Hi, i'm new here and want to say what a great site.

I'm looking for ideas to solve for a short in a run of mc cable. It's between the hot, neutral, and sheath on a 50' run that goes up and down and around a stairwell to an outside light and there's no way to fish a new wire. It's finished drywall and i suspect the culprit is a drywall screw. I isolated both ends from the boxes and fittings and energized the sheath and hot wire then attempted to find the offending screw with my 'hot stick' [non-contact tester]. the tester, which is not overly sensitive at other times, glows along the entire run one foot to either side.
Any ideas short of buying some expensive new tool? Thanks in advance, zero

You're sol. (simply out of luck). Either abandon, and run on the surface, or start tearing up the wall. Use nail plates the next time.

John, thanks for the reply. I've always applied nailplates liberally as well as using antishort bushings. Either i missed a nailplate and that's the one that bit me or it could be a mfg. defect.

I use Greenlee circuit tracer 2011 to find shorts. It can pin point short after a little hassle.

[This message has been edited by electricman786 (edited 10-27-2004).]

"energized the sheath and hot wire then attempted to find the offending screw with my 'hot stick'" not the safest idea!"glows along the entire run one foot to either side." 'cause you might be heating up the whole wall!


Do the same with a telephone inductive toner.

Or. rent a Time Domain Reflectometer. Tells you the distance in feet to the short in +/- 2-5%. Or, ohm it to the thousands or mili' and do the math for ohms per foot. Then do the same with a toner. Pull out offending screw, make someone else pay for your time and replacement!

Might be cheaper to re-run if not found easy.... Its a bummer though!

No worries, we've all been there. I work with some framers who shoot 16's for shear-wall, because they're too lazy to change the nails in the gun. Or say, "Oh we had to move your wire" and use a claw hammer to do it. Plumbers who solder against cables and conduit. etc etc

Finding a permanent short in a cable is as easy as pie. I'll write up something tonight ("pictures paint a thousand words" and all that). Stay tuned.

Marc.

Done!
http://www.marcspages.co.uk/tech/?5107

God bless,
M.

(p.s. try not shoot me down in flames this time!)

Leaving for Scotland in a matter of hours (teaching on lightning, bonding, & earthing, if you must know) - wife informs all appliances in kitchen are 'dead' - reset breaker thinking it was freak with everything wanting power at same time - short time later same scenario - have intermittent short on plug circuit - same circuit feeds boiler - d*mn!

Pull cover off consumer unit - find leakage on plug curcuit - no time to trace cables, must find short quickly so as to remove faulty piece - clock is ticking & plane is calling.

The above link is updated to show how I found faulty piece with enough time to make my plane and have lunch with the oh so much better looking half.

M.

No one here would do that!

I like the idea (not of shooting you down), but it seems desperate to remove the battery and head light from my truck. Will keep it in mind, though. Nice Tip.

TWH,

Although hauling the battery and headlamp out of the truck may appear a desperate measure, it is a far safer means than slapping a hefty 1.2V rechargeable cell across the faulty cable (as I did in my case, using only a lengthy piece of wire as a resistor to replace the lamp) because should a dead short be right there at the test points, the battery could explode - then I would need to change my username to "UnwantedHelp".

This way, I can be pretty certain the current is reasonably limited to about 5-10A (depending on the size of the headlamp. However, a 'lantern' battery and suitable lamp is just as workable - you don't have to strip the truck (hmm, I can suddenly see a number of folk modifying their lantern style torches with wires strapped across the switch for the test probes!).

M.

Why not just use a test lamp wired in series with the breaker?

Dereck,

Because:

1) the test lamp does not draw enough current to cause a large enough voltage drop on the cable that allows reliable detection

2) the short my well be to Earth, and the cable could become "hot" should the fault be intermittent

3) any induction on the cable from others in the vicinity could cause the fault to appear in another area (by using DC, the meters become insensitive to any AC influence on the cable)

4) the high current source must be fully isolated so as to not influence the readings - the mains input is not isolated (one leg, at least, is taken to Earth - well it should be!).

M.

They already have something like that...

www.mtadistributors.com/products/IN2-CT

I knew I'd seen that before!

as long as the globe drags about 5 to 10 amps, then that is perfect (on 3V, that would be about 15 to 30W - have a feeling torches are a little lower than that - lantern style with screw type batteries have huge globes hence the suggestion).

M.