WiremanWireman · Control fuse keeps blowing: troubleshooting for electricians · 2023 NEC ·
Control fuse keeps blowing
Separate a real short or ground fault from a fuse that’s too small for coil inrush.
Before you start
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How it works
Most motor control circuits run from a small control transformer (CPT), commonly 120 V or 24 V on the secondary, with fuses on the primary, the secondary, or both. The secondary is usually grounded on X2, so a control wire that touches the enclosure or conduit is a short to X2. Coils also draw several times their holding current for a moment when they pull in, so a fuse that’s too small or too fast blows on inrush even with nothing wrong.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When does the fuse blow?
- As soon as control power is onTest: Look for the shortExpect: Some resistance through coils and pilot lights (often tens to hundreds of ohms), not near 0 Ω. To ground: OL, unless X2 is the grounded conductor and you’re reading through a coil.
- Near 0 Ω to ground or X2A short or ground fault in the control wiring
- Normal resistanceIs the fuse the size and type the starter or panel drawing calls for?
- Smaller or faster than specifiedFuse too small or too fast for coil inrush
- Correct, but more loads were addedThe control transformer is overloaded
- Correct and nothing changedCheck the primary side and tap
- When start is pressed or a coil pulls inTest: Check the coil that pulls inExpect: A steady reading consistent with that coil (not near 0 Ω), and a coil rated for the control voltage.
- Near 0 Ω, or much lower than a good oneShorted or wrong-voltage coil
- Wrong voltage coilShorted or wrong-voltage coil
- Coil looks normalIs the fuse the size and type the starter or panel drawing calls for?
- Smaller or faster than specifiedFuse too small or too fast for coil inrush
- Correct, but more loads were addedThe control transformer is overloaded
- Correct and nothing changedCheck the primary side and tap
- Randomly, or only when wetAn intermittent fault in a field device or conduit
- As soon as control power is onTest: Look for the shortExpect: Some resistance through coils and pilot lights (often tens to hundreds of ohms), not near 0 Ω. To ground: OL, unless X2 is the grounded conductor and you’re reading through a coil.