Wireman · Fuse keeps blowing: troubleshooting for electricians · 2023 NEC ·

Fuse keeps blowing

Read the blown fuse, then find whether it’s a short, an overload, a motor start or a bad fuseholder.

Before you start

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How it works

A fuse is a metal link sized to melt at a set current. On a sustained overload the link heats slowly and melts at its weakest point; on a short circuit it vaporizes instantly and often blackens the window. Time-delay (dual-element) fuses add a slow-melting section so motors can start without blowing them. A fuse must match the wire it protects; a bigger fuse doesn’t fix anything and lets the wire overheat.

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

What kind of fuse is it?
- Screw-in plug fuse with a windowLook through the window of the blown fuse. What do you see?
  - Strip broken, window clearWhen does a new fuse blow?
    - Right away when installedA short circuit
    - When a motor startsIs the fuse marked time-delay or dual-element?
      - Yes, time-delayThe motor or its circuit has a problem
      - No, or not sureA fast-acting fuse on a motor circuit
    - After a while with several things onDoes the fuse size match the wire?
      - Yes, it matchesTest: Inspect the fuseholder
        Discolored, pitted or looseA loose or burned fuseholder
        Clean and tightOverload: too much on one circuit
      - The fuse is bigger than the wire allowsThe fuse is too big for the wire
  - Window blackened or smokyA short circuit
  - Looks intactTest: Test the fuseExpect: Near 0 Ω for a good fuse; OL for a blown one.
    - OL (open): it’s blownWhen does a new fuse blow?
      - Right away when installedA short circuit
      - When a motor startsIs the fuse marked time-delay or dual-element?
        Yes, time-delayThe motor or its circuit has a problem
        No, or not sureA fast-acting fuse on a motor circuit
      - After a while with several things onDoes the fuse size match the wire?
        Yes, it matchesTest: Inspect the fuseholder
        Discolored, pitted or looseA loose or burned fuseholder
        Clean and tightOverload: too much on one circuit
        The fuse is bigger than the wire allowsThe fuse is too big for the wire
    - Near 0 Ω: it’s goodThe fuse isn’t blown
- Cartridge fuse (tube shape)Test: Test the fuseExpect: Near 0 Ω for a good fuse; OL for a blown one.
  - OL (open): it’s blownWhen does a new fuse blow?
    - Right away when installedA short circuit
    - When a motor startsIs the fuse marked time-delay or dual-element?
      - Yes, time-delayThe motor or its circuit has a problem
      - No, or not sureA fast-acting fuse on a motor circuit
    - After a while with several things onDoes the fuse size match the wire?
      - Yes, it matchesTest: Inspect the fuseholder
        Discolored, pitted or looseA loose or burned fuseholder
        Clean and tightOverload: too much on one circuit
      - The fuse is bigger than the wire allowsThe fuse is too big for the wire
  - Near 0 Ω: it’s goodThe fuse isn’t blown