The fusible switch is a type of bus plug typically found in older homes or apartments. When electricity passes through the system, there is always a danger of overload. With no protection from overloads, your appliances would be destroyed and there is great potential for fires as a result. The fusible switch contains a fuse to protect the home. In the fusible switch bus plug, the fuses are designed with an element which is destroyed in the event of a power overload. When the fuse is intact, the circuit is complete and energy can pass through.
When the fuse element is melted, the circuit is broken and electricity does not flow. This keeps the overload from reaching your appliances. Once the element is melted, that fuse must be replaced. This type of bus plug is replaced in newer buildings by the more efficient circuit breaker system, which requires no replacement of burnt-out fuses.