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A fuse link is a "sacrificial" thermal overload, electric over-current, or short circuit protection device. The device depends on the melting of a metal strip or wire to break an electric circuit or activate a safety mechanism. The links or wires used in fuses are manufactured to melt at predetermined temperatures or current values making the level of protection they afford predictable. Fuse link devices fall into two basic categories: mechanical and electrical. A mechanical fuse link activates a safety mechanism such as a fire sprinkler and electrical fuse links break a circuit in the case of an electrical failure.
A fuse link is generally made up of a fusible alloy strip or wire which is attached to two connectors or terminals. These connectors are then inserted into a fuse holder in an electric circuit or used as a trigger mechanism for an appropriate safety device. In the case of a mechanical fuse, should the ambient temperature rise above a set point, the fusible alloy would melt, thereby triggering the safety mechanism. An electrical fuse will transfer current across the fuse link, until that current reaches a predetermined level, at which point the link would melt and effectively disconnect the circuit.
Mechanical fuse links are used in applications such as fire sprinkler systems and fire isolation doors and are generally only one part of a more complex mechanism. In these applications, the fuse link assembly is exposed to the open air in the protected area. If a fire should break out, the increase in air temperature would eventually reach a preset level and melt the fuse link. This then allows the rest of the fuse circuitry or mechanism to open the water supply to a sprinkler system or close fire isolation doors. The thermal fuse link is also used in the locking mechanisms of high security safes as a protection against intrusion using cutting torches.
Electrical fuse links are a critical part of any electrical circuit and protect the circuit or appliance, its users, and the environment the circuit is located in against damage, shock, and fire. Electrical fuses meant for circuit boards or appliances typically consist of a fuse link encased in a glass or ceramic tube with terminals on each end. These fuses are designed to safely carry their rated current flow but melt off to cut the power supply when exposed to a short circuit current spike. They will also carry any overload current for a short period before melting off to protect the circuit against damage. The glass tube type offers a visible cue when the link has burnt off; the ceramic type needs to be checked with a continuity tester to establish whether or not it has blown.
In general, fuse links are known as a sacrificial protective devices as they are usually discarded when they have blown. In the case of many older domestic appliances and distribution boards, fuse links are often replaceable. In these cases, rolls of fuse wire are available with specific amperage ratings from which replacement links can be cut to replace blown fuses. This type of fuse is no longer common, however, and is being replaced by pop-out resets and trip switches.