Safety Devices: Fuses

Fuses and circuit breakers are safety devices. Let's say that you did not have fuses or circuit breakers in your house and something "went wrong." What could possibly go wrong? Here are some examples:

• A fan motor burns out a bearing, seizes, overheats and melts, causing a direct connection between power and ground.
• A wire comes loose in a lamp and directly connects power to ground.
• A mouse chews through the insulation in a wire and directly connects power to ground.
• Someone accidentally vacuums up a lamp wire with the vacuum cleaner, cutting it in the process and directly connecting power to ground.
• A person is hanging a picture in the living room and the nail used for said picture happens to puncture a power line in the wall, directly connecting power to ground.

When a 120-volt power line connects directly to ground, its goal in life is to pump as much electricity as possible through the connection. Either the device or the wire in the wall will burst into flames in such a situation. (The wire in the wall will get hot like the element in an electric oven gets hot, which is to say very hot!). A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation. In a fuse, a thin piece of foil or wire quickly vaporizes when an overload of current runs through it. This kills the power to the wire immediately, protecting it from overheating. Fuses must be replaced each time they burn out. A circuit breaker uses the heat from an overload to trip a switch, and circuit breakers are therefore resettable.

The power then enters the home through a typical circuit breaker panel like the one above.