A circuit breaker is a resettable switch that opens and closes a circuit manually or automatically in the event of an overload or short circuit. They are necessitated by the governing body in virtually every location in the civilized world — the National Electrical Code (NEC) in the United States, the Canadian Standards Association (CSA) in Canada, the “Wiring Regs” or BS7671: Requirements for Electrical Installations in the United Kingdom, and the International Electrotechnical Commission (IEC) in the European Union, to name but a few.

In the live event production industry, we normally deal with low-voltage (600 volts and under) circuit breakers, and usually the molded case variety, although miniature circuit breakers are occasionally found in automated luminaires and similar devices. Circuit breakers can be further classified according to the mode of operation. There are thermal, thermalmagnetic, magnetic, and electronic circuit breakers.

The typical circuit breaker most often found in North American power distribution systems is the thermal or thermal-magnetic type. The mechanism by which a thermal circuit breaker senses overload current is a bi-metallic strip through which the current flows.

The strip is laminated with two dissimilar metals, each with a different coefficient of expansion. The current flowing through the metals causes them to heat up due to their resistance and they expand as they heat.

Because each of the two metal strips has a different coefficient of expansion, they expand at different rates, causing the laminated strip to bend. When the current reaches a predetermined magnitude the bi metallic strip will bend enough to trigger a spring-loaded switch, which then opens the contacts and breaks the circuit.

Thermal circuit breakers are affected by the ambient temperature because the amount of energy it takes to heat the bi-metallic strip to the tripping point depends on its starting temperature. Thermal breakers are somewhat forgiving of voltage spikes and surges because of the time it takes for the current to heat up the bi-metallic strip.

A spike or surge of a few cycles will have no obvious effects on a thermal breaker. For that same reason, they are relatively slow to react to faults and short circuits.

Therefore, some thermal circuit breakers have a magnetic relay to sense large currents and more quickly trigger the switch. These thermal-magnetic circuit breakers offer both overload protection (with a slower response) and short circuit protection (with a faster response time).

Magnetic circuit breakers are more common in Europe and are found to a lesser degree in North America. Rather than using a bi-metallic strip, magnetic circuit breakers sense current through a coil of wire or an inductor.

The magnetic field generated by the current flowing through the inductor attracts a moveable iron core, which in turn can trigger a spring-loaded switch when the current is strong enough. This solenoid is very fast acting and very accurate.

Some magnetic circuit breakers are filled with viscous fluid to introduce a slight delay to the reaction of the solenoid. This allows for brief periods of inrush currents and surges but provides for overload protection. Large short circuit currents are sufficient to overcome the hydraulic delay and trip the circuit breaker almost instantaneously.

Electronic circuit breakers replace the magnetic solenoid with a much faster and more accurate Hall sensor and digital processing. They are unaffected by the ambient air temperature and they can be designed to filter out harmonics, thus eliminating nuisance tripping due to harmonic currents.

A thermal circuit breaker has a bi-metallic strip that is calibrated to heat up and trip the switch in the
event of an overload. The two metal strips have a different coefficient of expansion and expand at
different rates due to the heat generated by the current flow.

A magnetic circuit breaker senses current with a solenoid. The magnetic field of the inductor pushes
a moveable core, which trips a spring-loaded switch at a predetermined current level. (a) A
cross-sectional view of a magnetic circuit breaker with no load. (b) The same breaker with some
load but under the trip threshold. (c) The same breaker beyond the threshold tripping current.


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