Roofing nail shorted which ceiling fan?

jimmysdown · 12-23-2008, 10:40 AM

I have 3 ceiling fans, with lights, on my porch, Wires to fans are not visible , I assume they are inside the top of the beams.
Fans are operated by any of 3, two way switches.
Had new roof installed. Now when turn on ceiling fan light, with one of the two-way switches, get a brown out, on fan light for 2-3 seconds then the breaker flips. I assume the roofer put a nail through the wires going to one of the fans, since it happened at the same time.
What is the easiest way to determine which fan’s wiring is shorting out? Or do I need to cut the wires at the junction of porch ceiling and attic and do it by process of elimination?

Stubbie · 12-23-2008, 11:14 AM

Jim

It does sound like you have a nail through a cable and that also makes sense since I'm understanding everything was fine with the fans before the roof was installed.

You pretty much are going to use the elimination method.

Also you probably have two 3 way switches and one 4 way switch.

If you have 3 single pole switches that operate the fans from any location someone has some real resourceful wiring going on and I would not be so sure that you have a nail thru the cable.

So would I be correct in assuming that everything was working fine before the roof was installed?

Also please verify the type switches being used.

jimmysdown · 12-23-2008, 11:36 AM

Yes, everything was working fine before the roof was replaced. The house was wired, when new , 25years ago , by a reputable company, I assume it was done correctly ,it’s 3 way, maybe a 4 way also .

Stubbie · 12-23-2008, 12:39 PM

Jim

The switched power to the lights comes from one of the 3 way switches. Alot of electricians will just isolate the switch leg and then each fan one at a time continually tripping the breaker until they determine what cable the short or ground fault is in. I'm not a fan of constantly tripping a breaker. I'd rather find it with a continuity check by disconnecting the cables and isolating each cable from the switch to the first fan and then the cables going to each fan isolating each cable from the fans and each other. Then take one end of a cable and check continuity between each wire in the cable including the ground wire. Any continuity between wires will tell you that the fault is in that cable. Take careful note of all connections and do these checks with the power off.

Stubbie · 12-23-2008, 12:57 PM

Jim

I should also state to be on the look out for anything that looks like it maybe loose or shorted in the ceiling fan/light boxes. All the vibration from installing the roof may have caused a wire to come lose and ground out or short to another wire or to a the metal of a box.

jimmysdown · 12-23-2008, 03:13 PM

Quote:

Originally Posted by Stubbie

Jim

I should also state to be on the look out for anything that looks like it maybe loose or shorted in the ceiling fan/light boxes. All the vibration from installing the roof may have caused a wire to come lose and ground out or short to another wire or to a the metal of a box.

This was Golden, thanks, I'll check for loose wires.

If all wires are tight, do you think I could use a continuity tester on each bulb socket(with the breaker off) to test for continuity to locate which fan/light has the short?

Stubbie · 12-23-2008, 03:20 PM

Jim

Not really you have to isolate the wiring otherwise you will get continuity when it is normal to have it and you won't know where the nail is located.

Yoyizit · 12-23-2008, 03:21 PM

Quote:

Originally Posted by jimmysdown

get a brown out, on fan light for 2-3 seconds then the breaker flips.

So it's not a dead short, maybe only several times the breaker's rated current, and maybe it's only a short at 120v, so an ohmmeter might not find it.
Since the roof is not grounded, it might be troublesome to find this one.
I'd find this one by inspection, not electrical testing.

jimmysdown · 12-23-2008, 03:46 PM

[quote
I'd find this one by inspection, not electrical testing.[/quote]

How would I go about this?

Yoyizit · 12-23-2008, 03:58 PM

Quote:

Originally Posted by jimmysdown

[quote
I'd find this one by inspection, not electrical testing.

How would I go about this?[/quote]

Find the nail, undo what the nail did.

A nail just scraping live conductors may have enough contact impedance between the nail and the conductors to give you this kind of heavy overload without being a dead short. This kind of short might disappear with temp. changes from day/night temp. cycling or just stomping around (intermittents are really hard to troubleshoot).

There is a more complex way to do this, without using
http://en.wikipedia.org/wiki/Time_domain_reflectometer

If inspection doesn't work, this other way may.

Stubbie · 12-23-2008, 04:19 PM

Jim

It would be very normal for a nail to cause this brown out and then trip the breaker in 2 or 3 seconds. I'm betting if it is a nail your continuity tester will find it, especially if you use a digital one. If not you can't just allow the breaker to trip as you isolate the wiring section by section reconnecting the tested section as you go.

We are also assuming that the problem is a nail caused by the recent construction and that makes sense but may not be the answer. If everything checks out in the ceiling fan boxes then it likely is a nail through a wire or even a nail through a fan box hard to tell.

If it is a nail through one of the cables you are going to have to replace that cable... so to some degree your going to have to do some type of inspection by removing the ceiling or whatever along the cable route to repair it. Knowing what and where the fault is will minimized the inspection part.
The unknown is how the lights were wired as there are a few ways of doing it. I've attached a diagram that gives the general idea of doing a continuity check. If you want to isolate by tripping out the breaker that's fine. The hardest part of this will be your ability to understand the wiring of the fans without tearing out any ceiling.

Wildie · 12-23-2008, 05:05 PM

In the good old days of plug fuse panels, we used to screw in a light bulb into the fuse socket. The short circuit would light the bulb. Then, it was just a matter of going along opening the chain until the lamp went out. Indicating the section that had the short. This can't be done with a breaker of course, but a pig-tail lamp, wired to the power feed with wire nuts would work quite nicely!

Yoyizit · 12-23-2008, 07:50 PM

Quote:

Originally Posted by Wildie

In the good old days of plug fuse panels, we used to screw in a light bulb into the fuse socket. The short circuit would light the bulb. Then, it was just a matter of going along opening the chain until the lamp went out. Indicating the section that had the short. This can't be done with a breaker of course, but a pig-tail lamp, wired to the power feed with wire nuts would work quite nicely!

And you can use the lamp as a milliohmmeter. If it passes 1A and you read 250mV across the short, it could be a dead short at the end of 50' of #14 Romex.

Two to three seconds could mean 3x to 7x rated breaker current. With a 15A breaker at 120v this would be 2.7 Ω to 1.1 Ω.
Maybe 200' of #14?