How does only part of a curcuit go down?

October 26, 2007 12:58 AM

Any electricians out there? How can a household circuit *halfway* stop working? I am confused. Details inside.

(Sorry for the length, but I am trying to be as detailed as possible)

In my house, there is a single 15 amp circuit that covers the garage, my home office, and the living room. From the garage, the circuit enters the office, feeds five outlets plus a ceiling light, and then carries on into the living room. More precisely, it enters the office at one outlet, the line carries one direction to another outlet and stops, and carries the other direction to the third outlet which is also switched to the wall switch and ceiling light, and then around to the rest of the outlets and on to the living room.

Last night while working late, the power went out in the office (and not the rest of the house) for a few seconds. After it came back on, I went to turn on the overhead light and immediately the power went out again.

Now here is where it gets weird. The circuit breaker was not tripped when the power went out. There was still power to the garage, and to the first two outlets in the office. From the third outlet (the one switched to the overhead light) and onward all the way through to the living room there was no power. I checked the breaker box, and although it had not tripped I did manually turn the breaker off and then back on. When that one breaker was turned off, there was no power in the garage or in the two outlets in the office. When I turned the breaker back on, power resumed in the garage and the two office outlets, but not in the rest of the office or the living room. After an hour like this, I gave up and went to bed, with the obvious plan to call an electrician in the morning.

In the morning, the power was back on everywhere. I still called the electrician, who recommended that I run a dedicated 20 amp circuit into the office because of all the computer equipment. I will definitely do this. But I still don't understand how *half* of a circuit can go down. If there was a short, shouldn't that by definition take out the entire circuit and trip the breaker? If it was too much load on the circuit, shouldn't something similar have happened? Can anyone explain this to me?

posted by Lokheed to home & garden (13 comments total)

I had a similar problem once. The electric company came and fixed outside - the power line was messed up somewhere.
posted by bigmusic at 1:12 AM on October 26, 2007

If the wire is broken but not shorted out, that could cut power to stuff after it. More likely than the wire itself being broken is in one of the junctions something's loose.

Loose connections I think can generate a fair amount of heat if you pull a lot of current through them, so this might be a fire hazard. I'd avoid putting any big power draws on the outlets that have been acting up.

You might (AFTER TURNING OFF THE BREAKER of course) open up the last good outlet and the first bad one (and the light switch?) and check if anything looks loose or tenuously connected, but getting an electrician to fix it is not unreasonable.
posted by aubilenon at 1:13 AM on October 26, 2007

What aubilenon said. This doesn't have anything to do with the breaker or with overloading the circuit; there's a bad connection somewhere. A loose wire nut or terminal screw can do just what you describe.
posted by jon1270 at 3:10 AM on October 26, 2007

You need an electrician to come and check this out NOW! One probable cause is a loose wire on one of the outlets or somewhere else in the circuit. Loose wires can cause sparks and sparks can cause fires. You have your dwelling on the line here.
posted by caddis at 4:45 AM on October 26, 2007

Seconding caddis. Call an electrician to find the bad connection before you burn your house down. I'm pretty suspicious of a wiring installation that has your garage and living room on the same 15 amp circuit, that can't be to code. Because of the indoor/outdoor nature of garages, they are usually on the their own circuit.
posted by octothorpe at 5:45 AM on October 26, 2007

I can't believe that all three of those locations aren't on dedicated circuits. 15 amps for all that stuff? Ouch.

Because most shared-circuit plugs/lights, etc, are wired directly through each other, when one goes they all go, sort of like christmas lights. It could be as simple as a connector screw in the first dead outlet has come off or loose, or it could be that you're running too much current through too little wire too far, and you're cooking your circuit, although the breaker would probably go at that point. It could simply be that you just can't suck enough power through the line and you're browning out without throwing the breaker. It could be a bad breaker. It could be several things.

Call the electrician and be prepared to pay through the nose.
posted by TomMelee at 6:48 AM on October 26, 2007

Call the electrician and be prepared to pay through the nose.

Fixing the existing circuit will be cheap, and if you know what you are doing is simple enough to do yourself. Rewiring to create new circuits might not be so cheap depending on how easy it is to access the walls etc.
posted by caddis at 7:13 AM on October 26, 2007

Oh, sure, FIXING it will be cheap. Running new circuits, not. Diagnosing the problem...definitely not.
posted by TomMelee at 8:12 AM on October 26, 2007

"I still don't understand how *half* of a circuit can go down."

This happens when a circuit uses the plug in as a junction instead of pigtailling the wire at the plug. (see picture at the bottom of this page) You definitely want to get this attended to as you have a potential fire hazard if there is a loose conection. You should check the plugs on that breaker and see if they are using the plugs as a junction. If they are install pigtails with wire marrettes (assuming you have copper wire).

If all the plugs aren't pigtailled you might want to plan on doing your whole house. It it's difficult but it'll take 5-10 minutes per outlet.
posted by Mitheral at 9:06 AM on October 26, 2007

PS: Pigtail style outlet connections are code required in Canada but some of the American codes are more permissive.
posted by Mitheral at 9:09 AM on October 26, 2007

Thank you, everyone. It seems very likely that Mitheral has the answer, I am almost certain the problem outlet is not pigtailed. A little later this afternoon I will shut off the breaker and open up that outlet, and rewire as necessary. I'll report back what I find....
posted by Lokheed at 10:45 AM on October 26, 2007

I doubt anyone is still reading this thread, but I will follow up now that the issue has been resolved. It turned out to be a loose connection with the hot wire on one of the outlets on the circuit. Although the problem appeared to be in the office itself, it turned out to be an outlet that was on the other side of the wall in the utility room. The loose wire had melted sheathing, and there were burn marks on the connection terminal. Clearly the wire had been loose for a long time, worked fine most of the time, but would periodically heat up and cause the connection to break. The electrician trimmed off the damaged part of the wire, replaced the outlet, and now everything works swimmingly. Thanks to everyone who pointed me in the right direction.
posted by Lokheed at 9:38 AM on October 30, 2007

Yay, I called it.
posted by TomMelee at 1:25 PM on October 30, 2007