How to Test a Slow-Blow Fuse

Written by naeem ahmed
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How to Test a Slow-Blow Fuse
Fuses come in two types: slow-blow and fast-acting. (Fuse panel image by Charlie Rosenberg from

A slow-blow fuse opens only during a continued flow of overcurrent through its coil. Unlike a regular fast-acting fuse, a slow-blow fuse does not react to high currents that last for a extremely short amount of time. If a slow-blow fuse blows, do not replace it with a fast-acting fuse, which may trip the system too frequently. Use a multimeter to test a slow-blow fuse.

Skill level:

Things you need

  • Multimeter

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  1. 1

    Place the fuse on a flat, dry surface.

  2. 2

    Set the multimeter to continuity testing mode, usually by turning a knob to a point marked with two small parallel lines.

  3. 3

    Insert the red multimeter lead into the multimeter's "V" socket, and insert its black lead into its "COM" socket.

  4. 4

    Turn the multimeter on, and test it by touching the two bare ends of its leads together. The multimeter should beep. If it does not, you may not have set it properly to the continuity mode, or the battery may have dies. Take appropriate action to fix it before moving on to the next step.

  5. 5

    Touch the two ends of the slow-blow fuse with the two leads of the multimeter at the same time. (The order of the leads does not matter.) If the fuse is good, the multimeter will beep. If you do not hear the sound, the fuse has blown and requires replacement.

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