|Circuit Protection Device|
It is a standard rule that all electrical installations must be protected against overcurrent or short circuit by means of devices that will operate automatically to prevent injury to persons and livestock and damage to the installation, including the cables. As such, the overcurrent devices must be of adequate breaking capacity and be so constructed that they will interrupt the supply without danger. Also, the cables must be able to carry these overcurrents without damage.
Fault currents arise as a result of a fault in the cables or the equipment. There is a sudden increase in current, perhaps 1O or 20 times the cable rating, the current being limited by the impedance of the supply, the impedance of the cables, the impedance of the fault and the impedance of the return path. The current should be of short duration, as the overcurrent device should operate.
Overload currents do not arise as a result of a fault in the cable or equipment. They arise because the current has been increased by the addition of further load. Overload protection is only required if overloading is possible. It would not be required for a circuit supplying a fixed load, although fault protection would be required.
Selecting protective devices
- the nature or type of load
- the prospective fault current P1 at that point of the installation
- any existing equipment
- the user of the installation, as a CB is easier to reset than a bolted type HRC fuse.
- Ics = is the value of fault current up to which the device can operate safely and remain suitable and serviceable after the fault.
- Icn = is the value above which the device would not be able to interrupt faults safely . This could lead to the danger of explosion during faults of this magnitude or, even worse, the contacts welding and not interrupting the fault.