How a Fuse Box and Fuses Work

  • 01 of 06

    The Fuse Box

    Old fuse box
    photographer Karl Berneli karl.b on flickr / Getty Images

    The fuse box is the equivalent of the circuit breaker's electrical service panel, in that it is a metal box with a hinged cover that houses and controls the incoming electrical service and distribution to branch circuits within the house. It provides fused over-current protection through the use of different fuse types and ratings.

    The fuse box will have threaded sockets into which the fuses will be screwed. These large threaded sockets look like light bulb sockets and are called Edison sockets....MORE They are named after Thomas Edison who invented them.

    There are several types of fuses that may go into these sockets. There are some fuses that have Edison bases that screw directly into the sockets, while other fuses fit into socket adapters that in turn screw into the Edison base.  These latter types are called "S" fuses and are also called "tamper-proof" fuses with Rejection bases.

    More on that in the next section...

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  • 02 of 06

    Fuse Bases: "Edison" and "Rejection" Types

    Fuse Base Types
    Some fuses require an Edison Base adapter to work in the fuse box. 2006

    Picking the right fuse can be confusing, but once you understand what the various codes mean, you can pick the right fuse for the job. The codes printed on a fuse specify  two things: the type of base and the degree of time-delay of the fuse.

    As already discussed, fuses have two types of bases:  the Edison Base (Type T fuses) and the Rejection Base (Type S fuses), also known as a "tamper-proof base".

    The Edison Base Type T fuse works with the typical socket used in the fuse box seen in older...MORE homes,  and the fuse base looks like a light bulb base.

    A Rejection Base Type S fuse actually consists of two components, an adapter base that screws and locks into the Edison socket in the fuse box, and the fuse itself. Each different amperage rating of a Type S fuse has a matching socket base adapter with a different sized thread that prevents mismatching the fuse. For example, it stops a person from putting a 20 -amp fuse in a 15-mp circuit—a potentially serious mistake.

    Mistakenly inserting fuses rated higher then the circuits were designed to handle was a common problem in older homes. Once inserted, it is hard to tell the fuse is the wrong size, and people often continued to replace blown fuses with the same wrong fuses. Using a Type S fuse with its rejection base adapter prevents tampering or mistakes from being made.

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  • 03 of 06

    Types of Fuses: Standard Fuse "Type W" (no time delay)

    Standard Plug Fuse - Type
    Type "W" fuses have no time delay feature. 2006

    Type "W" fuses are an older style fuse utilizing an Edison base,  and they are all but obsolete today They are general-purpose plug fuses and are "fast-acting"—that is, they have no time-delay fuse element but will quickly interrupt the circuit once the fuse's rated amperage is exceeded.

    These fuses are designed for use in general lighting and power circuits that do not contain electric motors. Electric motors draw additional current at start up and will blow these fuses if the motor is of any...MORE significant size. Because of this, time-delay fuses are now almost exclusively used.

    Rating: 120 volts; up to 30 Amps

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  • 04 of 06

    Types of Fuses: "Type SL" and "Type TL" (time delay; medium duty)

    Time Delay Plug Fuse -
    "SL and TL" fuses have a time delay feature. 2006

    SL and TL fuses are medium-duty time delay fuses and are now the most commonly used plug fuses found in the home. The only difference between the "TL" and the "SL" fuses are the types of bases. The "TL" fuse uses an Edison base and the "SL" fuse uses a Rejection base.

    These fuses work by having a plug of heat-absorbing solder attached to the center of the fuse element. This allows the fuse to absorb a temporary circuit overload, such as may be caused by a surge power draw when a motor starts up...MORE (e.g., garbage disposal). This ability to handle overloading starting currents of a motor without unnecessarily interrupting (blowing) is a key benefit of these types of fuses.

    Type "TL" and "SL" Fuses

    Rating: 120 volts; up to 30 Amps

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  • 05 of 06

    Types of Fuses: "Type S" and "Type T" (time delay; heavy duty)

    Time Delay Fuse - Type S and Type T
    Type S and Type T fuses are heavy duty fuses with a time delay feature. 2006

    S and T fuses are heavy-duty time delay fuses and used for circuits having critical or high motor loads or circuits having motors that cycle on and off often (e.g. a sump pump). These fuses have a longer time-delay feature than the TL or SL fuses. However, just like the TL and SL fuses, the only difference between the "T" and the "S" fuses are the types of bases. The "T" fuse uses an Edison base and the "S" fuse uses a Rejection base.

    These fuses work by use of a spring-loaded metal fuse link...MORE attached to a solder plug. If the overloaded circuit condition continues for too long, the solder plug melts and the spring pulls the fuse link free, cutting power to the circuit. This allows the fuse to absorb a longer temporary circuit overload.

    Type "T" and "S" Heavy Duty Fuses

    Rating: 120 volts; up to 30 Amps

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  • 06 of 06

    Types of Fuses: Minibreaker (converts a fuse to a circuit breaker)

    Minibreaker Circuit Breaker Fuse
    A "Minibreaker" circuit breaker fuse can be used to provide a level of circuit breaker protection to a fuse box. 2006

    Minibreakers are retrofit circuit breaker fuses that screw into Edison base fuse sockets. These devices are an improvement because they convert what was once a fuse into a push-button circuit breaker. These minibreakers have a little push button that pops out when the circuit is overloaded. All you need to do is snap the button back in to reset the circuit breaker fuse.

    These minibreakers are also designed for time delay so they do not trip unnecessarily when motors or appliances start up. Cooper-...MOREBussmann is a manufacturer that makes these minibreakers (Part No. BP/MB-15 or BP/MB-20).

    Rating: 120 volts; up to 20 Amps