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Finding the location of a short in EMT

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Sierra Me

Member
Location
Nor Cal
Occupation
Multi licenced contractor
Hi all,

Have an issue with a pull of 225' 4/0 6 cables. One of my cables did not make it. What is the best tool/procedure to locate the short within the EMT? Any help on this would be greatly appreciated.
 
Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
How would that help you? Are you planning on cutting away a section of pipe and patching it after some repair to the cable?

Why not just pull in a new cable with the old? I recommend lots of Yellow 77.

Or I suppose if you are planning on just wrapping a lot of tape around the break in the insulation, you can tie a rope on and pull it out then pull it back in with the rope.
 
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Sierra Me

Member
Location
Nor Cal
Occupation
Multi licenced contractor
Unfortunately there is no give in the 4/0. This is not a straight line. So yes, the plan is to find, cut, place a large box and re-connect. I need to find the proper detector in order to find the place of the short.
 
Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
It sounds like you just pulled these in. They should not be glued down by old lube. I suspect that with a tugger that cable will come out. Hardly any pipe run is straight and cables get replaced all the time. I'll leave it to others to talk about the equipment for finding the short. I know it exists, but I've never used it.
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Semi-Retired Electrical Engineer
It exists. It is called "time domain reflectometry." It basically sends a signal down the line and looks for reflections that would indicate an imperfection in the conductor. I don't know how to go about finding the equipment or the personnel capable of operating it.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I would be inclined to make you remove it all if you have damaged one that badly.

Do you have pull boxes? You should be able to identify the good conductors with a tone generator. Isolate the bad one at each pull point, cut and test. Repeat until you find the bad section. Pull all new in that section if you can't get the one out. Add pull boxes as needed.

Use larger conduits next time.
 
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oldsparky52

Senior Member
Location
Wilmington, NC USA
Sierra Me said:
Unfortunately there is no give in the 4/0. This is not a straight line. So yes, the plan is to find, cut, place a large box and re-connect. I need to find the proper detector in order to find the place of the short.
Click to expand...
How can there be no give? Didn't you just pull them in? A tugger can do amazing things (including pulling out a tree stump).
 
480sparky

480sparky

Senior Member
Location
Iowegia
If the conduit can be isolated from any grounding, it could be done with a fault locator. But it would probably be easier just to repull the wiring.
 
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oldsparky52

Senior Member
Location
Wilmington, NC USA
Just for grins, can you talk me through how you plan to do this? I mean I figure you are cutting the conduit and wire with an appropriate saw, then ... what, go to the closest fitting (coupling?), remove the conduit and cut to the correct length? If that's it, how are you going to get the box in place with all those conductors? Then, how are you splicing the conductors?

I guess it will cost you ... what ... $6 - $8 grand to pull out the old and pull in new? In the long run, that might be a more cost effective solution.

Everyone can have a nicked cable, eyebrows probably raise when junction boxes start being installed to keep from replacing conductors. Just sayin.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
480sparky said:
If the conduit can be isolated from any grounding, it could be done with a fault locator. But it would probably be easier just to repull the wiring.
Click to expand...

I can't find faults in a PVC conduit if there is a bare cu pulled in it. IDK how the heck it could be done in EMT.

Get an old welder, one lead to the emt and one to the damaged conductor, turn it up. Find hole. It should be near the smoke :rolleyes:
 
growler

growler

Senior Member
Location
Atlanta,GA
ptonsparky said:
I would be inclined to make you remove it all if you have damaged one that badly.
Use larger conduits next time.
Click to expand...

I think that to remove the cable is the easier choice anyway. I have pulled a lot bigger than 4/0 out and reused it if it was not damaged.

For myself I would like to see what kind of damage was done to the cable ( it may all be bad). There are some people that don't know how to run conduit and others that don't know how to pull wire.
 
E

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
ptonsparky said:
I can't find faults in a PVC conduit if there is a bare cu pulled in it. IDK how the heck it could be done in EMT.

Get an old welder, one lead to the emt and one to the damaged conductor, turn it up. Find hole. It should be near the smoke :rolleyes:
Click to expand...
I was thinking the same thing about the welder that is, probably would damage the other wire:):):)
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
It's been a long time since I was an industrial electrician, but the bad cable might've been jammed against the conduit by the other cables at a bend. So I would try pulling only the bad cable backwards from the way it was originally pulled just to see if you can get it loose. If so then maybe you can pull them all out.
 
H

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Buy this, put a battery on it, one lead to the emt, the other lead to the shorted 4/0, walk the length of emt with the receiver. The signal will drop off where the short is. Done it many times, you only need a 9 volt transistor battery in series with the tracer and short.

 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
ptonsparky said:
I have a TDR but haven't used it for years. I think the accuracy is about +-10%. Not sure how well it would work on 4/0. Equipment has got to be better now than the 30 year old unit I have.
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I have a VON TDR and a VON thumper. Basically the same but one sends a lot of joules at one time to arc across to ground creating a sound like a .22 rifle.

as long as the VOP (velocity of propagation) is set correctly it would get you to within 5‘ on a true break or end of the wire. As long as the wire isn’t nicked somewhere along the pull You should be OK.

now as far as the thumper feature, I really doubt you would want the cable insulation subjected to 800-900 joules Impulse.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
At first, I thought the OP was saying that one conductor was too short for the pull.

In my opinion, the wire should be pulled back from where it started, the bad wire repaired or replaced (better) and re-pulled.

It would be prudent to pull a mouse through and then inspect it carefully to make sure the new wire doesn't get damaged.
 
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