WiremanWireman · Test a breaker: troubleshooting for electricians · 2023 NEC ·
Test a breaker
What you can really check in the field, and when a breaker needs proper trip testing.
Before you start
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How it works
A thermal-magnetic breaker has a bimetal that bends with heat to trip on overload and a magnetic coil that trips instantly on a short. You can’t check those trip points with a multimeter. In the field you can check the breaker’s condition: how it looks, how hot it runs, whether it’s tight, and how much voltage it drops when closed and carrying load. Proving it trips at the right current takes primary-injection test equipment.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Why are you testing the breaker?
- It trips and I suspect the breakerTest: Measure the load on itExpect: Below the breaker rating, and below 80% of it for loads that run for hours.
- At or above its ratingThe breaker is protecting an overloaded circuit
- Well below its ratingTest: Look and check temperature
- Discolored, cracked, buzzing or much hotter than neighborsOverheated or damaged breaker
- Looks and runs normalTest: Measure voltage drop across the closed breakerExpect: A small fraction of a volt, similar to an identical breaker at similar load.
- Low and similar to its neighborsTest: Check the connections dead
- Loose, pitted or discoloredLoose or burned connection
- Tight and cleanPasses every field check
- Several times higher, or near a volt or moreHigh resistance in the breaker or its connection
- Low and similar to its neighborsTest: Check the connections dead
- It’s hot, buzzing or smellsTest: Look and check temperature
- Discolored, cracked, buzzing or much hotter than neighborsOverheated or damaged breaker
- Looks and runs normalTest: Measure voltage drop across the closed breakerExpect: A small fraction of a volt, similar to an identical breaker at similar load.
- Low and similar to its neighborsTest: Check the connections dead
- Loose, pitted or discoloredLoose or burned connection
- Tight and cleanPasses every field check
- Several times higher, or near a volt or moreHigh resistance in the breaker or its connection
- Low and similar to its neighborsTest: Check the connections dead
- It won’t reset or stay onReset problem: follow the reset guide
- Maintenance: will it trip correctly?Trip testing needs primary injection
- It trips and I suspect the breakerTest: Measure the load on itExpect: Below the breaker rating, and below 80% of it for loads that run for hours.