Underground Cable Fault Location

As a generic statement TDR can be much more powerful and much more useful than your paragraph states. But, I don't know anything about TDR as commercially available for cable fault detection. Your paragraph might be right for available equipment.

I have successfully used a home-built TDR with an oscilloscope and calculator to locate a fault in a signal cable that was over 1000 feet long- the fault was about two-thirds of the way to the end of the cable run, and the calculator was needed because I had to guess at the signal velocity in the cable. A TDR gives you a point where the impedance changes, and appears to be somewhat insensitive to any resistance at the break. If the line is open, you get one polarization of the return pulse; if it is shorted to ground, you get the opposite polarization. Commercial TDR's are much easier and simpler to use than my home-built version, but I think that is the best solution for your application.

Everything is relative: 10K is very high impedance compared to the cable: TDR will work just fine.

I agree with the general feeling.

TDR would be the way I would go, have used very effectively many times to trace and locate cable faults over distances up to 2 miles (obviously depends on the TDR used).

That said, I am assuming you know the cable route of the line, without that fairly simple bit of information, the distance from measured point to fault is accademic as you'd only get an area to search in, rather than a location.

Mike

TDR is the way to go. They are designed for LOW impedances, usually 75 ohms (for TV cable), or 50 ohms (for RF coax). Their accuracy of measurement for non-characteristic impedances (reflections) is pretty poor when you get above 1 Kohm, but it is good at locating the fault(s), and can usually discern a short (such as water damage), or an open (like a shovel cut it).

Here is a link to one brand:

http://www.mohr-engineering.com/ct100-metallic-tdr-cable-tester.php

Disconnect all downstream devices.

Apply 120 vac to the broken wire, hot side of the ac to the open side.Do this in late evening, and scan the area early the next morning, before dew dries, with an infrared scanner.The hot spot will show you where the problem is.If the leakage to ground is severe enough, there will be no dew over the area, or it will dry first, and you will not even need the scanner.With very sandy soil it may be nescessary to soak the area the night before to enhance conductivity before applying this method.

Good luck.

Someone has marked this off topic: I'm intrigued to know why. I can think of several reasons why it may not be a good idea, but, I can think of counter arguments to most of them.

The huge advantage it has over TDR is that you don't need to know the routing of the wiring.

Thinking about it there may be another slightly more direct approach:

Apply 24V DC between the "live" wire and a ground stake. Now probe around with a long prod on a DVM to find the point where the "ground" has the highest voltage. Obviously you need a long wire connecting the negative terminal on the DVM to the ground stake.

I have also used the "Breakdown Set" to locate troubles on "Pulp" cable. But, I would contact a local communications company to locate the trouble. Your ILEC (Verizon?) will not do this, but a local contractor should have the skill and tools to locate and fix your problems. A skilled technician should be able to put you right on the trouble, avoiding digging holes all over looking for the problem.

For location of u/g cable, the following steps may followed:

1) measure the fault resistance by a multimeter

2)depending on value of fault resistance, select the instrument to be used for prelocation of fault.For example, if fault resistance is approx. 100 ohms,TDR may be used;if fault resistance below 100 ohms,prelocation may be accurately done by AF Generator and AF Receiver set;if fault resistance is high, say several kilo ohms, High Resistance Fault Locator bridge may be used.

3) After prelocation of fault, pinpointing may be done by THUMPING Operation( in any case the thumping voltage should not be above five times the rated voltage of cables). Sometimes it may require to treatment of fault i.e.lowering of fault resistance;or alternatively aoption of SIM( Secondary Impulse Method) technique may give a quick good result.

first post, just registered.

i have been using a TDR for irrigation locating and troubleshooting since 02-03.

the first i owned was a bci, it was fried on a 2-wire system 2 years ago. i replaced the bci with a triplett (manufactured by bci) and have been happy with the instrument.

as far as locating faults with the TDR it's pretty straight forward, calibrate the vop with a like sized pair to an known length and go to work.

the TDR will see the first event on a pair and display the length, at that point the wire path is located and the ground fault locator is brought into play.

the TDR is used by me to narrow the area of search down to a minimum.

the fault is then located for repair.

I have worked in this industry for over 20 years and have found several thousand faults in irrigation cables.First the cable tracer to use needs to have a low enough frequency to reduce induction into the other cables less than 8khz second keep the tone generator less than 20mA always disconnect the common cable at the controller. the thumper to use should never be the audible type use a thumper with a frame the 3000V DC thumpers have good current control and wont damage the coils if there are multiple earth faults thump from both directions the fault generally wont pass the open circuit. Another good tool is a valve locator using a good earth attach to active wire this can trace the cable by generating a field either side and the null in the center although not pin point accurate it will fade out at around 10m of break. I use an AVO TDR which generally only works well on two wire or systems with multi core. If you can access a tracer with constant mA display this can also be useful this fault should take no more than 3 hours to find if less than a couple of Kms

forgive me for parsing but i wold like to address your post in full.

"I have worked in this industry for over 20 years and have found several thousand faults in irrigation cables."

your experience is noted and respected

"First the cable tracer to use needs to have a low enough frequency to reduce induction into the other cables less than 8khz"

the industry standard locators use 1748 hz frequency for tracing wire paths and locating solenoids, both the 521 and the pro-700

"second keep the tone generator less than 20mA always disconnect the common cable at the controller."

all wiring to be traced must be disconnected from either the controller or the junction box in some of the larger systems.

both the 521/700 use an analog meter/power levels 0-10, without testing the mA are unknown

"the thumper to use should never be the audible type use a thumper with a frame the 3000V DC thumpers have good current control and wont damage the coils if there are multiple earth faults thump from both directions the fault generally wont pass the open circuit."

i personally use a dynatel 573 dl w/Aframe for fault finding, it has both directional and audible prompts

fault finders such as the gfl-3000 and the pulser are 2400-3400 volt dc generators, not thumpers, thumpers are more akin to hipot testing.

ground fault locators locate the dc current as it leaks to earth seeking the transmitter ground.

"Another good tool is a valve locator using a good earth attach to active wire this can trace the cable by generating a field either side and the null in the center although not pin point accurate it will fade out at around 10m of break."

current movement creates an electromagnetic field which circles the conductor evenly, the vertical aerial recognizes this signal a null, a horizontal aerial recognizes the signal as peak. utility locators that have multiple aerials and use both peak and null modes use the two modes to backcheck the target path.

"I use an AVO TDR which generally only works well on two wire or systems with multi core. If you can access a tracer with constant mA display this can also be useful this fault should take no more than 3 hours to find if less than a couple of Kms"

i think if you would step down from the megger and into a non-graphical bci/triplett 3271 hand held you may have better results in the irrigation field.

with the 3271 i get constant readings to the first event on cables, twisted pairs, multi strand and 12/14-1, in regards to the 12/14-1 the wires have to be in relation to each other.

i too have located a few faults, paths and valves in my time and thoroughly believe that in using the TDR, locator and gfl that even the most stubborn issue can be resolved in much less time than you suggest.

in fact i flat rate my locates and usually it takes more time packing and un-packing than it does to locate the issue

the TDR alone has many times given me the distance to a compromise that was well within a couple of feet, many times a few inches. by using the TDR to minimize the search area, the use of the ground fault locator or valve finder is made more efficient and drastically improves the locate time and results.