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I know that sometimes a circuit breaker will be faulty and will not pass power, but I have a situation where I identified a wiring problem where I expected the breaker to trip but it didn't. Is this possible? How can I test that my breakers will trip when a fault is detected?

To explain the situation a bit better: I had a circuit where three outlets worked, while one was faulty and a switched set of lights didn't operate from the switch. When I investigated the faulty outlet I found a stray wire that had pulled out from the back. I removed the outlet completely and rewired the broken electrical chain together. To my surprise, the lights started to operate correctly.

If there was a fault in that outlet, why didn't the breaker trip? What conditions must be met to trip the breaker? I'm pretty sure a live-to-neutral short would trip it, but what about live-to-ground or neutral-to-ground?

Maybe the fault was something other than a short? I can't really fathom what the actual wiring could've been to have caused the symptoms that I observed. If memory serves correctly I believe it was a white neutral wire that was loose. My non-contact voltage sensor indicated that both slots of the outlet were hot.

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  • Neutral to ground won't trip a breaker, unless it's GFCI. But it can cause a great deal of current to flow in abnormal places. In fact that's a bit of a problem: neutrals do not have overcurrent protection. – Harper - Reinstate Monica Jan 24 '17 at 21:16
  • If the neutral wire had pulled out of the receptacle, the rest of the circuit after this point (assuming the receptacle is being used as a splice) will no longer work. When you plugged the neutral back in, you completed the circuit again. – Tester101 Jan 24 '17 at 22:33
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Neutral to ground will not blow a breaker. Live to neutral or live to ground should.

It is possible for a circuit breaker to fail to trip. This is a huge concern with old Federal Pacific breakers - some were recalled due to not tripping in a fault, and could cause a fire. Testing for it isn't really a safe thing to do - because if your breaker has failed, intentionally shorting it could cause a fire.

My guess would be that your breaker is fine and there was no short - one wire for the outlet had come disconnected. You said you wired the chain together after removing the outlet - that chain was broken with the wire missing, so any outlets downstream of that didn't get any power.

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  • Do you know of a safe way to test if the breaker will trip when necessary? – Octopus Jan 24 '17 at 19:44
  • @Octopus you could intentionally cause a short...if the breaker is good, it will trip. If it's bad, your house may catch on fire. Assuming it's not an especially fancy AFCI/GFCI breaker, they are usually pretty cheap, and your best bet is to just replace it if you feel it's suspect. Even if it was ok, you'll feel better knowing you have a new one in there. – Grant Jan 24 '17 at 19:50
  • But the outlets downstream of that were still powered, while the lights that were upstream (ie. closer to the breaker panel) were non functioning. Thats why I say this I can't really fathom the wiring. – Octopus Jan 24 '17 at 20:01
  • @Octopus just because they are closer to the breaker panel doesn't mean they are first in the chain. Easy way to tell would be to separate those wires again, and measure with a multimeter to see which pair has power. – Grant Jan 24 '17 at 20:03
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A fault where a wire is opened (severed) causes low or zero current, never more current than the load wants. It would never cause an overcurrent trip, though we hope it will cause an arc fault trip.

What aspect of the circuit's breaker functioning are you testing?

  • AFCI (if equipped), or arc fault circuit interruptor, which detects arcing in wires which typically start fires.

  • GFCI (if equipped), or ground fault circuit interruptor, which detects current leakage outside the normal hot-neutral loop, i.e. Voltage going through a human.

  • Magnetic trip, which detects very high-current or "bolted" faults many times the rating of the breaker, and trips immediately.

  • Thermal trip, which detects mild less-than-bolted overcurrents, with tolerance for a limited time of 2 seconds to 10 minutes, including normal momentary overloads such as inrush current from motor startup, incandescent lamps, ballasts, digital power supplies and the like.

Which are you trying to test? Each requires an entirely different testing method.

There is no overcurrent protection on neutral. If the grounding system is healthy, a hot-neutral fault will flow a considerable fraction of the current the equipment is using. It will also trip GFCIs and some AFCIs. If the grounding system is unhealthy, it can create arcing where the grounding problem is. In very large circuits, the ground wire is allowed to be much smaller than the neutral, so you don't want them splitting the current in normal operation.

A hot-ground fault will behave same as a hot-neutral fault if the grounding system is healthy. If the grounding system has a problem, the hot-ground fault can place dangerous voltage on parts of the building that are supposed to be grounded.

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