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Old 04-23-2011, 09:53 PM   #1
johnsdeere850j
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Default how do utility guys find broken wires underground?

Just spent the last 12 hours on the excavator digging up 200 feet of 4/4/2 direct burial going to my well-house to find that a gopher had chewed through one of the hots. Is there some fancy gadget i can buy so i dont have to dig it all up again? some kind of giant ohmmeter or something
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Old 04-23-2011, 10:03 PM   #2
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Default Re: how do utility guys find broken wires underground?

there are a few ways, a TDR can tell you how many feet away the break is, there is also a thing called a 'thumper' that can detect a break in an underground line but those can be dangerous (couple people have been killed by the high voltage used)
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Old 04-23-2011, 10:21 PM   #3
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Default Re: how do utility guys find broken wires underground?

We use a dynatel 965 dsp at work. We use this for telco wires, usually 19-26 awg. Not sure if it will work for the larger wire, but I'm sure there is a piece of equipment that is similar for the larger wire sizes. These pieces of equipment are usually alot of $$$.
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Old 04-23-2011, 10:22 PM   #4
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Default Re: how do utility guys find broken wires underground?

They disconnect the wire from power, hook one end up to a radio transmitter, and sweep the ground with an antenna. Generally. I'm not a sparky type. But, they do have a way to do it.
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Old 04-24-2011, 02:11 AM   #5
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Default Re: how do utility guys find broken wires underground?

I bet you run it in conduit before putting it back.

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Old 04-24-2011, 07:22 AM   #6
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Default Re: how do utility guys find broken wires underground?

As almost everyone says, for shorts there is a device that will measure the distance to the short. For opens same thing (usually same device) which gives the distance in feet to the fault.

The only thing difficult is when there is leakage which is not too bad, those are sometimes more difficult to pinpoint.
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Old 04-24-2011, 07:49 AM   #7
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Default Re: how do utility guys find broken wires underground?

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Originally Posted by nehog View Post
As almost everyone says, for shorts there is a device that will measure the distance to the short. For opens same thing (usually same device) which gives the distance in feet to the fault.

The only thing difficult is when there is leakage which is not too bad, those are sometimes more difficult to pinpoint.
A TDR will pinpoint (to within a few feet) all those problems and many more. They do take some knowledge and experience to use.
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Old 04-24-2011, 10:17 AM   #8
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Default Re: how do utility guys find broken wires underground?

We use a toner first to find the wire path and then mark it with upside down paint, then we use one similar to this to find the fault:

http://www.stayonline.com/detail.aspx?ID=170

We use the crap out of it during the startup of spring irrigation season when the farmers irrigation pivots don't work. We've used it on 2000' runs with no issues. Sometimes you'll find 2-3 bad spots in a run of wire before it's completely fixed....

On long runs we usually splice it back together and tell the customer he should look into getting it replaced. Especially if we find faults again the next year. On shorter runs with more than one-two faults, just replace it. And put it in conduit the next time.

Last edited by Aceman; 04-24-2011 at 10:21 AM.
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Old 04-24-2011, 01:43 PM   #9
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Default Re: how do utility guys find broken wires underground?

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We use the crap out of it during the startup of spring irrigation season when the farmers irrigation pivots don't work. We've used it on 2000' runs with no issues. Sometimes you'll find 2-3 bad spots in a run of wire before it's completely fixed....
Is direct bury wire for pivots typical in your part of the country? I've done the engineering on a few of those things, and we use that HPDE flexible conduit that comes on a big reel with the wire already installed. It's normally plowed in, but I had one project recently where they used directional boring.
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Old 04-24-2011, 04:39 PM   #10
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Default Re: how do utility guys find broken wires underground?

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Originally Posted by Stuart in MN View Post
Is direct bury wire for pivots typical in your part of the country? I've done the engineering on a few of those things, and we use that HPDE flexible conduit that comes on a big reel with the wire already installed. It's normally plowed in, but I had one project recently where they used directional boring.
Direct bury is all over around here. Probably 90% or more of the older circles. We've used HDPE(we call it Cable-Con) on the last couple of projects though. I love this stuff. We just got a couple reels in for a job that showed up with no UL listing on it though

Our cable is usually layed in the ditch alongside the water line out to the pivot, so we haven't had to plow/bore any in.
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Old 04-24-2011, 08:14 PM   #11
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Default Re: how do utility guys find broken wires underground?

Back around 1975 I watched my grandfather find a break in a 12/2 underground line using only an ohmmeter. I was only 12 or 14 years old so I don't remember exactly what he did. The line was about 80 feet long. He measured resistance in one or both legs (power off I'd guess), maybe from both ends, I can't be sure. Paced off the distance, calculated some ratios in his head, paced back part way and said dig here. Damned if he was right at the break. I do know he was using an analog Simpson meter, cause he later gave me one. I've always wondered exactly how he did it.
Lest you think he was crazy and "water witching" I have a framed original newspaper article hanging in my house from 1946 where as a telephone company technician he figured out how to bring service to 2697 households without adding any new switching equipment; which was not available at the time. He and other technicians made 46,000 wiring changes to accomplish the task. Southern Bell gave him a $500 award.
Not sure if this helps you but it sure brought back good memories for me!
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Old 04-24-2011, 08:53 PM   #12
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Default Re: how do utility guys find broken wires underground?

This happened to me about 15 years ago. Lost one leg underground, there is about a 150 foot run from the pole to the socket. I expected the poco to come in with a fancy TDR or other such fancy method. What they used worked perfectly and pinpointed the break exactly. There were three major parts to the apparatus, a low-end Fluke DMM, a metal stick, and a long roll of cheap speaker wire. The wire gets rolled out, and one end gets clipped to the neutral at the meter socket, the other plugged into the low terminal of the DMM. A short test lead goes from the stick to the volts input of the DMM. I tell the guys the path where the wire is buried (I staked this out for the poco when I built the place 22 years ago). The guy pokes the stick into the ground along the path of the cables. The closer he got to the house, the higher the reading got. From a couple hundred millivolts at the start, to about ten volts at ground zero. The target happened to be smack in the middle of a bush I had recently planted. I knew for a fact that the cabling was at least 3 feet under ground at that particular point (and it was actually more like 4 ft). Guys removed the bush, dug down to the cables, one hot was pulled away from the rest by my backhoe guy during the backfill and drainage installation, you could see the shape of the bucket tooth in the bend of the wire. The aluminum cable deteriorated over time until it finally opened. The guys crimped and sealed the cable, replaced the soil and the shrub (which is still there) and all has been fine since. The guys worked till after midnight, I provided them scads of light (house on a generator), drinks and snacks. Friendly guys, and I learned some stuff that night too.

While they did have at their disposal more advanced equipment, I was told that in most cases the poke-sick method finds the trouble spot on, and much faster. It's only in areas with lots of electrical noise such as substations and large commercial/industrial do they get out the sophisticated stuff.
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Old 04-24-2011, 08:57 PM   #13
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Default Re: how do utility guys find broken wires underground?

Conduit is your friend here. I'd never direct bury wire. I figure that when a wire breaks, I just want to pull it out of the conduit and pull in a new one.
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Old 04-25-2011, 05:51 PM   #14
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Default Re: how do utility guys find broken wires underground?

For us guys just "listening," can someone explain "TDR"?
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Old 04-25-2011, 06:29 PM   #15
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Default Re: how do utility guys find broken wires underground?

First: http://en.wikipedia.org/wiki/Time-domain_reflectometer

Basically (if you want a quick explanation) a pulse (very short good quality square wave) is sent down the wire. When it hits an open, some of the voltage is reflected back and the time it takes to get back tells you (since you know the speed of electricity in a copper wire) how far the short is. Same is true for an open, and some other faults. To test for opens, typically the far end is shorted, there will be a reflection at the open point, and another at the end.

Theoretically you can use a very high quality oscilloscope (lab quality) and a square wave generator to do this, and watch the returns. Accuracy is limited only by the ability to resolve very short periods of time.

(What that Wiki article calls a pulse is actually supposed to be a square wave, IMHO, as you can learn a bit more about the wire's condition with a square wave.)
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Old 04-25-2011, 06:51 PM   #16
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Default Re: how do utility guys find broken wires underground?

Thanks. I had a mystery wire in a wall that went nowhere. I energized it and tried to follow it with a non contact tracer, but lost it in the maze. Never did find it's source or what it may have been used for. It was terminated in a switch box next to a door and the device had been removed and covered with a blank plate. The outside light had no power. Sure could have used that wire if I could have found it.
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Old 04-25-2011, 10:54 PM   #17
johnsdeere850j
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Default Re: how do utility guys find broken wires underground?

Thanks all, good info
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Old 05-01-2011, 09:56 PM   #18
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Default Re: how do utility guys find broken wires underground?

Last summer, the buried primary from the street to the transformer near our house failed. The fuse about a quarter mile up the street blew, making a significant "boom". Although the CBYD crew mis-marked the location of the wire, the poco crew located the bad spot close enough that they dug a hole about 4x8' and the failed spot was right in the middle (they had to wait for another CBYD crew to mark it correctly before they dug). The entire operation took about a day and a half; in the meantime, they ran a bypass cable alongside our driveway (about 400') so that we (and our neighbor on the same transformer) would have power.
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Old 05-01-2011, 10:38 PM   #19
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Default Re: how do utility guys find broken wires underground?

Aceman and Stuart, Cablecon for pivot service is the ONLY way to go. On any new installs we do (or I used to do) we use it exclusively. Remote panel machines were most critical because you send 7 #12cu wires and 4 #4al wires to the center point in a 1 1/4" 'duct'. The control panel is on the edge of the field near the well (usually) and there is no need to go to the center point to operate the pivot.

Some farmers tried to cheapen things up by putting the control panel at the center point and just sending 4 #4al and 2 #12cu wires to the center point. The hassle of having to go to the center point (over a sometimes muddy pivot trail) quickly negated the lesser wire expense.

Most of out applications have the wire duct thrown in the same ditch as the water pipe, which is usually at least 4' deep. If trenching was done, we would go as deep as the trencher would go. We have had NO problems with rodents or anything else getting into the duct and disturbing the wires.

We use a PVC compression coupler below grade at each end of the wire run to transition from the duct to PVC liquidtight conduit, which terminates nicely in panel and j-boxes with proper liquiditite fittings. How do you terminate the ends of the ducted wire runs?
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Old 05-02-2011, 12:29 AM   #20
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Default Re: how do utility guys find broken wires underground?

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We use a PVC compression coupler below grade at each end of the wire run to transition from the duct to PVC liquidtight conduit, which terminates nicely in panel and j-boxes with proper liquiditite fittings. How do you terminate the ends of the ducted wire runs?
We use sch. 80 pvc for the stubups. This is the transition fitting we use underground:

http://www.adtechnologies.com/pdf/Sh...%20coupler.pdf
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