Jonathan Van Owen, age 15, of Hattesburg, Miss., for his question:

WHY IS A CIRCUIT BREAKER USED?

A circuit breaker is an automatic switch. It is used to protect electric motors, household wiring, long distance power lines and other electric circuits against damage caused by too much electric current. Too much current may flow in a circuit as a result either of a fault in the circuit or of an outside event, such as lightning. A circuit breaker prevents damage.

Every circuit breaker is designed to allow a specific maximum amount of electric current to pass. If the current exceeds this limit, an automatic mechanism inside the circuit breaker throws open a set of contacts and stops the current. Mechanisms used to open the switch include electromagnets and temperature sensitive devices similar to a thermostat.

Some circuit breakers are about as small as an ordinary light swtich while others are as large as a small two story house. A large circuit breaker can interrupt current up to 40,000 amperes at 345,000 volts. It can open a circuit in less than one thirtieth of a second.

In an air blast circuit breaker, a blast of compressed air blows out the arc. In a magnetic arc suppression circuit breaker, a magnetic field deflects and breaks the arc.