WiremanWireman · Primary trips when the transformer is energized: troubleshooting for electricians · 2023 NEC ·
Primary trips when the transformer is energized
Inrush, faults, secondary loads and protection settings when the primary breaker won’t hold.
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How it works
When a transformer is switched on, its core can draw a magnetizing inrush current of roughly 8 to 12 times rated current for a few cycles, depending on its size, design and where in the AC cycle the switch closes. Magnetism left in the core from the last shutdown can make it worse, which is why it may trip only sometimes. The primary breaker has to ride through that inrush while still protecting the transformer within the limits of 450.3(B).
Every question and answer in this guide. Tap “Step by step” to walk through it.
- How does it trip?
- Every time, instantlyTest: Test the transformerExpect: High megohm readings and all taps matching.
- Low insulation readingWinding insulation fault
- Taps don’t match between coilsMismatched taps
- Tests goodWith the secondary disconnected, does it still trip when energized?
- Holds with the secondary offSecondary-side fault or load
- Still trips with no loadWhat protects the primary, and how is it sized?
- Standard breaker or fuse near 125% of primary ampsNormal inrush on tight protection
- Adjustable electronic tripInstantaneous or short-time setting too low
- Larger breaker, and it still tripsBad breaker or hidden fault
- Sometimes, not every timeWhat protects the primary, and how is it sized?
- Standard breaker or fuse near 125% of primary ampsNormal inrush on tight protection
- Adjustable electronic tripInstantaneous or short-time setting too low
- Larger breaker, and it still tripsBad breaker or hidden fault
- After it’s been on a whileOverload, not inrush
- Every time, instantlyTest: Test the transformerExpect: High megohm readings and all taps matching.