Cause Of High Resistance In Wire Line

Discussion in 'Irrigation' started by ed2hess, Nov 7, 2005.

  1. ed2hess

    ed2hess LawnSite Fanatic
    Posts: 13,306

    I had a valve that didn't work, the resistance measured around 39K at the controller. I am wondering what the cause of this high resistance. Is it due to insulation breaking down on a sharp edge somewhere and it shorts to wet ground? The locator was not very useful in locating this valve but with some logic and a lot of luck I found the valve. I ran another line out to the valve box and verified which line was the problem. I simply put in a new line. I have had no luck trying to find the location along the wire where the problem is located with a locator? Has any one else been able to find exact point of the problem?
     
  2. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    Can't answer the technical parts about resistance but I do know that when you encounter major ground faults the locating process goes haywire and it's very difficult to locate a particular valve. I use the Progressive 521 Locator to pinpoint the exact wire route (as far as I get a strong signal) and then switch to the ground fault indicator to locate nicks and breaks. I have found ground faults where gophers have stripped isulation off the wiring but the loss wan't great enough to stop the valve from working. Seems that the GFL is so accurate that it finds every nick along the wire. Sometimes gets frustrating to repair a nick and find out it's not the culprit. Then I move down the wire again and keep finding/repairing nicks until the one that is actually causing the problem is located and repaired. This is why I'm a proponent of installing extra wiring in the trench for our type of applications. Easier to switch wires, keep the valve operating and then work on ground faults over the winter when I have more time to spend on these types of complicated problems.
     
  3. ed2hess

    ed2hess LawnSite Fanatic
    Posts: 13,306

    Tell me a little more about ground fault indicator? Is it another function on 521?
     
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,992

    A bad line that is not completely cut is hard to trace, because the signal tends to 'jump' across the point of resistance - I've had bad lines come temporarily back to life after a trace.
     
  5. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    It's a different piece of equipment called a Pulsar (I forget the #) but I can't find the book on it right now as I'm not at work. You connect the wires from the transmit box to a wire's end and ground it like the 521. The box sends out high voltage pulses (DO NOT TOUCH!!! WILL SHOCK YOU!!!) that travel down the wire. The receiver is like an A-frame with spikes in the end of the legs (to stick in the ground) and an indicator box (with needle) at the top of the frame. Every time the pulse occurs the needle picks up the signal and strongly jumps. By working the receiver frame down the wire path (first located by the 521; we mark with orange dots) with spikes in line with the wiring and over the wire route the needle will jump in the direction of the ground fault. If you pass the GF point the needle will start jumping the opposite direction. You just work it back/forth until the needle stays steady and you're right over the bared/broken wire. It's a pretty cool machine but it takes a LOT of patience. I have never failed to find at least one ground fault location with it. As noted earlier, the patience comes when the GF you find is not the major one (or complete break) and you have to keep repairing these minor GFs to get to the problem one.
     
  6. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    It seems that these "jump points" are when I tend to get ghosts or extremely weak signals that are hard to follow with the 521. Pisses me off. :p
     
  7. Planter

    Planter LawnSite Member
    from Utah
    Posts: 214

    I had the same problem last month. The ESP controller showed a fault on number 14. Chased wires for a few hours and found the valve. I toned it and disconnected the entire bundle of 7 wires at the clock. There were 4 sets for seven strand in the clock. I was getting high resistance from the station wire and the common. I disconneced both at the valve and still had a high resistance on the pair (and all the other pairs in the bundle). Keep in mind that all the wires were disconnected from the clock and valves, they should have been completely open.

    I followed the path with the 2003 pulser and located a fault about half way between the clock and the valves, near another valve box. We had recently pulled a new line there and I thought we had nicked the wires.

    We dug down 18 inches and found the bundle all spliced together with butt connectors and regular wire nuts. No nicks from the pipe pull. They were just connected and dropped in the trench way under the PVC main where they would be safe. All the connections were corroded. A few splices and a junction box and we were back in service.
     
  8. Planter

    Planter LawnSite Member
    from Utah
    Posts: 214

    I ran into this a lot and recently had a job where we hit one fault after another. Dig up the wire and find a small nick in the skin. To be sure they needed repair, but they were time consuming. After doing two on this line I decided to just run the pulser clear to the valve. I marked every nick with a flag and then I really hit the big one. With the sensitivity knob backed way low I found a major fault near the valve. This one was it and we dug it and repaired it. That solved the problem. We'll check the ones we left when we have more time.

    Next time I'll try walking the whole line and look for the worst one, fix it and then check the line to see if I got it. That should let me pass some of the small nicks.
     
  9. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    Good experience. I know the "dwell" (or whatever they call it) can be adjusted but in my circumstances I usually have to work the wire run in segments. Too many kids and other things around and I usually have to station a guy at the box to make sure some kid doesn't accidently shock themself.

    On one particularly long run of over 1000' with twists and turns I'd find a conveniently located nick, dig up the wiring and then cut into the proper control wire after using the toner to make sure I've got the right one. Then I'll take a solenoid and hook it up as a tester. If I can get it to fire at that point then I move on to another convenient GF further along the wire route. It's a lot like chasing a sewer clog between clean-outs where I can isolate an area between two points where the wiring ceases to function. Then I'll take the GF locator and work that section real good.

    Only problem is when you have a wire that has so many minor nicks and no major one that the accumulation of GFs causes the wire to stop working. I've also come across some wiring that has been stripped 12"-18" of its insulation and the thing is still working. "Yippee I found it" turns to "dammit" after the repair is made and the wire still doesn't work.

    We've had some areas (maybe a couple hundred feet) that are notorious for gopher hits. We'll go back to a convenient place, put in a makeup box and then conduit the wiring between valves.
     
  10. BTW...don't use your hands to dig up the "nick" or "fault" while the pulsar is running.....even if you think you can time the "between the pulses" to grab the wire...boy does it hurt when your timing is off!

    Yeeehaaaawww!
     

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