Electricity can be dangerous. It is vital to use a reliable method of cutting off current and isolating a circuit when adjusting electrical equipment. Switch disconnectors perform this function, cutting off the electricity supply to power down a length of the electrical circuit so it can be serviced.
Fused switch disconnectors are a type of isolator, a standard electrical safety device, but they differ from other varieties in combining isolating and current switching functions. Fuses within their enclosures allow sections of an electrical circuit to be quickly and safely shut down and then re-energised when the required maintenance has been completed.
Isolator switches are also distinct from circuit breakers, although they have a similar function. Circuit breakers stop the flow of current to an entire circuit, while isolator switches cut off only part of the circuit.
Isolator switches are offload devices, meaning that the requisite part of the circuit is only isolated after the current has been stopped. By contrast, circuit breakers are on load, so the current continues up until the moment of the break.
It is relatively common to use both a circuit breaker and an isolator switch for additional safety in higher voltage environments. The former cuts off the current to the whole circuit and the latter then isolates a section for safe access during servicing.