Screw-in Fuse Bases
Fuses for standard circuits (not high-voltage appliance circuits) are called plug fuses and have screw-in bases. There are two different types of bases and screw-in fuses: the Edison base (found on Type T fuses) and the rejection base (found on Type S fuses).
The Edison base (Type T) looks like a light bulb base and fits the standard sockets found in old fuse boxes. Rejection base (Type S) fuses will work with Edison-type sockets only when combined with an adapter base that screws and locks into the Edison socket. The Type S fuse then screws into the adapter.
Rejection bases are also known as “tamper-proof,” and they were developed to prevent homeowners from using the wrong type of fuse for a circuit. Each Type S fuse of a specific amperage rating has a matching base adapter with a specific size of thread that prevents mismatching the fuses. For example, it stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially serious mistake. This condition is called over-fusing and can result in the fuse failing to blow before the circuit wiring overheats and potentially catches fire.
A 15-amp Type S fits only a 15-amp base adapter. By contrast, a Type T fuse can fit into any Edison socket, regardless of the circuit’s amperage. If you have an old fuse box with Edison sockets, switching to socket adapters and Type S fuses makes the panel much safer.
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