Electrical Switchboards – Overcurrent protection – overload, short circuit, discrimination and cascading

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Overcurrent Protection

Overcurrent protection comes under two main categories, Overload protection and short circuit protection.

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Overload protection

Overload protection aims to protect conductors from the thermal effects of passing more current than they are rated for. Here the current flowing is higher than the nominal current, however not high enough to be classified as a short circuit fault. For instance, where a 630A circuit breaker will trip in 1 hour when passing 650A and will trip in 1 minute when passing 800A. This protection normally follows an IDMT curve (Indefinite mean time) which means that the time it takes to trip is dependant on current flowing, hence indefinite. This protection is most commonly carried out with the thermal part of a thermal magnetic circuit breaker, however can also be programmed into an electronic circuit breaker or a protection relay using CTs to measure the current. For overload protection, it is ideal to have only the smallest section of the electrical system trip as possible. It is also common to have definite mean time (DMT) protection using protection relays. Here there is a given current rating, and if exceeded, a definite amount of time before the trip.

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Discrimination between electrical switchboards and circuits

Discrimination studies are often completed to compare trip curves of circuit breakers with the aim of minimising overlap. This is so that in the event of a overload fault only the affected section of the electrical system if tripped off.

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Short circuit protection

Short circuit protection prevents catastrophic faults by forcing the circuit breaker to act as quickly as possible, in often less than one cycle. In thermal magnetic circuit breakers, this is the magnetic function, where a large current flow will force the magnetic section to trip the circuit breaker without the delay of the thermal protection mechanism. Short circuit protection for circuit breakers is commonly activated at around 10x the nominal current. In the event of a short circuit fault, it is common for upstream circuit breakers to trip as the fault is to be isolated as quickly as possible due to its destructive force. This protection can also be carried out with the use of a protection relay and adequately rated protection CTs. Metering CTs are not fit for this purpose as they will saturated and the high fault reading will not be transferred to the protection relay.

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Cascading

It is common for circuit breaker manufacturers to integrate cascading protection into their range of circuit breakers. This allows circuit breakers to break faults higher than their fault rating by including allowance the upstream breaker to assist in breaking the fault as they are both expected to trip together.