Circuit breakers protect homes by stopping electric current when a circuit is carrying more than it can safely handle. This matters because wires heat up when too much current flows through them, and overheated wires can damage insulation or start a fire. A breaker is designed to act before the wiring reaches a dangerous temperature.
It is a reusable safety switch built into the home electrical panel.
Key Facts
- Electric power is P = IV, where P is power, I is current, and V is voltage.
- Current can be found from I = P/V for devices connected to a known voltage.
- A 15 A breaker on a 120 V circuit should not supply more than about P = 1800 W at once.
- Wire heating increases with current according to Pheat = I^2R, so a small current increase can cause much more heating.
- A thermal breaker trips when a bimetal strip bends from heat caused by sustained overcurrent.
- A magnetic breaker trips very quickly when a short circuit creates a large sudden current.
Vocabulary
- Circuit breaker
- A circuit breaker is a safety switch that opens a circuit when current becomes too high.
- Overload
- An overload occurs when too many devices draw more current than a circuit is designed to carry.
- Short circuit
- A short circuit is an unintended low-resistance path that allows a very large current to flow.
- Bimetal strip
- A bimetal strip is two bonded metals that bend when heated because they expand by different amounts.
- Trip
- To trip means for a breaker to snap open and stop current flow through the circuit.
Common Mistakes to Avoid
- Adding device wattages without comparing them to the breaker limit is wrong because a circuit trips based on total current, not on each device alone.
- Assuming a 20 A breaker can replace any 15 A breaker is wrong because the wall wiring may not be rated for the higher current.
- Resetting a breaker repeatedly without unplugging devices is wrong because the overload or fault may still be present and could overheat wiring.
- Confusing an overload with a short circuit is wrong because an overload is usually too many normal loads, while a short circuit is a dangerous low-resistance fault.
Practice Questions
- 1 A 120 V kitchen circuit has a 15 A breaker. What is the maximum power it can supply before reaching the breaker rating?
- 2 A space heater uses 1500 W and a microwave uses 1000 W on the same 120 V circuit. What total current do they draw, and will a 20 A breaker be overloaded?
- 3 A breaker trips instantly when a lamp is plugged in, even though no other devices are on the circuit. Explain why this is more likely a short circuit than a normal overload.