Fuses are protective devices that use a metal strip or wire to break an electrical circuit or activate a safety mechanism. They come in mechanical and electrical types and are used in fire sprinkler systems, high security safes, and electrical circuits to protect against damage, shock, and fire. Fuses are usually sacrificial and replaceable, but are being replaced by latching resets and trip switches.
A fuse is a sacrificial thermal overload, electrical overcurrent, or short circuit protection device. The device depends on melting a metal strip or wire to break an electrical circuit or activate a safety mechanism. The links or wires used in fuses are manufactured to melt at predetermined temperatures or current ratings, making the level of protection they offer predictable. Fusible devices fall into two basic categories: mechanical and electrical. A mechanical fuse activates a safety mechanism such as a fire sprinkler, and electrical fuses interrupt a circuit in the event of an electrical fault.
A fuse generally consists of a strip or wire of fusible alloy that is connected to two connectors or terminals. These connectors are then inserted into a fuse holder in an electrical circuit or used as a triggering mechanism for an appropriate safety device. In the case of a mechanical fuse, if the ambient temperature were to exceed a pre-set value, the fusible alloy would melt, triggering the safety mechanism. An electrical fuse will transfer current through the fuse link, until that current reaches a predetermined level, at which point the link will melt and effectively disconnect the circuit.
Mechanical fuses are used in applications such as fire sprinkler systems and fire isolation doors and are usually only one part of a more complex mechanism. In these applications, the fuse assembly is exposed to the open air in the protected area. If a fire were to break out, the rise in air temperature would eventually reach a predetermined level and blow the fuse. This then allows the rest of the circuit or fuse mechanism to open the water supply to a sprinkler system or close fire isolation doors. The thermal fuse is also used in the locking mechanisms of high security safes as protection against intrusion by cutting torches.
Electrical fuses are a critical part of any electrical circuit and protect the circuit or appliance, its users, and the environment in which the circuit is located from damage, shock, and fire. Electrical fuses intended for printed circuit boards or appliances generally consist of a fuse enclosed in a glass or ceramic tube with terminals on each end. These fuses are designed to safely carry their rated current flow, but will melt to shut off power when exposed to a short circuit current surge. They will also carry any overload current for a short time before melting down to protect the circuit from damage. Glass tube type offers a visible signal when the link is blown; the type of ceramic should be checked with a continuity tester to determine whether it is blown or not.
In general, fuses are known as sacrificial protective devices as they are usually discarded when blown. In the case of many older appliances and distribution boards, the fuses are often replaceable. In these cases, rolls of fuse wire are available with specific amperage ratings from which spare links can be cut to replace blown fuses. This type of fuse is no longer common, however, and is being replaced by latching resets and trip switches.
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