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Workforce LibreTexts

1.4: Circuit Protection Devices

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    Protection is required to prevent damage to expensive components and all the equipment wiring if a circuit should overload due to excessive current flow. Protection can be provided by:

    • fuses
    • fusible links
    • circuit breakers
    • thermal limiters


    The fuse is the most common circuit protection device. Fuses are available in different shapes and sizes and are rated to burn out or blow out at a specific amount of current flow. The material within the fuse provides excellent conductivity as long as the current flow stays below the rating of the fuse.

    Once the current flow exceeds the rating, the material will melt and open the circuit. Blown fuses indicate a circuit fault that must be located and repaired. Simply replacing a burned fuse will not correct the problem. A burned fuse should never be replaced with another fuse that has a higher rating than what is recommended by the manufacturer.

    Figure \(\PageIndex{1}\): Fuses (CC BY-NC-SA; BC Industry Training Authority)

    Fuses can be located in a fuse box or in-line in a special holder. Small fuses, such as the glass tube fuse, are available in different lengths and sizes and have the amps rating printed on the end. Bayonet-type fuses are colour coded and have the amperage rating printed on the case. Larger cartridge fuses are used in high-voltage industrial applications. Plug or screw type fuses have a clear window on the face to check them. Figure \(\PageIndex{1}\) shows these four types of fuses.

    Figure \(\PageIndex{2}\): Fusible link and ratings (CC BY-NC-SA; BC Industry Training Authority)

    Fuse link wire size

    Color code

    20 GA


    18 GA

    Brown or red

    16 GA

    Black or orange

    14 GA


    Circuit Breakers

    There are three different types of circuit breakers:

    • cycling
    • non-cycling
    • manual reset

    Cycling circuit breaker

    Cycling circuit breakers contain an arm constructed of two different types of metal: one that expands quickly when heated and one that expands more slowly. This allows the circuit breaker to cycle from open to closed automatically.

    There is a contact point attached to one end of the arm that provides a closed circuit when it touches a fixed contact. If the current is too high for the circuit, the arm will heat and begin to bend, lifting the movable contact away from the fixed contact to open the circuit. When the arm cools, it will straighten out and touch the fixed contact again, closing the circuit. This is the type of circuit breaker that is used in automobile lighting systems.

    Non-cycling circuit breaker

    Non-cycling circuit breakers use a high-resistance wire around the arm that will carry current when the contact points open. This provides heat that prevents the circuit breaker from cycling. To reset this type of circuit breaker, it must be disconnected from the power source so that the arm can cool.

    Manual reset circuit breaker

    Manual reset circuit breakers must be reset by pushing a button or reset bar. They will not cycle automatically. This type of circuit breaker is used in residential wiring.

    Thermal limiters

    Thermal limiters are designed to melt if the device they are protecting exceeds a pre-set temperature. For example, the heating elements in a 120 volt portable electric heater are protected by a thermal limiter. If a thermal limiter melts, it must be replaced, just like a fuse.

    Figure \(\PageIndex{3}\): Circuit breakers (CC BY-NC-SA; BC Industry Training Authority)