I had one last electrical box to install a outlet in, on a live residential circuit, which I had "completed" earlier by wire nutting the wires together to "complete" the circuit. But instead of flipping the breaker to 'off' I decided to try and short the circuit with a screw driver. (many people would find this a foolish practise.) I was surprised to discover that this attempt at shorting failed. I tried again and failed again. So, I think to myself that I might have a bad breaker that doesn't trip when overloaded, but I don't know how to find out for sure. I decide to test another circuit on another breaker, to see if I can overload it. Instead of shorting it, I'll put sufficient load on it to trip it, I think to myself. What I use is two 1500 watt portable heaters and two circular saws, maybe 15 amps each on start-up, I guess. So lots of juice going through the breaker for about 20 seconds. I don't want to fry the wires with a heavy load too long. The breaker does not trip. I repeat a few times, after a cooling of wires period of time in between, but this breaker also does not trip. My thoughts are that it is rare that breakers are faulty and fail to trip. Does anyone have experience with breakers failing to trip due to faulty breakers? Does anyone else have thoughts and opinions on this situation? How can I reliably/cheaply/confidently find out which breakers, if any, are faulty? I wouldn't want to unnecessarily replace all breakers if they weren't faulty. How can I test my breakers?

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    What make/model is the breaker in question? Oct 20, 2017 at 0:06
  • If you want to test your breakers without stressing your wiring, short them out at the service panel. Connect an insulated alligator clip to one end of a heavy (10 gauge if you have it) insulated wire and strip only 1/16" of insulation from the other end. Clamp the alligator clip onto the neutral bar. Stab the line terminal on the breaker with the other end of the wire. - Wear gloves. Oct 20, 2017 at 0:27
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    @A.I.Breveleri And eye protection for the arc flash. Oct 20, 2017 at 3:20
  • The test I describe is for massive overload, i.e. a dead short, only. - For proper testing with loads near the breaker's rating, follow Harper's advice below. Oct 20, 2017 at 3:45
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    Who is the manufacturer of the breaker? If it's something like an old style FPE it may never trip without damage. That's why we recommend replacing them. Oct 20, 2017 at 11:53

1 Answer 1


Here's how you test a breaker

First, you need to get some hard numbers as to the actual load you are putting on the breaker. So get a Kill-a-watt and see what your various loads actually do draw in amps (test them individually).

Now you are looking to test the breaker, not set the wiring in your home on fire, so don't draw this through the house's wiring. Build a little test box out of a 4x4 metal junction box, #10 cable, and 2 twin receptacles, and strip the #10 cable back about a foot. Ground to ground bar, neutral to neutral bar, and hot directly to the breaker under test. Plug all your loads into the receptacles.

OK, you want to test the breaker. Go get the breaker's trip-curve chart.

enter image description here

Look up the amps you will be applying, say it's 40 amps. That's 200% of the breaker rating, so find 200% on the horizontal chart for your breaker (not this example)... and follow the 200% line up to the number of seconds the line enters and exits the "gray area". In this example, it's 9 seconds and 35 seconds.

Turn on your loads and stopwatch... your breaker should trip between 9 and 35 seconds later. (maybe a smidge longer if the load is causing voltage to sag).

That's a thermal trip mode.

The magnetic trip is an instant trip based on high excess current. That is the horizontal blob at the bottom of the chart. Flow 15x rated current and you should get a nice instant magnetic trip inside one AC cycle (<=20ms).

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    Just keep in mind that testing the magnetic trip can be quite hazardous due to the high energies involved. Oct 20, 2017 at 3:46
  • @ThreePhaseEel assuming you test at 120V. Magnetic trip should care about current, not voltage, so you only need enough voltage to push electrons through the breaker. Oct 20, 2017 at 16:41
  • good point. testing with low voltage is somewhat safer. Oct 20, 2017 at 22:22

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