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Ouroboros
06-06-2012, 10:26 PM
I would appreciate any advice on diagnosing and repairing a fairly simple issue. I have a three-conductor cable, directly buried partially under open ground and partially under stone pavers, from a timer to two valves approximately 150 ft away. Two of the conductors appear to be shorted together. I am reading 4 ohm from the one side and 4.5 ohm from the other side. Since I cannot easily examine the cable, especially under the stone pavers, I need an indirect method of finding the fault location. I understand that a ground fault locator and a time domain reflectometer may be useful, but I cannot seem to a find location that rents them.

1idejim
06-06-2012, 11:00 PM
I would appreciate any advice on diagnosing and repairing a fairly simple issue. I have a three-conductor cable, directly buried partially under open ground and partially under stone pavers, from a timer to two valves approximately 150 ft away. Two of the conductors appear to be shorted together. I am reading 4 ohm from the one side and 4.5 ohm from the other side. Since I cannot easily examine the cable, especially under the stone pavers, I need an indirect method of finding the fault location. I understand that a ground fault locator and a time domain reflectometer may be useful, but I cannot seem to a find location that rents them.

turn your DMM to 2000K and do a short to earth measurement test of each wire at both ends. to begin with. if the measurement is above 700,000 ohms the short is insulated from the earth. eliminate above ground connections and exposed wiring for bad splices or cuts. i have assumed this to be mvlti strand wire btw. if i am right you're best bet is a valve locator. if the measurement is below 700,000 and hopefully way below 700K you might get away with soaking the yard area, turn the controller on and walk around the wire path barefoot. if you get a tingle you are close to the fault. you can set the DMM to 200VAC and insert the probes in the ground for a volt reading but you won't get a directional indication because it's AC and not DC. i won't say that you can find the fault for sure but unless you get a pro with a locator....it's worth a shot
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1idejim
06-07-2012, 12:08 AM
I am reading 4 ohm from the one side and 4.5 ohm from the other side. .
if these measurements are from the clock and from the valve the 4.0 measurement would be closest to the point of measurement. this could be interesting. please keep us in the loop.
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Ouroboros
06-07-2012, 12:28 AM
These are solid-core conductors. I am not certain how to accurately measure the resistance to ground. Using a long screwdriver inserted into wet ground, I am getting readings of mostly 4 Mohm but occasionally 300 Kohm to all of the conductors on either end. Using the metal manual valve on the input water pipe to the valves, I am getting a reading of less than 50 Kohm to the common conductor but still 4 Mohm to both of the shorted conductors. Perhaps this indicates that the fault is near the water pipe? Strangely, I am also occasionally getting a reading of less than 50 Kohm between the common conductor and both of the shorted conductors--perhaps this is capacitance?

1idejim
06-07-2012, 12:34 AM
18 ga in a jacket or 14 ga.
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1idejim
06-07-2012, 12:43 AM
These are solid-core conductors. I am not certain how to accurately measure the resistance to ground. Using a long screwdriver inserted into wet ground, I am getting readings of mostly 4 Mohm but occasionally 300 Kohm to all of the conductors on either end. Using the metal manual valve on the input water pipe to the valves, I am getting a reading of less than 50 Kohm to the common conductor but still 4 Mohm to both of the shorted conductors. Perhaps this indicates that the fault is near the water pipe? Strangely, I am also occasionally getting a reading of less than 50 Kohm between the common conductor and both of the shorted conductors--perhaps this is capacitance?
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Ouroboros
06-07-2012, 12:45 AM
They appear to be approximately 1 mm, 2 mm with jacket, so probably 18 gauge. I should also mention that the short resistance measurements have changed. I am now reading 5.3 ohm and 5.9 ohm, respectively--perhaps a change in temperature or moisture level?

Ouroboros
06-07-2012, 01:10 AM
If the total cable length is 130 ft, then these measurements are consistent with a short of 4.8 ohm approximately 43 ft from the first end. Unfortunately, this location is under the stone pavers, which I am not going to lift without a more precise measurement.

1idejim
06-07-2012, 01:27 AM
If possible just insert the black lead into the ground. if the supply is galv. pipe the DMM is measuring the pipes contact with the earth also. the resistance of the short still is short so don't fret the change, it is consistant. your KOhm measurements indicate earth contact. try removing 1 wire from the controller and activate the attached zone. my guess is both valves activate if the common is clean. if the valves dont activate, the common is involved. youre bringing more to the table than most but i cant see you test so i am guessing about some of your results.
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1idejim
06-07-2012, 01:45 AM
If the total cable length is 130 ft, then these measurements are consistent with a short of 4.8 ohm approximately 43 ft from the first end. Unfortunately, this location is under the stone pavers, which I am not going to lift without a more precise measurement.

that is dependent upon the age of the wire the temp and the number you use for the resistance of the wire. 6.39 or 6.51 per M according to 2 books and i kow of a couple more without referance. if youre only gonna move things or dig at 100 % you need a locate
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Ouroboros
06-07-2012, 01:56 AM
With the black probe directly in the ground, I am still reading 4 Mohm to both of the shorted conductors but 2 Mohm to the common conductor. With one conductor connected to the timer, both valves do operate, which is how I discovered the issue in the first place. Regarding the location of the fault, you are correct that the variation in the resistance can significantly affect the result. I failed to consider this in the previous post.

1idejim
06-09-2012, 12:50 AM
I have been pondering on your orignal resistance measurements. if you are measuring from both ends (you said sides) you have 4.0 and 4.5 respectively. close to half way. you also said that the path was 130 and 150 call it 140. i might want to look near the middle of the path minus 15 ft ea way. without a locator you could try the AM radio tuned between stations, voodoo sticks, wet yard / barefoot method all of the above might work. or not. if you don't have the right tools you have to improvise
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Sprinkus
06-09-2012, 08:18 AM
In addition to the barefoot method you could also try pouring gravy along the wire path and let your dog lick it up until he yelps. :laugh:

1idejim
06-09-2012, 08:56 AM
In addition to the barefoot method you could also try pouring gravy along the wire path and let your dog lick it up until he yelps. :laugh:

i doubt that we will hear from the OP again. too bad, he brought more information to the table than most. btw sprinkus, i got the barefoot idea from you.
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Sprinkus
06-09-2012, 09:09 AM
i doubt that we will hear from the OP again. too bad, he brought more information to the table than most. btw sprinkus, i got the barefoot idea from you.


OK, now that I've had two cups of coffee and am fully awake.....how about valve locating/wire tracing/fault finding shoes! No more bulky equipment to lug around. Just connect the transmitter to the controller, slip on the receiver shoes and go!

FIMCO-MEISTER
06-09-2012, 09:18 AM
How about running new wire and installing it properly?

And use a decent wire like single strand 14ga.

Duekster
06-09-2012, 09:30 AM
In addition to the barefoot method you could also try pouring gravy along the wire path and let your dog lick it up until he yelps. :laugh:

I almost pee'd myself :laugh:

Duekster
06-09-2012, 09:31 AM
How about running new wire and installing it properly?

And use a decent wire like single strand 14ga.

140 feet is not too bad.

FIMCO-MEISTER
06-09-2012, 10:26 AM
If the op lacks the proper tools and doesn't want to hire a professional who has the proper tools then running new wire is his best option.

1idejim
06-09-2012, 07:25 PM
If the op lacks the proper tools and doesn't want to hire a professional who has the proper tools then running new wire is his best option.

all fine and dandy and i agree but that's not what the OP asked for. :)

Ouroboros
06-11-2012, 11:12 PM
After several days of hot dry weather, I am reading only 50 Kohm between the two shorted conductors. In the case that there is no ground connection, I was considering opening the short using a small 12 V battery, but this is no longer possible. In any case, a contractor has installed a second cable in a conduit--I was unable to find a contractor with locating equipment and expertise before this occurred. Unfortunately, I do not own a dog, so I will likely not be pursuing this issue much further. Thank you for all of the information.

1idejim
06-12-2012, 12:05 AM
After several days of hot dry weather, I am reading only 50 Kohm between the two shorted conductors. In the case that there is no ground connection, I was considering opening the short using a small 12 V battery, but this is no longer possible. In any case, a contractor has installed a second cable in a conduit--I was unable to find a contractor with locating equipment and expertise before this occurred. Unfortunately, I do not own a dog, so I will likely not be pursuing this issue much further. Thank you for all of the information.

glad you hired a contractor and brought us to speed on your decison.

your 12 vdc idea comes from the utility field and is called HIPOT short for high potential testing. a very serious generator induces enough voltage onto a line to make the earth thump at the fault. this is AKA thumping
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