I am wondering if there are any tricks electricians use before
I start ripping walls to find a short. (Sigh).
I have isolated the short between the 2nd and 3rd light fixture
in a hallway. The 2nd fixture is a recessed lighting box.
The 3rd fixture mounts on a simple round wall box. There
is no visible short in the accessible parts of the wire at the
two ends. All three wires (hot, neutral, ground) are shorted
This is relatively new construction, finished about 1.5 years
ago. The wire is standard 15A or 20A romex.
I tried measuring the resistance of the short from the two
ends to see if I could tell where it is. My voltmeters measures
down to 0.1 ohm which is a lot higher than a short length of
romex. So I rigged a circuit with a 100W lightbulb to push
about 1A through the short, and tried to measure the voltage
drop from each side. This is tricky because a lot of the
resistance is in the contacts and it varies a lot depending
on how good they are. I was using alligator clips. The
approximate readings are 220mV from the 3rd fixture
and 200mV from the 2nd fixture.
Any tips/suggestions before I start breaking the wall
near the 2nd fixture?
The easy way is with a cable fault finder. There is currently one on
eBay for about $100. Auction# 250019824403 I got mine for about $70.
These send a pulse down the wire and calculate how long it takes to
come back which is indicated on a digital read out as the distance to
the short or open circuit. They are extremely simple to use and are
also handy for measuring how much cable is on a reel. They only work
with 2 or more conductors and will not work with a single wire.
All three? Yikes.
Telephone servicemen (and presumably some computer network geeks)
have special instrumentation that allows them to "echo" check
shorts and figure out how far away they are. Electricians
wouldn't. Unless you can know of a tame phone serviceman, you're
All three wires is a pretty catastrophic cable failure.
Eg: getting "scissored" between two structural elements,
really badly crushed under a box clamp, or punctured by
With the segment isolated:
1) loosen the box clamps and wiggle the cable. See if
the resistance becomes intermittent.
2) Try to isolate the ground from the boxes. See if there's still
continuity from ground to the box (clamp crush or insulation
3) Using some sort of metal detector (eg: a compass or magnetic
stud finder), find the fasteners in the ceiling (eg: drywall
screws), and see if there's continuity from the wire end to the
fastener (you'll need a very sharp probe). If you find any,
you've found the short.
4) Commence chopping ceiling.
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
run new cable between areas.
the short is likely at a box but might be a screw thru romex or other
best to just replace that cable run.
yeah there will be colateral damage and perhaps some holes.
tink of closets etc on other side of wall, if its drywall its easily
Thanks to all for the quick replies.
The cable fault finder is a good idea. I know of at least another
place where I can use it (a short in the wire that turns on the
central vacuum when the hose is inserted in a plug). The TS100
accuracy could be better (2 to 5 feet, depending on cable length)
but it's still useful I guess.
I wasn't able to unclamp the wire at either end but I will try
harder. Not much room to work with in the recessed box.
I'd try pulling a new wire too but if it's not at the boxes whatever
pierced/crushed it may keep the wire from being pulled. Another thing to
keep in mind, it might not be just one place. My mothers storm door became
electrified after having a door lock installed. When I pulled off the
paneling to get at the wiring found several of the nails holding the
paneling had also nicked the wires . Each job is going to be different.
Good luck to the OP.
wire may well be stapled to insude of wall framing, in which case you
cant pull it anyway.....
fixing stuff right sadly sometimes involves colaterral damage.........
fix the problem then fix the wall.........
You don't need to follow the path of the old wire. Just cut off the
power to the existing wires and pretend they don't exist any more. Run a
new wire and it may well require running it in a totally different area.
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