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What Is a Magnetic Circuit Breaker?

A lightning strike would cause a magnetic circuit breaker to trip.
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  • Written By: Henry Gaudet
  • Edited By: A. Joseph
  • Last Modified Date: 21 October 2014
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A magnetic circuit breaker is a safety device designed to cut an electrical current in the event of a power surge, thus protecting electrical equipment and circuitry from damage. Overloaded circuits, loose or faulty wiring and lightning can cause the circuit breaker to trip. There are other devices able to break a circuit during a surge or short circuit, each with its own limitations. Fuses have a filament that burns during a surge, meaning that they will not work after blowing once and have to be replaced immediately to restore power. Other commonly available circuit breakers are heat-sensitive and require a cooling period after a surge, but a magnetic circuit breaker can be reset immediately, restoring power without delay.

All circuit breakers require some method of detecting the strength of an electrical current so that they can respond to a surge. Some breakers rely primarily on temperature and trip when overheated, but a magnetic circuit breaker uses an electromagnet, or solenoid, to generate a magnetic field that is used to gauge the current’s strength. When the strength of the electric current increases, the solenoid’s magnetic field increases as well. This field pulls on a metallic lever in the breaker. This lever is held in place by a spring, and under normal conditions, the magnetic field is not strong enough to make the lever move.

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The magnetic circuit breaker has a rating, a preset limit on the maximum strength of current that it will allow to flow. When the current exceeds the breaker’s limit, the solenoid’s magnetic field increases to the point where it is strong enough to move the lever. The breaker trips, the contact points move apart, and the circuit is broken before any damage can be done. There no longer is any current flowing through the circuit, so the solenoid loses power as well, and losing that, it also loses its magnetic field. This means that the circuit breaker can be reset immediately.

Although a magnetic circuit breaker is excellent for dealing with spikes from short circuits and large power surges, power will remain uninterrupted if a surge does not exceed a breaker’s limit. Prolonged low-level surges can cause equipment and circuitry to overheat, potentially causing damage to devices or causing fire. A thermal magnetic circuit breaker addresses this danger by using a pair of metal strips that cause the breaker to trip if overheated.

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hamje32
Post 5

@Charred - Circuit breakers are very useful indeed, and they do come in a wide variety of types depending on the application needed. I’ve heard that for household appliance like washers and dryers for example, they have a 2 pole circuit breaker.

This just basically means that it can monitor the two poles or the two live wires from your appliance, so that if you get a surge on either wire, the breaker will still trip. Otherwise you could have a surge on one line and not the other, and if the unprotected wire has the surge your appliance would be damaged.

Charred
Post 4

@allenJo - I think there is a delicate science to the circuit breaker devices, whether you are talking about magnetic circuit breakers or relays or whatever. You have to set the threshold at which the device will trip, and you need to set a reasonable range.

If you set it too high, then a power surge could escape through the lines undetected and cause a lot of damage. On the other hand, if you set it too low, then your device will constantly be tripping all the time, which will lead to a lot of false alarms and eventually wear down your equipment.

My guess is that constant testing and some historical analysis will tell you what the range should be for your circuit.

nony
Post 3

@allenJo - That’s interesting. Most of us have no direct experience with circuit breakers except for fuses. I’ve had to replace a broken fuse a time or two when power went out because of an electrical surge.

I prefer the magnetic circuit breaker concept however, because you don’t have to keep “fixing” it like you do with a fuse box. The circuit breaker resets itself whereas the fuse needs to be replaced. It’s a small thing to replace it, but it’s still a hassle.

allenJo
Post 2

I work in the utilities industry and am quite familiar with electrical circuit breakers. Substations have a whole bunch of them that are used to protect them in the case of a spike.

It should be pointed out that, as the article indicates, these are not the only devices that function in this capacity. For example, we also have what are called protective relays.

The functionality is similar in principle. The relay will trip if the circuit exceeds a certain level. With relays however you have a wide variation for the test conditions. You can have over current relays as well as under current relays for example.

So relays bring a little more to the table than the electric circuit breaker but as I said, it’s the same basic idea.

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