What is a Circuit Breaker and How Does it Work?

When I started my electrical career, the only thing I knew about circuit breakers was that they tripped at the worst possible time. Stepping out of the shower? Check. Getting ready for a wedding? Ditto. (Who knew hair dryers and knob-and-tube wiring are a recipe for disaster?) Well, after installing hundreds of circuit breakers— and troubleshooting even more— I’m now a big fan.

Circuit breakers protect you, your family and your home against dangerous electrical situations. Read on if you’re curious about what they are and how they work.

What is a Circuit?

A circuit is a closed loop of electrical conductors and loads (lights, devices and appliances) that allows electricity to flow, kind of like a circular racetrack that carries electricity instead of cars. The electricity coming into your home starts out at the transformer, either outside in a green transformer box or on a power pole. From the transformer, it travels through underground or overhead conductors to your home’s electrical panel, where it splits into many different pathways called “branch circuits.”

These branch circuits are different sizes, depending on NEC rules and what they’re powering. Your kitchen and bathrooms, with high-watt appliances like hair dryers and microwaves, are on 20-amp circuits, while your living room and bedrooms are more likely to be 15 amp circuits. Major appliances like electric furnaces, ranges and clothes dryers have branch circuits that are 30 and 50 amps.

Electrical current always wants to return to its source, so it circles the branch circuits from the transformer to the panel to your devices and back again endlessly — until something stops it. That “something” could be a simple switch that controls a light, or another type of switch called a circuit breaker.

What is a Circuit Breaker?

A circuit breaker is a safety switch that automatically “opens” (breaks) a circuit when a triggering event occurs, such as an overload, short circuit or ground fault. Every branch circuit in your home, as well as the main service conductors, are protected by circuit breakers (or fuses, if you have an older home, although that’s not as common anymore). Because they’re safety devices, circuit breakers have to work automatically. (Of course, you can also operate them manually, which you should always do when doing DIY electrical work.)

What Causes Circuit Breakers to Trip?

Here are three scenarios where a circuit breaker will close the circuit and prevent a dangerous situation.

Overloads

A circuit overload means too many things are running simultaneously on the same circuit. When I moved into my current house, I was so excited about the much-improved kitchen that I ran out and bought an air fryer. Unfortunately, the only place it really fit was next to my microwave, and that section of the countertop was served by one 20-amp circuit.

The microwave needed 10 amps to operate, and the air fryer 14 amps, for a total draw of 24 amps—just for those two appliances! What happened when I tried to make toast while I was heating up my coffee? Yep, the breaker tripped. Putting 24 amps on a 20-amp circuit caused an overload.

Overloads don’t necessarily trip right away, especially for certain motors, which are designed to have a brief surge when they start up.

Short circuits

Short circuits happen when electrical current bypasses the regular load and takes a shortcut back to the source. Let’s say a mouse crawled through your attic and chewed through some electrical wires. This will cause arcing, or electrical discharges, that jump from hot to hot, hot to neutral or hot to ground. This is a dangerous situation that causes a very high current because there’s no light or appliance to offer any resistance. Short circuits are a major fire risk, and circuit breakers must be able to protect against them.

Ground faults

A ground fault is a type of short circuit where a hot wire comes into contact with a non-current-carrying part of an electrical circuit, like a metal junction box or even an appliance frame. Imagine a hot wire inside your washing machine that jostles loose during a move. If you start a load of laundry and the wire finally breaks free of its splice, touching the metal housing, that current could go right through you if you’re still standing there. (And if not, it could go through a pet or any other unsuspecting passerby.)

That’s why electricians install equipment grounding conductors, which instantly carry the stray current back to the panel, tripping the breaker.

How Do Circuit Breakers Work?

There are many types of circuit breakers, but the ones you see in your home electrical panel likely have thermal-magnetic operation.

If you overload a circuit, like I did when I plugged in my air fryer and microwave, a “bi-metallic strip” inside the circuit breaker starts to heat up. The bi-metallic strip consists of two different metals bonded together, and these metals have different thermal expansion rates. (In other words, they heat up at different rates.) Overloading a circuit causes the bi-metallic strip to heat up, bend and eventually break when it hits a pre-determined limit.

For short circuits and ground faults, you can’t wait for metals to heat up, bend and break. To protect against these very high currents, circuit breakers have an electromagnetic coil inside. This coil generates a magnetic field inside the breaker that’s proportional to the amount of current that will trip it. When that threshold is reached due to a sudden inflow of current, the magnetic field trips the breaker nearly instantaneously.