DB cable fault?

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residentuma

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I have about a 150' run of direct bury cable going to the end of the driveway. From what I can tell, it is not in conduit. It was disconnected when I purchased the house. From how it was originally wired (stapled to studs), I assume it has been there since the house was built about 30 yrs ago.
Initially I wired it into a GFCI which immediately tripped when I turned on the breaker. I though okay, that is probably just current leak given the length of buried wire.
I then wired it into a normal outlet and it did not trip the breaker. Went out to end of driveway and there was power there (unfortunately did not try my multimeter or put a load on it, just the noncontact tester). I disconnected it and left it there as I had a few other things to tackle before this.
Yesterday, I wired it permanently to a new breaker, but this time I am not getting any power at the end of the driveway. I double checked my wiring and I am getting the noncontact lighting up all the way to the end of the circuit in the house where it exits underground. The breaker and wiring inside of the house appears fine.

I am guessing water has infiltrated this at some point? Hence the working a month or so ago and not yesterday? Any other ideas that I can do to help diagnose the problem? Is it common to have voltage half the line but not at the end yet not trip the breaker?
 

WorthFlorida

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Circuit breakers trip on shorts or with a continuous overload. GFCI trips when the hot and the neutral wire current are not equal. My guess is somewhere the wire has been cut by a shovel or a critter has chewed into it. You can stick the hot wire directly in the dirt and it will not trip a circuit breaker, it will trip a GFCI. With such a long driveway there might be another underground electrical box for perhaps a light post?

If it is UF cable, it is pretty tough and it should at least 12" below grade. https://www.familyhandyman.com/project/how-to-bury-underground-cable/
 

Reach4

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I am guessing water has infiltrated this at some point? Hence the working a month or so ago and not yesterday? Any other ideas that I can do to help diagnose the problem? Is it common to have voltage half the line but not at the end yet not trip the breaker?
A bad splice could match your symptoms, but you should probably characterize the fault.


You have 3 conductors? Can you isolate the ends? Do what you can.
1. What is the lowest resistance between conductors? Looking for Ohms.
2. What is the lowest resistance of the conductors to an earth ground?

Based on those, I suspect we could devise a plan to find out where the failure is.
 

residentuma

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OK I was able to do the above tests tonight. I'm not sure these are helpful because they all seem to be quite similar +/- 0.5.
I put them in a quick excel graph to hopefully explain what I did better. to further clarify, these wires were all connected on the same wire that goes to end of driveway- not to the romex coming from the panel (except for the ground)

Also for a sanity check, I hooked up everything to see if it was working again by some odd chance... no change. Getting noncontact lighting up at the wall that exits the house but none at the end of the driveway

driveway.jpg
 
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WorthFlorida

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With a non contact tester it senses voltage, not necessarily 120 volts but they are very convenient. I have one that can be set for low voltage or line voltage. On your readings there should infinite ohm reading (open) between the black and white wires. All wires should have been disconnected at the juncture. The wire leaving the house, is it gray in color? UF is gray, if not and it's white it is NM and not suitable for buried work.

Do you get voltage at the juncture box? Where is the juncture box?
Two test you can try. The first is basic and simple. The second is if you have an open between the WHT and BLK no matter what.

1) Turn the power off. Disconnect the WHT and BLK wires at the juncture. An ohm test at the driveway end should read an open. If not then there is a short. If you have an open connect the WHT and BLK together at the juncture and you should now read near zero ohms, maybe 10 ohms as you had at the driveway.

2) Turn off the power. At the point where you can read voltage, maybe at the juncture box, use the green wire as a spare for this test. At the end of the driveway wire in an outlet and plug in a light or lamp so you can see it from the juncture box. With it wired normally, turn on the power. If the light doesn't light up, turn off the power. At both ends disconnect the white and green wires. Then connect the green wire as a substitute for the white wire from the house and the green wire in the white screw at the outlet. Turn power on. If no light then do the same with the black wire using the green and reconnect the white. What we're trying to prove that either the black or white has an open, maybe both.

If you really need power at the driveway it may be easier to bury a new wire. Rent a trencher and use PVC conduit or UF cable.

klein-tool-voltage-tester-01.jpg
 

residentuma

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Yea line leaving house is gray. It's definitely has the thick UF jacket

I'll test the voltage tonight at the driveway while it is wired to the panel and on.

The ohm readings are what I don't understand - with all wires disconnected (control test if you will), resistance should be infinite yet it is the same in any configuration. Does that indicate they may be shorting of white-black (or maybe even all 3) somewhere along the buried route? Shouldn't that trip the breaker?

Junction box is just outside of the panel connecting to an outlet (that was easier/safer to splice into rather than pulling the panel to put in a new breaker). It is not currently powering anything. I did confirm the outlet does power a light bulb so I know it's not something inside of the panel.

Your first test is one that I did but got the same reading with everything disconnected at both ends and also with black/white on the same wire connected.

I will try #2 tonight

Thank you for your help trying to diagnose this. I'm afraid you may be correct about burying new conduit!
 
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WorthFlorida

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With the ohm meter on and no leads touching, that is an open reading. The cable can be cut where on the house side they are open but the other side they are shorting. There are companies that can trace the wire and may even be able to find the cut.

meter.jpg
 

Reach4

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The ohm readings are what I don't understand - with all wires disconnected (control test if you will), resistance should be infinite yet it is the same in any configuration. Does that indicate they may be shorting of white-black (or maybe even all 3) somewhere along the buried route? Shouldn't that trip the breaker?
10 Ohms or 10 kOhm?

10 kOhm may not even trip a GFCI.

10 Ohm would probably not trip a breaker. The heat generated at the leak spot when powered might be hot enough to show up on a FLIR camera.
 

residentuma

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Reach4

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It was 20k Ohm setting.

Here is another test. Do you have an earth ground, such as a ground rod, a metal pipe that goes underground, or something else? What is the resistance from that to each of the isolated wires.

I am thinking of putting a DC voltage (little 9 volt battery or car battery, for example) between an earth ground and your wires. Then measuring the DC voltage between the earth ground and various points in the dirt along the path. Where that voltage is highest, I think that is where the leaky place most likely is.

A test like this calls for what is essentially a 50 ft (for example) long probe. You could use one conductor of a 50 ft extension cord to extend your probe. Stick the test probe tip into one end of the extension cord, and clip on to the corresponding blade for the other end.
 

residentuma

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Interesting--- I'll add that to my list of testing and let you know. thank you!
 

WorthFlorida

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......I bought a tester that looks like some kind of tone generator off amazon- my expectations are very low but for 30 bucks I'll try that in addition to what worthflorida mentioned above tester I am referring to: https://www.amazon.com/gp/product/B00F5WA6QQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I have this tester and was just using it tracing underground sprinkler wires. Besure there is no power on cable. It will give you the position of where the wire travels but because it is cheap, it won't tell you where an open might be. With the two transmitter wires, one wire goes to a conductor, the other to a earth ground, not another conductor. I wrap a screwdriver with a copper wire, stick the screw driver into the ground and connect the copper wire to the transmitter. The signal will bleed over to the other conductors so if one is open, the signal may still be present past a cut but it will weaken. To use it you swing the sensor like a pendulum. The dead spot between the tones as it swings is the location of the cable. For the money it is a fun tool and at least you know where you can start digging.

For irrigation systems it now states in the instructions that it cannot locate valve solenoids. As you trace the wire using a pitch fork into the area you might hit on the valve cover, in your case hopefully there is a junction box underground.
 
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