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


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According to JohnF :
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

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.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

Chris Lewis wrote:
[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.]

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

According to HorneTD :
Chris Lewis wrote:
[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.]

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.

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).

If you really want to
test the rough wiring then rent a MegOhmMeter.

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...]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

Chris Lewis wrote:
According to HorneTD :

Chris Lewis wrote:

[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.]



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.


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).


If you really want to
test the rough wiring then rent a MegOhmMeter.


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...]

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


"HorneTD" wrote in message
k.net...
Chris Lewis wrote:
According to HorneTD :

Chris Lewis wrote:

[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.]



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.


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).


If you really want to
test the rough wiring then rent a MegOhmMeter.


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...]

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

"JohnF" wrote in message
...
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


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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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 KB2Xc.103047$Lj.49961@fed1read03):


"JohnF" wrote in message
...
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


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.710 / Virus Database: 466 - Release Date: 06/23/2004

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.



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.