Finding%20Short%20in%20BX%20Cable%20(8/18/99)
 

Mike I would like your thought on a problem I have on a job but have not seen discussed in any books. In a riser from main disconnect to apartment unit I have a short that is probably caused by a screw through the wire. Itís a sheet rock wall that has 6/3 BX running through. Is there any way to use a toner wire-tracking device to locate more or less the spot of the short? I was told that in the Category 5 cabling world they have a tracking device that locates shorts between 2 wires to the exact spot so please let me know your opinion

Thanks, Leon Sabel, a.sabel@worldnet.att.net

Response No. 1

I know from working for DOD for 15 years that the technology exists. The Telephone Company uses devices such as this and finds breaks within a certain distance. The application is outside and the accuracy may not be applicable to the length of an interior line (typically less than 100'). Also the cost is at least a few thousand.

As technology improves and reduces in price such a device that identifies the break, or short, and provides an accurate distance to say within 2 feet, will likely come about.

Peter, Apvjohnson@aol.com

Response No. 2

The devices used in the data/comm world do this by measuring the impedance on the wire to the short. They are calibrated for the "per foot" impedance of the wire, and then all that is done is a little math. Haven't seen anything like this in the big wire world, though I sure could have used it a couple of months ago. I imagine that if you did a little research and had a sensitive meter you could accomplish the same thing. Just remember to divide by 2 for the distance, as you are actually measuring a loop.

Bill Ellis, IESNA bellis@candelacontrols.com

Response No. 3

This sounds like a problem for the "Murray or Varley loop" tests as used to locate shorts and/or grounds in telephone-telegraph cables.


These tests involve placing the defective cable into a Wheatstone bridge circuit. If you can not find a reference in terms of "Murray or Varley loop" references, I can fax you some pages from a book that I have. These are referenced in "Principles of Electricity applied to Telephone and Telegraph Work" November 1938. This is a text prepared for "A Training course Text prepared for Employees of the Long Lines Department of ATT. I am quite certain that these tests were described in many textbooks of the 1930-1950 era.

Dale Rummer, P.E. dprummer@falcon.cc.ukans.edu

Response No. 4

I have been able to locate shorts in outlet circuits by using a fairly precise ohmmeter. By reading resistance in the shorted outlets, the smaller the resistance between hot and ground or hot and neutral (whichever is smaller), the closer I am to the shorted outlet. My suggestion would be to take two resistance readings between the shorted wires, one from the top end and one from the bottom end, then a good estimate of the distance to the short may be calculated by using the following formula: Lb=LTX(Rb/(Rb+Rt)) where:

Lb is the distance from the bottom end to the short circuit.

LT is the total length of the cable.

Rb is the resistance read from the bottom end.

Rt is the resistance read from the top end.

The theory says it should work, and as I said I have used a similar approach. I would like to know if it worked out. Please let me know if it did.

Eliud EVillanue@cs.com

Response No. 5

For a wire to just short inside BX suggests that there could be other damage that isn't obvious right now? But assuming your ground is the BX casting, it should be possible to use a signal tracer approach-inject a signal between the H and G at one end and follow it with an AM radio. It should terminate or at least get weaker at the short. You can also buy a tracer from Greenlee, closed circuit tracer 2007/36924 or circuit seeker 2011/00521. I know these would work unless they are metal studs in the wall. Price ranges from $300 to $450 or just rent one.

Another approach to getting close: Measure the resistance between H and G at each end-the ratio will also be the ratio distances. There is also something called a time-domain-reflectometer which sense a pulse down from one end and measures how long it takes to bounce back from the short. Usually these things happen at junction boxes or fixtures or when someone bangs a nail into the wall. Also check the ends going into the boxes. This is where most of the time the wires get cut on the armor flexing. Hopefully this will help you and good luck.

Jim W. JDWueb11@aol.com

Response No. 6

I don't think you can use a telephone tone to locate shorts in a wall or pipe but there is a circuit tracer that is made by amprobe that does and I have used them with great success.
Jim Kimsey JimK94@aol.com

Response No. 7

Find somebody that has a "time domain reflectometer" see page 1222 in catalog of a company called "TESSCO" 1-800-472-7373 or WWW.TESSCO.COM in the internet. Price ranges from $600 to $3,000. It will locate any short or open to the exact foot.

John Hilliard <commsig@greenapple.com

Response No. 8

Although it is not wise but try changing wires.... neutral for hot.....This will help you find which wire the short is in. If the short is between two wires ... hook a light bulb between the return wire and the neutral buss. Measure the resistance at the light bulb. Then calculate the amount of resistance/volts per foot and locate the short. You have to know though how you ran the wire. This is the long and mathematical way but it works. I had to do is a couple of times although I had to give the measurements to an engineer to figure out

John Landry jlandry@exalted.com

Response No. 9

Yes, they have a "toner" that can detect a broken wire. The signal will stop after you pass that point. But, more than likely the screw is shorted to ground so the signal will be all over the place. The best solution is to remove the riser pipe and reinstall new wire.

Keli Bos <kelibos@palacenet.net

Response No. 10

Communications cable testers can pinpoint the distance to a bolted short, but the communication cable is more uniform to provide the high-speed connection. And your short is probably not a solid fault, so wire length meters based on resistance will give a false reading, depending on the resistance of the short.

The radio frequency type traces should provide the answer. You will have to determine which conductors are shorted: phase-to-phase; phase-to-neutral; phase-to-ground; neutral-to-ground. Then apply the signal to the two conductors that are shorted together; and trace, and hope. Some leakage signal will be passed beyond the short due to coupling to the undamaged conductors, but there should be a large reduction in signal above and below the short.

I have used the 'Phaser Tracer" that is now sold by Greenlee. There are several choices, you may need to be sure you can work off external power and inject a signal into a shorted pair. There is equipment made that the Phone Company uses that allows a man to drive along a (rural) road and listen till the tone drops off, stop, and search to locate exactly.

Lynn Adams ladams@powerh.net

Response No. 11

Thatís what they say, Dog & hound, the short finder, etc. I have never used one. I would give it a try or use your meter and time.

Fred Percival save@io.com

Response No. 12

A tone tracer wouldn't be of much help, unless you want to find another cable pair.

A TDR (time domain reflectometer) can locate wiring shorts and splices (discontinuities). It is calibrated for the type of wire. It sends an electrical pulse down the wire and looks for the reflection from a discontinuity. Knowing the propagation time (what you calibrated) the distance is then displayed. Some allow an o-scope to be used to view the pulse.

We have one in our shop we use to measure lengths of wire on spools. The cable TV companies use them to determine how many drops a customer has on their connection. The TDR requires access to both ends of the wire, at least ours does.

Tom Baker tjbaker@ix.netcom.com

Response No. 13

I have used an instrument that my shop has to measure wire on spools to located faults in cables. I am not sure of the brand, but it works quite well. To locate the fault, first isolate the two shorted wires, then attach the wire-measuring machine and enter the wire size. The machine will give you an estimated length of the cable, divide that length in half then measure along the approximate path of the wire. (This can be aided by using a simple toner). To get more accurate results duplicate this starting from the other end of the cable; the short should be in the center between the marks.

Mike White Mwhite2690@aol.com

Response No. 14

Perhaps this will help. Progressive Electronics Inc., www.progressive-inc.com, makes an open and short circuit locator, 77AT Transmitter & 200AT Receiver, page 22 of their recent catalog. Have not personally used it but I do know they are a reliable company.

Michael Draggett l1electrical@yahoo.com

Final Results

First let me thank you for the help, I did not locate the short because none of the answers provided a real solution to the problem. Itís funny that with all the technology, there is no simple solution to find a short between one hot and the metal jacket of BX cable installed within metal stud walls. With a toner you get a tone all over the building. The only thing that might work is an ohmmeter but since the following happened I could not test it any more. I'm definitely going to experiment some where else to see if it works. Since i couldn't find the short I decided to test my luck and cut the wire 20ft from the panel box and work my way down until I found the short. I was lucky it was in the last 20ft of wire, so I made an accessible junction box and ran a new wire from there. When i will do the experiment's I will let you know the result.

Note: The company Microtest that makes the Penta scanner said that by the end of the month they should have a new product out called Omniscanners that will help locate shorts in such situations. Check out www.microtest.com/.

LEON SABEL A.SABEL@WORLDNET.ATT.NET

 

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