Household Circuits

• Electricity enters our homes through a pair of cables each carrying 120 volts of electricity. The cables enter a service panel or breaker box and connect to conductive poles or “bus bars” in the service panel. The breakers are clipped to one or both of the bus bars and the branch circuit wires connect to the breakers. Electricity flows through the breaker and wires to outlets and fixtures.
  
  

Inside a Breaker

• At the heart of the circuit breaker is a spring-loaded switch held in place by a strip made of zinc and copper. The metal strip keeps the switch closed against the pull of the spring. When current flows normally the bi-metal strip remains in place; however, when a fault occurs in the circuit, the strip of metal begins to heat. Because zinc and copper act differently when heated, the metal strip bends, causing the spring to contract and open the switch. The fault has caused the breaker to trip and it will remain tripped until reset.
  
  

Types of Breakers

• Most household circuits require only 120 volts of electricity for power. For these types of circuits a single-pole circuit breaker is used. As the name implies, these breakers clip to just one of the hot bus bars. Heavy-duty appliances like ovens and dryers use 240 volts to operate, which requires that power be drawn from both bus bars. Double-pole breakers which are attached to both bus bars are needed in this case. There are a limited number of slots in a service panel, so in some cases an electrician will use a miniature breaker that takes up half the space of a normal single-pole breaker to save space. Another type of breaker is the ground-fault circuit interrupter. These breakers contain a microchip that continually monitors the flow of electricity through the circuit. If the chip detects the slightest difference between the hot and neutral wires in the circuit--usually about 1/200 of an ampere--it trips the circuit and kills the power.
  
  

Faults

• Two types of faults cause circuit breakers to trip: short-circuits and overloads. Shorts occur when the wires in a circuit come in contact with a conductive material like metal or water (or you). Overloads occur when too many appliances or fixtures are drawing power through a circuit. Overloads are easy to fix: unplug some appliances. Shorts are harder to find and are usually caused by worn out wires. The most common culprit are wires melted by the heat of a light bulb.
  
  

Caution

• Circuit breakers are designed to control a specific amperage, shown on the outside of the breaker. In the unlikely event you have to replace a breaker, always replace it with one of equal amperage. Better yet, call an electrician and let them do it.