# physics

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A circuit breaker uses an electromagnet.
Explain why the current is cut off by a circuit breaker if it is larger than a certain value.

• physics -

Actually, most household circuit breakers do NOT use magnets. Take one apart and investigate, they use a bimetallic strip which heats when excess current flows, bends, and trips a spring loaded latch holding the actuator arm On, moving it to the off position. It is amazing what one discovers when one looks.

There are some different types of large circuit breakers which do use electromagnets, or a combination of electromagnets and bimetalic strips. A small minority of these release the switch latch by excess current going through the electromagnetic coil. In nearly all cases the actuator lever is spring or hydraulic loaded to close.

Most circuit breakers do not use just an electromagnet, because current surges can trip those easily, and one doesn't want that very often.

The design challenge in circuit breakers is not the design of the tripping of the latch mechanism, any high school student could do that. The issue really difficult is that as the contacts are opened, and gap exists, wherein an arc developes. Arcs ionize air (Nitrogen,mainly), which then the ionized air makes a very easy path for electricity to travel (as in a lighting bolt). So even with the contacts open, electricty still flows easily across an "air" gap, when it shouldn't. So the design challence is to snuff out the arc as the contacts are opened. Usually this is done with a magnetic field which you know electricity flowing in a magnetic field has a force on it. So if designed correctly, the arc develops, and then that current flows through an electromagnetic coil, which creates a strong magnetic field perpendicular to the arc, and the arc is "blown" away from the contacts, and extinguished. Neat engineering goes into this, as it cannot ever fail, or the switch will never stop electric flow, even with the switch in the "off" position.

So all the engineering and thought is focused on the arc suppression, rather than the latch release mechanism.

Take apart a few circuit breakers, any electric shop has some old ones laying around they will give you.

• physics -

I don't particularly like circuit breakers tripping. It usually means I have a lot of work to do and money out the flare stack. Of course if it doesn't work then the fire department has work to do, and I have a lot of work to do.

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