Circuit breakers are simply switches that have an internal mechanism that senses an overcurrent in the circuit. This could be caused by overloading the circuit with too many appliances connected and operating at the same time. It could be caused by one piece of equipment or appliance that uses a higher current beyond what the circuit was designed to handle. It could also be caused by a short-circuit. The circuit breaker is designed to trip (open) under these conditions. Circuit breakers are designed to protect persons and property from a possible fire. If the circuit breaker itself is defective and does not open or trip when there is an overcurrent, it could cause a fire. Standards have been set up for satisfactory circuit breaker manufacturing.
When checking circuit breakers you need to check visually and manually the exact position of the switch handles. A breaker is ON when the switch handle is fully in the ON position. A breaker may sometimes look ON but it is OFF. A breaker that looks ON and is somewhat in the midway position is OFF and it needs to be reset. Some breakers after they trip, may need to be reset by switching it in the OFF position first, then switching it to ON. You may touch the handles of the circuit breaker switches and you will not be shocked. The handles of a circuit breaker are insulated; thus carry no electrical current.
Older homes used fuses for overcurrent protection. Today, many older homes are upgraded to circuit breakers. In homes with fuses, screw type plug fuses were used. They were made with glass or the top being transparent. This makes it easy to see if the circuit opened by a burnt and melted fuse link. Fuses should always be replaced with the proper sized current protection; replace a 15 amp fuse with a 15 amp fuse; a 20 amp with a 20 amp; and so on. The problem with ordinary plug fuses is that you can increase the amp size on a circuit that may not be designed for the increased amp load. This could pose a fire hazard. One solution to this problem is to use Type S fuses. Type S fuses are rejection or tampering resistant; that is, they have a rejection design which offers resistance with tampering with the established branch circuit design by rejecting certain higher amp rated fuses from being inserted in the holder. You insert a properly rated adapter into the fuse baseholder. The appropriate size fuse screws into the adapter. These adapters limit the ampere rated sizes of fuses and will not allow an oversized fuse to be used. Caution: once the adapter is installed it cannot be removed.
If you want to simply disconnect a circuit, remove the fuse from the baseholder by unscrewing it in a counterclockwise direction.
If it is not noticeable through the transparent material that a fuse link has blown or melted open, you may check the fuse by using a voltage tester. Check that the fuse is firmly inserted by twisting it in a clockwise direction. Next, hold one test lead on the neutral bus bar or adequate ground and with the other test lead touch the screwshell of the fuse or if visible, the corresponding circuit wire terminal (usually, horizontally left or right of the fuse).
In residences with pool motors, air conditioning or other equipment, cartridge fuses may be used in disconnects or fuse boxes to protect motors, compressors and associated equipment from overcurrent conditions.