How to Select Circuit Protection Devices?

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

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

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.

For example, a circuit load supplying a 7.2 kW shower will not increase unless the shower is replaced, when the adequacy of the circuit must be checked against the new load criteria. A distribution circuit supplying a number of buildings could be overloaded by additional machinery being installed in one of the buildings supplied. 

Overload currents are likely to be of the order of 1 .5 to 2 times the rating of the cable, whereas fault currents may be of the order of 10 to 20 times  the rating. Overloads of less than 1 .2 to 1 .6 times the device rating are unlikely to result in operation of the device. 

British Standard 7671 and IEC 60364 requires that every circuit be designed so that small overloads of long duration are unlikely to occur. It is usual for one device in the circuit to provide both fault protection and overload protection. A common exception is the overcurrent devices to motor circuits, where the overcurrent device at the origin of the circuit provides protection against fault currents and the motor starter will be providing protection against overload.

Selecting protective devices

The type of protective device chosen will depend on a number of factors, including:
  • 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.

Breaking capacity

There is a limit to the maximum current that an overcurrent protective device (fuse or circuit breaker) can interrupt. This is called the rated short-circuit capacity or breaking capacity. BS 7671 and IEC 60364 requires the prospective fault current under both short-circuit and earth-fault conditions to be determined at every relevant point of the complete installation. This means that at every point where switchgear is installed, the maximum fault current must be determined to ensure that the switchgear is adequately rated to interrupt the fault currents.

Circuit breakers  have two short-circuit capacity  ratings. 
  • 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.
Any faults that occur between these two ratings will be interrupted safely but the device will probably require replacement.

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