Finding short between two conductors of buried cable

Discussion in 'Irrigation' started by Ouroboros, Jun 6, 2012.

  1. Ouroboros

    Ouroboros LawnSite Member
    Posts: 6

    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.
     
  2. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

    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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  3. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

    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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  4. Ouroboros

    Ouroboros LawnSite Member
    Posts: 6

    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?
     
  5. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

    18 ga in a jacket or 14 ga.
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  6. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

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  7. Ouroboros

    Ouroboros LawnSite Member
    Posts: 6

    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?
     
  8. Ouroboros

    Ouroboros LawnSite Member
    Posts: 6

    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.
     
  9. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

    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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  10. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,339

    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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