How to Test a Fuse

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Some older homes still exist where the electric is protected by fuses rather than more modern circuit breakers. With this type of older wiring system you may have to test the fuses from time to time to check for continuity. You can also find cartridge fuses in use in many electrical disconnects like those feeding your central air conditioner or secondary sub-panel. Here is how to test a fuse.

Things You'll Need

  • Multi-tester
  • Fuse pullers (for cartridge fuses)

How to Test a Cartridge Fuse

  • Turn the circuit OFF. If you are working on a sub-panel, then turn the breaker off that feeds the sub-panel. If the power is unable to be interrupted, then turn the disconnect switch OFF, but remember that the wires coming into the panel will still be HOT!!

  • Clamp your fuse pullers on the mid-section of the cartridge fuse you need to test. Gently pull the fuse out of the fuse clamps. Set the cartridge fuse down on a flat, level surface.

  • Set your multi-tester to read continuity. Then touch one probe to one end of the cartridge fuse and the other probe to the other end. Be careful not to touch the metal tips of the probes with your fingers or you could read continuity through your body, resulting in a false positive.

  • If the fuse is good, the multi-meter should display a zero reading. Some multi-meters will also sound an audible signal as well. If you get a reading of infinity, then the cartridge fuse is bad.

  • Be sure to use the fuse pullers to insert the new fuse into the panel's fuse clamps. Never use your hands to insert a cartridge fuse in case the feed wires are still hot!

How to Test a Glass Fuse

  • Glass fuses are removed by unscrewing them counter-clockwise. Once you remove the fuse, place it on a level surface, flat side down so that the threads are on top.

  • Set your multi-meter to read continuity. Touch one probe to the copper center contact and the other probe to the metal screw shell on the side of the fuse.

  • If the multi-meter reads infinity, then the fuse is bad. If it displays a reading of zero and sounds the audio signal (if applicable), then the glass fuse is good.

  • Glass fuses can also be checked without removing them by shining a flashlight into the glass "window" on the front of the fuse. If the window is burnt or extremely cloudy, then chances are the fuse is blown.

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