As you might expect from their name, circuit breakers protect us against loose electrical current that can ruin appliances or fixtures, start fires or even kill. A circuit breaker acts as an automatic switch that detects faults in the flow of electricity and opens the circuit, preventing the current from continuing to flow. Double-pole circuit breakers are used on heavy-duty circuits.
How Circuits Work
To understand a circuit breaker, you must first understand how electric circuits work. Electricity is formed by the flow of electrons through a conductor--in most cases a wire--that forms a closed path or circuit. Electrical power flows from the source, powers a fixture or appliance, and continues on through the conductor back to the source. When something interferes with that circuit and the electricity does not complete its path, a short occurs, which can cause a shock or start a fire. Putting a special switch in the circuit--a breaker--that cuts off the power when it detects a short prevents shocks or other harmful effects.
How Electricity Enters a Home
The electrical current that powers our computers and lights and other appliances enters our homes through a pair of cables that each carry 120 volts of electricity. These wires are attached to service panels, which contain either fuses or circuit breakers. Modern homes use circuit breakers because they do not need to be replaced when a fault occurs. Inside the service panel are a pair of thick copper wires called buses, or poles to which the 120-volt cables attach. Circuit breakers clip on to these buses and the wires that run to branch circuits around our homes connect to the breaker.
Single- and Double-Pole Breakers
Each hot bus carries 120 volts of electricity, which is enough to power most household appliances and fixtures. Certain heavy-duty appliances such as electric dryers and ovens require 240 volts of current. To provide this extra power, the circuit must receive current from both hot buses. A double-pole breaker acts as the "bridge" that combines the current from both buses. A single-pole breaker is only in contact with one of the hot buses.
How Breakers Detect Faults
Inside each circuit breaker is a bimetal strip that holds a spring-loaded lever in place. The strip is made of two layers of different metals that react differently when heated. When a fault occurs in a circuit, the wires become hot, which causes the breaker's strip to heat as well. The heated strip bends, releasing the lever, which opens the circuit and stops the flow of electricity. The breaker remains tripped until it is manually reset.
After the Breaker Trips
The most common cause of a tripped breaker is an overloaded circuit--too many appliances drawing power through a single circuit. If a breaker trips repeatedly, disconnect one or more appliances or fixtures and see if this fixes the problem. Short circuits occur when the insulation in a cable breaks down and the copper wire comes in contact with another conductor. Heat from light bulbs is a frequent culprit.