Failure Detection In A Long Cable.

Its a problem. marking it.

1]you can use a precision resistor meter or electric bridge to get resistance and calculate its distance.

2]use stand wave method.

3] cable measure instrument which use burst sound method.

My instructor taught us a technique for finding internal cable cuts (the insulation is still intact but the core has become severed). He used a length of wire and a pin.

He connected one probe of his resistance meter to one end of the cable and to the other probe connected to a length of wire which had a pin at the other end. He began stabbing the cable (without power of course) and checking the reading on the meter. Eventually he narrowed down the location to within a couple of centimeters.

Now, 100meters is a long length so this idea might not be practical but if you have another length of intact cable handy, you can use this method.

Another method would be to flex the cable. The break will show as a soft spot.

I've heard that time-domain-reflectometers can determine the distance to the break but I've never used one.

"I've heard that time-domain-reflectometers can determine the distance to the break but I've never used one."

I used one in the 1980's, and it worked great. The models out there now should be cheaper and better.

Many modest price multimeters have a capacitance measuring function, if only one conductor is brocken measure the capacitacne of that condutor to the remaining condutors and the distance to the break will be in proportion.

If all condutors are brocken simply measure the capacitane between any pair from each end.

I've seen that technique used on coax cable. The distribution of capacitance in a shielded cable makes this technique very applicable. I'm not sure how accurate it would be for multiconductor cables (i.e., unshielded) though. I would think that it might not be very accurate. Perhaps accurate enough, I don't know.

Syhprum how will u determine the distance were the break is? Using the capacitance to give the distance.

You can find the distance by measuring capacitance this way:

http://cr4.globalspec.com/comment/127764/Re-Innovative-Applications-for-Multimeter

Simple cables have a given capacitance of xx picofarad per foot. I am thinking here of something like coax or twisted pair. If you can come up with the capacitance per 100 feet (for example) from the manufacturer, you can then calculate distance to the break.

If the cable has say... Oh!! this gets easier... I just dug out a catalog for Alpha cable, and it gives its capacitance per foot (or meter). For a particular RG58 coax, the capacitance is 28.8 pf per foot. If you measure a capacitance of 288 pf, your cable break is 10 feet from where you are measuring. I also noted that in their umpteen pair shielded cables, they also give capacitance measurements.

Now I won't say how accurate this is, but it will give you an idea as to where to dig the hole.

Bill

The best solution to your problem will depend on the particular circumstance:

• Can you access the entire length of cable? In other words, is it in directly buried in earth, underground in conduit, above ground in conduit or overhead in open air?
• What are the failure symptoms? Is the conductor shorted to ground, shorted to another conductor in the same cable or open (separated) but insulated from ground and other conductors?

If the conductor is open and the cable is accessible, a simple non-contact voltage detector will work. Disconnect both ends of the cable, apply AC voltage (as little as 110V) to one end of the open conductor, and use the detector to find where the voltage stops.

If the cable is in conduit, attach a rope to one end, and pull the cable out the other end of the conduit. You now have access to the entire cable for visual inspection and/or voltage detection as above. Once repairs have been made, use the rope to pull the cable back into the conduit.

If the cable is direct buried you have a few options.

• Use any of the capacitance or reflective detectors mentioned above.
• Use a "thumper". (See drawing).

The thumper uses a DC source and adjustable spark gap to build voltage on the bad conductor until it arcs at the point of fault. Start with the spark gap closed (or nearly closed). As the DC source voltage increases, sparks will occur across the gap. The sparking with discharge the voltage source, seen as a downward spike of the voltage. Gradually open the spark gap until sparking stops. Check the source output. If it is still experiencing spikes, the sparking is now occurring at the failure point of the cable. If the voltage is stable, slowly raise the voltage until you see the voltage spike downward regularly. If the sparks are occurring at the spark gap again, open the gap further. It may be necessary to alternate between raising the voltage and widening the spark gap until the arcing transfers to the failure point of the conductor.

The arc creates a noise in the ground which can be heard by the human ear. If the cable is deeper than 1m, you may need a microphone to amplify the sound to detectable levels.

The DC source must be capable of exceeding the breakdown voltage of the cable, but must not have significant current capacity. The idea is for the arc to knock down the voltage, which then must rebuild on the capacitor before arcing again through the fault. The capacitor creates a time delay to make the sound a recognizable "thump" instead of a continuous arc. Since I usually deal with 15KV and higher cables, we use a DC Hi-Pot as a source. We have used this system for many years to locate faults in direct buried cables.

WARNING: THE CAPACITOR AND DC SOURCE WILL PRODUCE HAZARDOUS VOLTAGES. SAFETY PRECAUTIONS FOR ENERGIZED CIRCUITS MUST BE OBSERVED.

OK, your thoughts are welcome. I have an underground 'Radio Fence' for my dog. It is a single antenna wire, buried about an inch deep, around the perimeter of our 1.3 acre lot. There are several cacti and bamboo plants that are encroaching on the buried wire, severing it. To find the break, I typically have to dig to find the wire, then run a jumper for 100' or so and dig to find the wire again, then jumper between the two points. If the system works, then I know that the break is between the two spots I've dug up. It has sometimes taken half a day to jumper across breaks and I never actually found the last one. Any thoughts??

I would try a tone generator & probe amplifier like phone company techs use to locate a specific pair in a cable bundle (in the trade they're called a "buzz box" and "banana probe"). Ideal industries makes a nice compact unit you can get for under $50. Connect the tone generator to an accessible section of your wire, using the provided RJ11 to alligator clip adapter. You only need to connect one of the clips to the wire. Note: If the antenna wire is a closed loop, you must disconnect one end so the signal is only on one side of the break. Turn on the tone generator and test the probe near the connection to verify the system in functioning.

Since you know the route of the antenna, stick the sensing end of the probe amplifier into the ground directly above the wire and depress the test button. The probe will beep, indicating continuity of the antenna to that point. Continue testing every 20 feet or so along the path of the antenna. As you get farther from the tone generator, you may have to increase the volume on the probe. When you no longer get a signal. you have passed the open point. Check between your current location and last good signal location in shorter increments until you find the point at which the signal stops. That's your break.

With a little practice, you should be able to locate the break in under 30 minutes. I have used these testers on 22 AWG wire 3.5 miles in length, so it should work on your yard. If you don't want to spend the money until you know it works, you might find one at a tool rental shop for a try-out. Just make sure you put fresh batteries in both the tone generator and the amplifier.

Replying to Comment by steve45: (Use Copy & Paste or drag text to quote the original text.)

OK, your thoughts are welcome. I have an underground 'Radio Fence' for my dog. It is a single antenna wire, buried about an inch deep, around the perimeter of our 1.3 acre lot. There are several cacti and bamboo plants that are encroaching on the buried wire, severing it. To find the break, I typically have to dig to find the wire, then run a jumper for 100' or so and dig to find the wire again, then jumper between the two points. If the system works, then I know that the break is between the two spots I've dug up. It has sometimes taken half a day to jumper across breaks and I never actually found the last one. Any thoughts??

Your Message:

A ring,tip signal thrower as used for checking phone lines will pick up the generated noise from the buried wire on a dry day. (Price $30), provided the wire is near the surface,remove the loop from the dog signal transmitter, and Feed the phone trace signal into one or other side of the loop, preferably on the side you think the break is nearest , following the wire and listen to the signal with ear phones. when you loose the signal start digging.

This will only work well for a single buried wire.

GF

In the winter, The electric utility looks for the place where the snow is melted to find the failure.

Not knowing if your cable is in conduit, is buried in ground, is underwater, ...

1. Remove power from the cable, Disconnect the submersible pump from the cable. Place the end of the cable so it is safe and not shorted against anything.

2. Take resistance readings to ground from the phases. If any phase is shorted or has a reduced resistance reading identify this. The operating voltage plus 1000 is a typical "operating" dielectric test voltage.

3. Now short two phases and do a resistance comparision reading, then short the next two phases, and then the third pair, to verify continuity.

4. If the cable is not in a conduit. Verify the cable is disconnected from the submersible pump and the ends are safe, Energize the cable, walk above the cable with an AM radio, if there is arcing there will be radio interference, decrease the radio volume and tip the antenna so the end (least sensitive point) points at the defect, use care around the cable end connectors.

5. If the cable is in conduit use a dry wooden dowel or insulated screwdriver to listen for arcing.

6. If the cable is underwater, be very careful as the deadly distance can be miles!

Before getting into expensive test gear, try witching for the fault. I'm not kidding! I did say WITCHING! ) I once worked as the underground cable splicer for a cable company. i also worked as the underground fault locator for a power utility company at one time. I am familiar with TDR, resistance bridges, and various types of radio locator machines.

An old power line splicer taught me how to "witch" for broken cables and faulted (partially shorted to earth) cables. Seemed to wrk equally wel lon shielde coxaial cable dua lcore phone wire and single core power wires.

Use two pieces of coat hanger wire or any other wire cut to about the lenght of your forearm (cubit) from elbow to fingertip.

Make a right angle bend at the end on each wire. Hold the two wires loosely in each hand. Tuck your elbows into your sides to hold the arm steady. The fingers of the hand are circled loosely so the short bent piece is straight up and down. The long part slopes slightly down and forward so it point straight ahead. As you walk across the line of the buried cable, the two wires wil pull together and cross.

Once you know where the wire is trenched in; follow the run of the wire. The wires will point forward until you cross the fault point. The two wires in your hands will swing together and cross each other as you pass above the fault.

If you feel foolish doing this, imagine how I felt the first time after my $10,000 worth of high technology had failed to find a definite indication of the buried fault. But sure enough, the old timer proved his system worked, and when I dug down there was the cut wire. Over a period of six years after that, I repeatedly tried it and found it to be 100% effective. Sometimes more so, than my fancy equipment which would give vague and indeterminate fault indications in very wet or in frozen ground.

Good luck

If what you say is true, you're wasting your talents finding broken cables.

Around the world there are many "Skeptic" organisations that have a standing challenge - just demonstrate your ability (under randomised, controlled conditions) and pick up a substantial prize.

In Australia it's a cool million dollars and the local skeptics made a video about one of their trials. They got (expert and highly successful) dowsers (I think 20 off) to search for gold, water, moving water and oil. There were 9 empty pipes with 1 filled pipe and the pipes were changed around at random. After the tests they asked the dowsers "Was it fair and what will be your success rate". All dowsers replied "Fair" and predicted excellent results. But, when the results were tabulated by an independent witness, all dowsers did no better than chance. Perhaps you'll do better. ffej

My technique works for energized wires, not necessarily for gold, water, or moving water and oil. Quite a difference. Energized wires involve a magnetic field around the wire. Nor did I claim 100% success. Only that it works the majority of times and was often more sucessful than my sophisticated equipment. Your milage may vary. But for a quickie test which cost practically nothing except a few minutes od your time, its a nice back-up to the expensive test equipment.

I compared results against my other equipment often enough to convince myself (a sceptic) that there was something to it. I do not claim to have a valid explanation. I can only report a much higher than 50 % sucess rate. Can you flip a coin and get it to land 100% the same way each time? If so, how?

I sold tow of this instrument years ago. either made in Britain, or in China ourself.

they are widly using in electric plant and communication. but I forget its type. its not cheaper. ours is cheaper than britain or usa.

you can get it on market, no problem. if you need, I can find its type from my old recorder.

As a lineman working down on the tube railway in London, I was shown a technique for finding a faulty cable when the cable is buried (in a bundle of other cables)which line both sides of the tunnel. we would connect a high DC source to the faulty pair and listen and look for the "flash and the bang", which could occur hundreds of metres distant.

The DC source was the 600 traction voltage .which was not always available.

A gentler controlled method is recommended, a spark producing device fits the bill.

My buried dog fence wire broke recently, some where in it.s 1000ft length. I charged up a hefty bundle of motor start capacitors in parallel and fed the charge into the circuit from both ends of the loop, This procedure, reconnected the underground break by welding. "What luck".

This was ok for my dog fence which is only an antenna.

GF

Kilowatt0 is correct. A TDR (or time domain reflectometer) is easy to use and very accurate. It sends a pulse down the cable and receives a reflected pulse from the break. It will show on the instrument's crt. Tektronix has made them for years and you can rent one pretty cheaply from a test equipment rental co. or buy one if you run into this alot. Good luck!

Are you having short circuit or open circuit problem?There are two methods are available one is Murray Loop method and another one is Varley Loop method to find the exact location of fault.Depends on type of fault i can give easy solution for you.

It is short circuit