Fuse Terms

A fuse by any other name

The fuse comprises one of the most basic safety elements of an electrical system. A fuse functions like a gatekeeper. It contains a metal wire or strip that will melt when heated to a predetermined temperature by electrical current, thereby opening the circuit of which it is a part and powering down the system. This protects the system from any kind of overcurrent. Properly selected fuses or other overcurrent devices form an essential part of a power distribution system and prevent fire or damage due to overload or short-circuits.

Measuring up fuses

Fuses for power circuits are available in a wide range of ratings. The normal amount of energy cycled through the circuit, the circuit’s voltage, and the maximum level of current available on a short-circuit all figure into a fuse’s grade. For example, in the United States, a so-called "code" fuse may only be safely used in circuits with no more than 10,000 amperes available on a short circuit (an ampere is a measure of how much electricity is moving through a conductor).

In North America, fuse boxes, metal boxes that contain the fuses that regulate electric current in a house, were prevalent in homes wired before 1950. Fuses for these panels were screw-in, "plug"-type, set in holders with the same threads as Edison-base incandescent lamps, with ratings of 5, 10, 15, 20, 25, and 30 amperes. To prevent installation of fuses with too high a current rating for the circuit, later fuse boxes included restrictor features in the socket. Some installations feature miniature thermal circuit breakers that screw into the fuse socket. Plug fuses are no longer used in new residential or industrial construction, but are often found as branch circuit protection for electric cooking units and dryers.

Fuses compared with circuit breakers

Fuses are often less costly and easier to operate than a circuit breaker. The blown fuse must be replaced with a new device, a less convenient solution than simply resetting a breaker. This inconvenience might encourage improvements to the system, rather than the simple flip of a switch. On the other hand, replacing a fuse without isolating the circuit first (most building wiring designs do not provide individual isolation switches for each fuse) can be dangerous in itself, particularly if the fault is a short circuit.

"Current-limiting" fuses operate so quickly that they limit the total "let-through" energy that passes into the circuit, helping to protect downstream equipment from damage. These fuses clear the fault in less than one cycle of the AC power frequency. Circuit breakers cannot offer similarly rapid protection.

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