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I am installing a set of recessed lights in a closet, replacing the existing fixtures one at a time. I removed power to the lights while installing each one by using the light switch, not the circuit breaker at the box (after verifying that the the switch was wired correctly with the hot side being switched, not the neutral).

I successfully installed the first light, and had just pulled the second one down. I turned the lights back on to see better (since the other fixtures still worked), but didn't notice that the now bare and exposed hot and neutral wires from the fixture I just pulled were touching.

When I turned on the switch, the lights did not come on and you could hear a deep growling noise somewhere in the house, very similar to water hammer noise. I turned off the switch right away, not knowing the cause. I turned on the switch again with the same result. After turning off the switch, I inspected the wiring from the fixture and discovered the short.

As I understand it, the time it takes a breaker to trip is basically inversely proportional to the current draw. A dead short should be VERY high current, and I would have expected the breaker to trip instantly. It did not do so. Questions:

Shouldn't the breaker have tripped? If so, I can't trust the breaker and should replace it. But, how do I go about testing the other breakers in the box? They are all old... probably original to the 1964 house. What was that noise?

Niall C.
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bobfandango
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3 Answers3

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That noise was a wire or junction/splice somewhere vibrating and heating up. It could have also been the breaker (trying to) trip. There's a good possibility that if you turn the breaker off, it will refuse to turn back on.

At a minimum, you should replace that breaker. You should also inspect that entire length of the wiring on that circuit as you may have melted a wirenut or wire insulation somewhere.

If this were my house (and my family's life) I would call an electrician and have them inspect for damage.

longneck
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A dead short would have very high current. Fortunately, a dead short is not possible in a typical home.

The wires have resistance as do the connectors, wire nuts, and switch. There is probably nearly one ohm in your configuration. This limits the current to 120 amps—initially. As the point of closing the unwanted circuit, the contact gets hot, perhaps sparking. That considerably increases the resistance which further limits the current.

You probably were hearing the breaker trying to trip. I agree that it should be replaced. If it is not a GFI nor AFCI, it should be inexpensive ($3–$7).

It would be useful and perhaps interesting to measure the short: Disconnect the wire from the circuit breaker, connect the neutral and hot wire which accidentally touched before, turn on the switch, and then use an ohmmeter to measure the resistance of the circuit between the wire (formerly) connected to the breaker and a neutral bar in the service panel. Then measure the meter's own resistance by touching the leads together. Subtract the latter from the former. (Example: suppose the full circuit shows 0.75 ohms and the meter leads show 0.05 ohms. The circuit resistance is 0.75 minus 0.05 which is 0.70 ohms.)

120 volts (in North America) divided by the resistance is the current flow. (For the example 120 / 0.7 = 171.4 amps.) If the current is more than 1.5 times the breaker rating, it should trip pretty quickly, not much more than a second if it is rated for motor starting, otherwise much less than a second.

wallyk
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Does it really need (not should) to be replaced: Is there a lifetime failure count for a breaker? -StackEx

Here's some extra Yahoo answers nonsense, from my attempted search in finding a way to safely make a breaker pop.

The other answers to your question here bring valid concern about replacing a questionable breaker. I'd be more concerned with the condition of the wire, which I believe was the noise (rattling in the pipe). Circuit breakers are pretty good about failing in a safe state, I.e, it just won't work anymore. I wouldn't be so concerned that a monetary/slight (crappy) contact didn't trip the breaker; you did not achieve a dead enough short (see wallyk's). It was not a good thing to have happened (although not the worst) and if you're not going to pull the wire it may be safer to replace the breaker with an arc fault.

THQL1120AF, an arc fault for $50, except it's not a tandem. No good unless it happens to be the one single you have there. aplussupply.com sells standard replacement breakers for your panel. I was unable to find a THQL1120 (AF or GFCI) tandem breaker. FYI, I looked up a 100amp AF for that panel, it's $800...

You could swap that single for a new regular tandem, freeing an entire slot (by taking the other wire from the bad breaker (provided that it was on the same leg originally) for a 1" AF, now hooked to the circuits' questionable wire. Request clarification if you do not understand the importance on which of the legs a breaker is on and how to properly relocate them.

Go big or go home. Meaning, as discussed here, replace that panel or leave it alone as much as possible.

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Mazura
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