What Is a Circuit Breaker?
A circuit breaker is a protection switch that automatically opens the circuit under over-current or a short circuit, then can be reset by hand (or automatically in some types).
How Does It Work?
Inside it there are two tripping mechanisms: a thermal part (bimetal) that trips by heating under a sustained overload, and a magnetic part that trips instantly on the sudden current of a short circuit. When it trips the lever "flips" and the circuit is cut.
Difference From a Fuse
A fuse blows once and is replaced; a circuit breaker is used again and again by raising the lever. Also, a circuit breaker serves as a switch, able to open and close the circuit by hand.
| Property | Circuit Breaker | Fuse |
|---|---|---|
| Reuse | Yes (reset) | No |
| Switch function | Yes | No |
| Cost | High | Low |
Tripping Curves
Circuit breakers are referred to by tripping curves like B/C/D; this states how much tolerance they show to short-term high current (like motor startup). A curve suited to the load type is chosen.
Where Is It Used?
Home/building electrical panels (miniature circuit breaker), industrial distribution, automotive and power lines where overload can be frequent.
Common Mistakes
- Wrong curve: choosing a low-tolerance curve with a motor load causes nuisance tripping.
- Too high a rating: too high a rated current leaves the cable unprotected.
- Breaking capacity: a breaking capacity above the fault current is needed.
Frequently Asked Questions
Is a miniature circuit breaker a circuit breaker?+
Why does it sometimes trip instantly, sometimes late?+
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