Underground Cable Break

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  #1  
Old 10-26-13, 07:50 PM
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Underground Cable Break

My outdoor post lamp no longer works. I checked the wires feeding the socket of the fixture and there is no voltage. I know there is 120 V where the wire exits the house because there's a junction box there that powers some low voltage lighting that does work.

A couple years ago some landscapers were digging in the area of the buried cable and hit the edge of the direct burial Romex so that you could see exposed copper on one of the conductors. I didn't have time to fix it at that time. It only powers one 20 watt CFL bulb so I wasn't overly concerned about it. Two questions.

1) Is it likely that galvanic corrosion on that exposed conductor could have caused it to open up? We really don't use that light all that much so the wire only has voltage on it for a pretty short time.

2) Is there some way I can localize the break? I don't have a ~1 MHz RF oscillator I could modulate and then use a portable radio as the receiver. Nor do I have a time domain reflectometer lying about. Any other tricks I can try?
 
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Old 10-27-13, 03:39 AM
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Hopefully this circuit is protected via GFCI somewhere, and it has tripped. If not, it should be protected. Finding the break may be a task, but if you suspect where you saw a break, then you can make an underground splice of all three wires using a kit for that purpose. You can get them at box stores or electrical supply houses.
 
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Old 10-27-13, 05:56 AM
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If you have a pet containment system or have a neighbor or friend that has one that is willing to allow you to borrow the transmitter and one collar you can pinpoint the line and the break easily. Works just like the 5k pipe locator systems sends a RF signal through the metallic line and the collar picks it up. It is fun too and gets lots of laughs out the neighbors and kids. Note: Isolate the circuit before hooking up the transmitter and just hold the collar without touching the shock prongs (do not put it around you or your dogs neck LOL) If you need more details let me know the key is one side of the transmitter goes to a ground rod perpendicular to the line and the other side of course goes to the line to be located.
 
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Old 10-27-13, 07:54 AM
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Ding Ding Ding--We Have a Winner!!

What an outstanding idea!! We do have a neighbor/friend with a pet containment system. (Actually, I think he bought it in an attempt to keep his wife away from shopping malls.) That sounds like a much better idea than digging up planting beds hoping to find something. I guess I need to separately energize each of the two conductors since I don't know which one is open. As far as who holds the collar, I usually ask my wife to help me with things like this..... (JUST KIDDING. Please don't call 911 or protective services. Really, I'm just kidding!)

I'm not at all familiar with these things. Is there a light on the collar that lights up when the collar is near the line with the signal? Will this work if the cable is buried 12" or 18" below ground level? I have a long spike I use for putting in driveway markers for the winter that will work great as the grounding rod. I cannot wait to try this out.
 
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Old 10-27-13, 06:26 PM
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I guess I need to separately energize each of the two conductors since I don't know which one is open.
In your first post you said you had no voltage at the light, but you did have voltage where the cable exited the house. That would indicate the break to be on the black conductor. Did you actually test the wiring at the light with a meter to make that determination or did you just test it with a light bulb? IF you didn't test it with a meter, it could be either the hot (black) ot neutral (white) conductors.
 
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Old 10-27-13, 07:04 PM
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I used a DVM set on AC for the voltage check. If my memory serves me correctly (it's been several months since I made the measurement) I actually measured something like 5 or 6 volts at the socket, which baffled me.

If the neutral/white conductor were open wouldn't I still measure zero volts? I measured between the black and white wires.
 
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Old 10-27-13, 07:10 PM
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I actually measured something like 5 or 6 volts at the socket, which baffled me.
Thats why we recommend an AVM. Cheaper DVMs have too high an impedance to cancel out meaningless induced voltages. An AVM would have probably read 0v.
 
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