Everything you need to know about circuit breakers

Electrical contractors and builders, do you know how circuit breakers work?

Circuit breakers are an essential element of modern homes because of the safety they provide. These devices are automatically-operated electrical switches which have been designed to protect your electric circuits from damage due to an overload or a short circuit. A circuit breaker identifies a fault and then interrupts the flow of current in order to avoid overheating appliance wires, and possibly causing a fire.

How does it work?

The basic circuit breaker has a switch connected either to an electromagnet or to a bimetallic strip. The circuit’s hot wire connects to the two ends of the switch, and when the switch is flipped to the on-position, electricity can flow through the circuit breaker.

• In a circuit breaker with an electromagnet, this flowing electricity magnetises the electromagnet. When current increases to unsafe levels, the electromagnet is strong enough to pull down a metal lever which breaks the circuit and shuts the switch off, disallowing the flow of electricity.
• In a circuit breaker with a bimetallic strip, too much current will bend the thin strip, breaking the circuit and so disallowing the flow of electricity.

Some circuit breakers use an explosive charge to break the circuit. When current rises above a certain level, the charge ignites explosive material which causes a piston to flip the switch to the off-position.

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Standards and ratings

• The maximum current a circuit breaker is designed to carry continuously, at an ambient air temperature of 30°C, is defined by the international standard, IEC60947-2. The most common maximum currents of low-voltage circuit breakers, used in domestic, commercial and industrial applications, are: 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A and 63A.
• Circuit breakers differ in their instantaneous tripping current too. This is the minimum current value it takes for the circuit breaker to trip without intentional time delay (in less than 100ms). Instantaneous tripping current is measured in rated current (In). Type B circuit breakers have an instantaneous tripping current of above 3 In up to and including 5 In; type C circuit breakers have one of above 5 In up to and including 10 In; and type D circuit breakers trip instantaneously above 10 In up to and including 20 In.
• Finally, circuit breakers have a fault current rating (kA rating), determining the maximum amount of current a circuit breaker can handle under fault conditions. For example, if a circuit breaker has a fault current rating of 6kA, it can withstand 6000A of current during the brief time it takes to trip. Ratings of 3kA and 6kA cover most applications downstream from the main circuit breaker.

Refer to SANS 10142-1:2012 for all circuit breaker requirements.

Written by Daniella Favis for Builders’ Space. Sources: VETI and How Stuff Works.