Parts of a Circuit Breaker
In a perfect world, a building would be wired for electricity, and every time we turned on a switch, the electricity we needed would flow through the wires without interruption and operate the radio, lamp or hair dryer. In the real world, however, problems crop up from time to time, and the smooth flow of electricity through wires and cords can hit an obstacle, called a "fault." Circuit breakers are needed to stop the flow of electricity in the event of a fault, to avoid damage to the lamp or appliance, injury to a bystander or even a fire.
Types of Faults
There are two main types of electrical faults: overloads and short circuits. An overload is caused by a lightning strike or another source of a power surge---perhaps the malfunction of another device on the same electrical circuit---resulting in overheating of the wires.
A short circuit occurs when the path of electricity is diverted from what was intended. One of the most dangerous characteristics of electricity is that it follows the shortest path to ground. It will find that path if there is even a tiny hole in the cord, and your lamp won't turn on because of this electricity "leak."
After it enters your home or office, electricity is divided up among all of the circuits that exist behind the walls to serve the electric appliances and devices that are plugged in. For example, there may be a circuit for one bathroom, multiple circuits for the kitchen because of the demand of the many appliances and one to serve a couple of bedrooms.
In a typical home or small business, a circuit breaker box---it looks like a metal cabinet---hangs on a wall in the basement, garage or closet near to where electricity enters the building. Inside the box is a number of switches, much like those used to turn on a light. This is where the electricity that comes into the building is divided among all the building's circuits; each circuit has its own switch (shown at top of image).
Under normal conditions, electricity enters at the terminal connector through a connecting wire, flows through the closed contacts, exits the breaker and continues on its journey through its particular circuit. There are two common ways that a dangerous fault is detected: either a bimetallic strip senses too much heat and bends, tripping the mechanism that holds the circuit shut, or an electromagnet reacts to too much electric flow by increasing its magnetism, which pulls on the metal mechanism that keeps the circuit closed. In both cases, the contact is lost and the flow of electricity stops immediately. At the same time, the electricity will "arc," or spark, between the contact points, and an important function of the circuit breaker is to extinguish the arc.
Obviously, the inner components of a circuit breaker must be able to withstand high temperatures because of the heat produced by the process, and they must be made of an effective conductor of electricity, usually copper or a copper or silver alloy.
Types of Circuit Breakers
Although a molded case circuit breaker (shown at top) is the most common residential breaker, there are many sizes of circuit breakers, each designed to handle a particular amount of electricity, called "load." For instance, factories use circuit breakers very much different from those used in a typical home. There are even circuit breakers that serve the electric load of entire neighborhoods. The method of arc control is a major distinguisher among the varying nonresidential types.
You should try to determine the cause of the tripped circuit breaker response before attempting to turn the switch back to the closed position. Was there a lightning strike? Did someone connect a power-hungry machine to the household electric supply? In the example of the lamp with a hole in its cord, someone may have heard a pop or smelled something burning, or you may have an idea that the lamp was the culprit because the breaker opened, or "tripped," right at the moment you turned the switch. Don't tamper with the circuit breaker, because it is energized, or "hot." Instead, call a qualified professional if the cause isn't readily apparent.