A circuit breaker is an automatic safety switch that breaks (opens) a circuit when the current exceeds a safe level. It performs the same protective function as a fuse but can be reset after tripping.
How a circuit breaker works
When the current through the circuit breaker exceeds its rated value, an electromagnetic mechanism trips a switch that opens the circuit. The current stops flowing.
In some circuit breakers, a bimetallic strip bends when heated by excessive current and opens the contacts. In others, an electromagnet pulls a lever when the current is large enough. Modern residual current devices (RCDs) detect imbalances between the live and neutral currents and trip within milliseconds.
Advantages over a fuse
| Feature | Circuit breaker | Fuse |
|---|---|---|
| After tripping | Can be reset by flipping a switch | Must be replaced with a new fuse |
| Response time | Very fast (especially RCDs) | Depends on how quickly the wire melts |
| Convenience | Quick to restore power | Need to find a replacement fuse of the correct rating |
| Cost | Higher initial cost | Cheaper to buy |
Where circuit breakers are used
Modern consumer units (fuse boxes) in homes use miniature circuit breakers (MCBs) instead of fuses. Each circuit in the house (lighting, sockets, cooker) has its own MCB with a rating matched to the cable and expected load.
RCDs protect against earth faults and can cut the circuit within 30 milliseconds, fast enough to prevent a fatal electric shock in many situations.
Need help understanding this?
A 0625 specialist can explain this concept using the student's own questions and show how it appears in exam papers.