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, firstname.lastname@example.org
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.
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 email@example.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
Dale Rummer, P.E. firstname.lastname@example.org
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.
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 <email@example.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 firstname.lastname@example.org
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 <email@example.com
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 firstname.lastname@example.org
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 email@example.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 firstname.lastname@example.org
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
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 email@example.com
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