Hi all........what tests does one perform to determine if a short is someplace in a cable run in the wall and not in the outlet box. There is no power as yet in the cable and before the walls are sheetrocked I would like to insure that no staples or crushing or whatever has occurred. Thanks for the as usual good info......John
According to JohnF :
Most electricians don't bother "testing" circuits. Power the thing up with nothing connected and see if the breaker trips. [Nod to Tom: Given that it's a branch circuit, the available short-circuit current should be well within the interrupt-rating of the breaker.]
This is assuming that the circuit is complete (all devices installed):
If you really want to test, and you have a volt-ohm-meter of some sort, disconnect the hot from the breaker, and test for low resistance between the hot wire and a good ground. The resistance should be very high.
If you really want to test, and you have a fuse panel, find a low wattage
120V bulb (say 15W) with a standard screw base. Screw it into the fuse panel in _place_ of the fuse. If the lamp lights, you have a short.If the circuit is incomplete you really want to test before installing switches and outlets, test for low resistance between hot (usually black) and ground (bare), and hot and neutral (usually white) on each cable segment. At least on the cable segment attached to the panel, the resistance _should_ be low between ground and neutral, so don't worry about ground-neutral resistance.
You can test using an ohm meter. If the conductors are open between all groups then there are no shorts. (H-G,H-N,N-G). This will require all of the connections to be made in the outlets and switches and not in the panel.
I have used a megger to see if the insulation is ok. Really need some experence with the machine to know what it is telling you. Would not recommend that you megger the wiring, pay someone to do it for you.
On Wed, 25 Aug 2004 08:53:15 -0700, SQLit wrote (in article ):
I sometimes use a "bell box", which consists of a 12V lantern battery with one terminal connected to a dc buzzer and leads with alligator clips connected to the other terminal and other buzzer connection. One of the leads is about 50' long. It's very useful for finding low-impedance ("bolted") shorts and for identifying multiple conductors in the same conduit.
should be well within the interrupt-rating of the breaker.]
That would depend on the location of the fault in the branch circuit would it not. Given that this is a new installation there is a very low likelihood of an inadequate withstand rating. If you really want to test the rough wiring then rent a MegOhmMeter. A MegOhmMeter applies a test voltage to the wiring that will cause a current flow in there is a fault. The Meter is calibrated to show the resistance to current flow in MegOhms. In brand new cabling the meter can be set to 500 volts but a three hundred volt setting should be high enough for your purposes. The reason that such testing is done with a MegOhmMeter is that the current available from such a test instrument is not large enough to be destructive. In commercial and industrial wiring projects a requirement for insulation resistance testing is often written into the contract.
-- Tom Horne
current should be well within the
All true, but the likelyhood of exceeding withstand ratings is sufficiently low that I wouldn't worry about it. For example, making a reasonable presumption that any dead shorts would be at least 10 feet of wire away from the main, the down and back resistance of 20' of 12ga would limit the available fault current to no more than 3750A, and probably considerably less due to resistance in the main feed. Which is below even the worst breakers (I think).
By far the best - even detects crimped/damaged insulation that isn't quite yet a short. Question for you: do you test new circuits with one? Do residential electricians commonly do it? Should they? Does "professional practise" say they should?
It's worth remembering that electrical codes theoretically bury the wiring deep enough that nailing/screwing wall covers shouldn't reach it, and damaging a cable with a staple/strap should be visually obvious and remedied before attempting to power up.
The only time I've had dead faults on new circuits have been:
1) A carpenter dead-ended a complex circuit by wire-nutting the hot and neutral together. Shoulda checked first that he had done what I instructed (capped off the wires separately). A bit of a puzzle until we finally found it. No damage... 2) a defective piece of romex straight off the roll - it was laid in a raceway and _couldn't_ have been damaged during installation. 3) I trusted my grandfather to follow wiring instructions... ;-) [long time ago, on a fuse panel, _way_ out in the boonies on a Sunday - no fuses within 4 hours drive.... Developed the lightbulb trick just before we ran out of fuses...]
should be well within the
No I usually don't use a megger on new residential work. I test it clear with a meter prior to closing any brakers though. Megging is comonly done on large feeders with large available fault currents. It is rarely done on residential branch circuits.
-- Tom H
Wire shorts? They sound uncomfortable... ;)
What is the 'lightbulb' trick?
Dave
short-circuit current should be well within the
Probably more bearable than Barbed wire underwear.
:-Þ
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MUADIB®
MUADIB® wrote in news: snipped-for-privacy@4ax.com:
Probably functions well as a chastity belt.
According to Dave :
Simple way to test fuse circuits for shorts:
Find a small wattage lightbulb, screw it in place of the fuse. If it lights, you have a short. If it doesn't light, you don't.
Make sure there's nothing else turned on the circuit.
I tried that once, but I have breakers. Where do you screw the bulb into a breaker?
According to :
Obviously, you can't.
If you have nerves of steel, sufficient foolishness, and a low wattage
120V lightbulb in a lamp holder in a pigtail, you can accomplish the same thing by disconnecting the circuit from the breaker, turning on the breaker, and bridging the breaker screw to the hanging circuit wire with the pigtail lamp.But that would be foolish.
Slightly less foolish would be to disconnect the circuit, run one pigtail wire under the breaker screw, and the other pigtail wirenutted to the circuit wire, _then_ turn on the breaker. If the lamp lights, you have a short.
I don't bother doing that. It's just very convenient with fuses, and fuses only work once. Breaker don't work just once. Usually.
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