What is causing a hum and tripped breaker in my house?

December 6, 2015 3:13 PM

I had my roof replaced last week. Coincidentally(?) I had a breaker — for the circuit which serves the upstairs — trip, and trip again when it's reset. There's a buzz or a hum, coming from somewhere upstairs when the circuit is powered for a second or two, before it trips again. The electrician who came to check it out concluded I had an exhaust fan or some other motor that's seized up. The only problem is, I don't have an exhaust fan that I'm aware of. What could this be? Details aplenty inside.

You are not my electrician. My electrician already looked at this and his conclusion is below. I likely need a second opinion, which I will get, but in the meantime you might enjoy speculating.

Some background: The roof could be a red herring, but it seems relevant. The breaker tripped while the roofers were working, though I didn't notice it until later. They did replace some decking, not just shingles. I have a 1940s vintage house. The problem circuit is on a 15-amp breaker, and serves too much stuff (again, 1940), including the upstairs finished attic's lights and outlets. I haven't added any new load to that circuit for probably a couple years and it's never tripped before this.

The electrician checked the breaker, and said it was fine. He said he didn't think it was a short (I guess from the behavior of powering up for a second or two before the breaker trips). He said the circuit was pulling 70-some amps when powered, and suggested it was a motor that had seized.

When the circuit is powered, there is a hum that sounds like an electric motor of some sort (think fridge compressor, electric shaver, that sort of characteristic). This is accompanied by a vibration in the wall near where the sound seems to be coming from.

I opened up the wall and ceiling near where the noise seems to be coming from, and haven't been able to find the source. Moreover, there's no place that would even make sense for an exhaust fan to be — there's no vent on the roof or under the eaves. And also, if it is a motor that's seized, that means that either it's been powered the whole time I've owned the house without my knowing it, and just seized now, or it's been seized the whole time, and just somehow became powered.

I did find that the some of the Bx runs very close to the roofline. In some places that I can see where this is the case, there are some new-looking nails which come very close to touching the Bx. Which seems…not good.

So…

Is it likely I have a mystery motor somewhere? Would a nail through Bx cable cause that behavior? Or is there a bonus third option I'm not considering?

What would you do in this situation?

(If the bonus third option is "get another electrician", that's already in progress).

posted by brentajones to Home & Garden (19 answers total)

what does Bx mean?

edit: oh, bx cable seems to be some usian standard for armoured cable.
posted by andrewcooke at 3:25 PM on December 6, 2015

Maybe an attic vent fan? A bathroom vent fan into the attic? Your first electrician does not inspire confidence.
posted by Bee'sWing at 3:41 PM on December 6, 2015 [1 favorite]

Nails anywhere near wiring would suggest your wiring is not up to code. There has to be at least a 1.25 inch space between wire and any possible nails. Your electrician should have pointed that out so you may want to look into getting a new electrician who cares.

I suggest you get a good electrician and start upgrading your wiring because wiring that is not up to code can completely void your home insurance coverage if you have a fire.
posted by srboisvert at 3:44 PM on December 6, 2015 [2 favorites]

I think I would get a stethoscope and a metal detector.
posted by HuronBob at 4:15 PM on December 6, 2015

I shorted a pair of AC wires together one time and blew the breaker. When I did, there was a loud hum from the wall and then it blew. My guess is that they put a nail through the AC line and the wires are shorting together to blow the breaker.
posted by Slinga at 4:32 PM on December 6, 2015 [6 favorites]

An electrician who is willing to walk away without solving the problem aftern being presented with those symptoms is one that I would pay off, send packing, and then never hire again.

Your breakers are there for a reason and if they're tripping repeatedly that's very definitely something you should be concerned about.
posted by Nerd of the North at 5:09 PM on December 6, 2015 [10 favorites]

A nail stuck in the hot side of the circuit and either draining to the BX or through the frame of your house, most likely the latter, since a dead short with low resistance would trip the breaker more quickly.

Examine every inch of the circuit for damage, because it is almost certain that there is a wiring problem given a known good breaker.
posted by wierdo at 5:17 PM on December 6, 2015 [3 favorites]

What wierdo says. And until you have a different electrician on site looking at everything, keep the breaker off, because if it is a nail into a hot circuit it is very dangerous.

I'd think the electrician would begin disconnecting cable runs sequentially away from the breaker panel until they can identify the one that is shorted.
posted by BillMcMurdo at 5:50 PM on December 6, 2015 [1 favorite]

I concur with the two previous posts. Do NOT go poking around that BX with the breaker on.
posted by humboldt32 at 6:55 PM on December 6, 2015

Yeah, speaking as an amateur electrician (and network engineer) who has done lots of wiring, the nail scenario seems likely. Or maybe an old piece of wire just got jiggled enough to finally make contact between conductors, or with a nearby ground.

Electricians (real ones, not clowns like you seem to have hired) have a process of troubleshooting, and various tools (like line testers and metal detectors and RF/mag detectors) and can figure this out.

Also, this has been said but maybe not emphasized enough: Treat that breaker as a "START THE HOUSE ON FIRE" switch until it's been properly fixed. DO NOT TURN IT ON EVER.
posted by mmoncur at 6:57 PM on December 6, 2015 [4 favorites]

Yup, I'd guess someone accidentally put a nail through a wire and shorted things in a way where the short is marginal enough not to trip the breaker immediately. This can happen if a wire gets nicked, and the offending nail is just barely touching the wire so that the connection it makes is kinda crappy.

By the way, your roofers should probably have realized that they'd done this, apologized, and gotten it fixed for you. I've accidentally cut a couple of wires myself (it occasionally happens, despite all care being taken to avoid it) and that has always been the protocol. You own up to it and make it right. Anything else would be unethical, and dealing with the occasional mistake like this is part of doing business in the building trades.

It's possible that your roofers didn't know they'd done anything, since the wire was on the underside of the roof, it likely wasn't powering anything they were using, and a marginal short might not make a noticeable spark or popping noise. Still, if your second electrician finds the problem (I agree that your first electrician should not have walked away from this) and it does turn out to be a roofing nail, I would call/email the roofers and let them know. They might be willing to help you out somehow on the repair as a gesture of goodwill, and in any case they should know that they need to be more careful.
posted by Anticipation Of A New Lover's Arrival, The at 6:59 PM on December 6, 2015 [2 favorites]

Response by poster: Checking back in; first, thanks for all the suggestions. As I said, another professional will be taking a look. Feel free to keep the theories, anecdotes, etc. coming.

To defend the first electrician a bit: he didn't actually see any of the wiring; I hadn't cut open the wall yet at that point. His contribution was testing the breaker, hearing the noise and testing the current. Sounds like he probably shouldn't have been so quick to dismiss the possibility of a short, though.

And to defend the roofers a bit: I'm guessing they didn't realize anything had happened; they definitely weren't using that circuit for power and they wouldn't have had any way to know it tripped.

I certainly won't be poking around at it myself, and the breaker has been and will be off until someone who's more knowledgeable than me figures it out.
posted by brentajones at 7:22 PM on December 6, 2015 [1 favorite]

BX -> Type AC -> American MX cable

brentajones: "He said the circuit was pulling 70-some amps when powered, and suggested it was a motor that had seized."

70A of LRA current on 115V is a couple horsepower; you'd know about that motor if it was regularly running. Why didn't he trace down the motor?

Given the symptoms I'd bet on nail through BX armour and touching the hot. The armour is a fairly high impedance path for fault current (especially on a home run) and contrary to popular belief a short that only results in 5x handle current can take a while to trip. Especially if it is an arcing type fault.

In the electrical trade roofers are notorious for using fasteners that are way longer than required and easily long enough to penetrate the 1 1/4" that wires are required to be set back from surfaces (electricians also make mistake sometimes putting wires in harms way).

The hum is caused by induced movement of the cable from the high current.

Keep that circuit off until you find the problem even if you manage to get the breaker to hold because dangerous potential for arc fault could still exist.

The roofing could also just be a coincidence, that kind of thing sometimes happens. Or the cause could be some existing problem that was aggravated by people walking on the roof deck.
posted by Mitheral at 7:23 PM on December 6, 2015

I had something similar happen. It ended up being a faulty doorbell.
posted by dhens at 8:22 PM on December 6, 2015

I just want to reinforce that there's no reason to think this has to be a motor: "electricity managed badly" sounds exactly like a small motor, at least to me. There was a very motor-ish noise just before the last time my stepdad almost electrocuted himself.

He was scared enough by the shock that he no longer screws around with wiring, at least while barefoot in a damp basement. That noise means "stop right now and call for professional help" as far as I'm concerned, unless you can actually see the motor.
posted by SMPA at 9:30 PM on December 6, 2015 [1 favorite]

To defend the first electrician a bit

What? No. He left you with a wild-ass theory he didn't bother testing and a 70 amp short somewhere. He left you with a fire or electrocution hazard and didn't even tell you it would be dangerous to energize that circuit until you found a real electrician. Don't hire that guy again.
posted by ctmf at 10:37 PM on December 6, 2015 [7 favorites]

He said the circuit was pulling 70-some amps when powered, and suggested it was a motor that had seized.

70A on a US circuit is *huge*, the largest you're likely to find in a house is 30A, and that only to things like a older clothes dryer or to the central air conditioning system (where it's likely a 30A 240V circuit)

If the nail theory is correct -- and I'm good with that theory -- the hum is the wires literally vibrating at 60Hz (and the 30&120Hz harmonics.) Changing electric currents create changing magnetic fields and you change the current 60 times a second in US AC power.

A loud hum followed by the breaker popping means there's a dead short. The "electrician" who took a guess as to where was either incompetent or unpaid. A competent one finds the short and either fixes it (if in the wiring) or disconnects whatever's attached to it that's shorting (if it's something connected to the circuit, rather than in the circuit.)

They do not say "probably an exhaust fan" and walk away.

And, yes: Treat that breaker as a "set house on fire" switch until a competent electrician finds the fault and either fixes it (if in circuit) or disconnect the broken thing (if attached to the circuit.)
posted by eriko at 5:41 AM on December 7, 2015 [2 favorites]

70A on a US circuit is *huge*, the largest you're likely to find in a house is 30A, and that only to things like a older clothes dryer or to the central air conditioning system (where it's likely a 30A 240V circuit)

This is not entirely crazy. An induction motor on startup will pull about 20 times nominal current for the first cycle and then 5 to 10 times nominal current for the first several seconds.

Because of this, circuit breakers have a trip delay curve. The breaker will handle up to 10 times rated current for one second and 3 to 5 times rated current for 10 seconds without tripping.

The fact that the breaker takes about 2 seconds to trip indicates that the load is probably around 5 times the breaker rating which would be in the neighborhood of 70 amps. It would take about 150 amps to trip instantaneously (in the first cycle).
posted by JackFlash at 9:14 AM on December 7, 2015 [1 favorite]

I once had an instance of a wire breaking in BX - forget if the neutral or hot - but the result was the circuit was using the grounded metal cable sheathing as a neutral return path. This was near a switch, the cable had not been messed with at all, it dated to around the 1930's.
So just wiggling the BX - or all the vibration from nailing, could have broken a wire at a connection.
posted by rudd135 at 10:18 AM on December 7, 2015