables must be greater than or at least equal to the
rating of the protective device.
Short Circuit Currents
Short-circuit currents may be several hundred, or even several thousand times normal circuit current.
If a short circuit occurs the circuit protection must break the fault current rapidly, before damage is done to cables and equipment through overheating or mechanical stress. The current likely to flow under short-circuit conditions is called the prospective short-circuit current, the value of which can be measured using a special test instrument.
If the device used for overload protection is also capable of breaking the prospective short circuit current safely, it may be used for both overload and short circuit protection. In practice both fuses and circuit breakers meet these needs.