120v -w/no load, 0v w/load.

[Fred B]

Got a call, no power in a garage. Simple no lights or receptacles working. Detached garage run with a single branch circuit. Tested with meter - 120V L/N, with no load. Added a heater load initially started then gradually died. Then unplug and still read 120V. Added load back and this time it didn't run. Then tried a smaller load, a 60watt incandescent bulb, initally lit then faded. Unplug load and again had 120v. This time added meter inline at same time as applying load. Meter spike momentarily to 120v then faded to 0v while load applied, and stayed at 0v as long as load was applied and only rebounded when load removed. No power issues reported in main house, and no issue at breaker.
Additional readings, L/G had 0v, N/G 0 ohm, open ground.
HO reports this history:
Questioned apparent driveway excavation I saw when I arrived. HO states it was 6yrs old and power had been ok until recently. Excavation was at least 6ft deep and crossed probable buried branch feeder location, based on depth of visible culvert exit from excavation. Possibly the buried feeder was damaged during excavation and only now presenting.

Thoughts?

 

[zbang]

Sounds like a wet underground fault that fails to high resistance when current flows. I'd disconnect the circuit from the house, isolate both ends, then use pairs of leads to look for an open or high-resistance with a low-impedance meter. Or even use a car battery and a 12v bulb that draws a couple of amps and see if you can isolate which wire is bad.

However you do it, might be just be easier to replace the entire circuit than fix anything.

 

[Dennis Alwon]

You have a wire that is either broken or hanging on by a few threads. There is enough connection either thru the soil or in the wire that voltage can be read but no equipment or lights will work. Classic case of a broken wire.

 

[Hv&Lv]

I bought a couple of Dynatel 573A off eBay a few years ago. Paid for it on the very first job locating a break in a well line on a farm. Service was about 500’ and the guy really didn’t want to dig in a whole new line.
I found the break and had it dug up in about 20 minutes.

 

[Fred B]

Sounds like a wet underground fault that fails to high resistance when current flows. I'd disconnect the circuit from the house, isolate both ends, then use pairs of leads to look for an open or high-resistance with a low-impedance meter. Or even use a car battery and a 12v bulb that draws a couple of amps and see if you can isolate which wire is bad.

However you do it, might be just be easier to replace the entire circuit than fix anything.

Kind of what I thought. HO says and indications are that it is a direct buried UF cable. So a new ditch and new run probably the most cost effective vs trying to locate the break and maybe fix.

 

[ptonsparky]

Kind of what I thought. HO says and indications are that it is a direct buried UF cable. So a new ditch and new run probably the most cost effective vs trying to locate the break and maybe fix.

If you plan on staying in business, refer to post 4 and add a megger to your list of tools. This project may not warrant repairs but others will.

 

[Fred B]

If you plan on staying in business, refer to post 4 and add a megger to your list of tools. This project may not warrant repairs but others will.

Now I've heard that a megger will damage a wire that otherwise was marginally ok. Got no first hand experience to say one way or the other as to megger damaging a wire.
What would a megger tell me that I don't already know? That the existing wire is compromised.
What @Hv&Lv looks like is referencing is a Locator.

 

[ptonsparky]

The Fluke 1507 does not damage the wire. It does tell us the resistance at a certain voltage level. It may indicate ,3 meg at 27 volts or .3 meg at 320. (randomly chosen numbers).
Process is to verify that the cable is faulted and to what degree.
Locate the cable.
Locate the fault
Access the fault and cut it out.
Do not make repairs until you have checked cable or wire for additional faults. Both directions. Repeat as needed.
At some point you can suggest repair or replace. Eventually even 1/4 mile runs are better off with replacement

Your current situation may not warrant the expense but others will.

 

[GoldDigger]

The main thing that a megger would be useful for would be to check for a high resistance fault to ground consistent with a damaged cable sheath that has led to corrosion failure of the cable. Is the "direct buried UF" copper or aluminum?
Repair versus replace may be influenced by any later construction (planter beds, concrete walkways, etc.) across the path of the feeder.

 

[Hv&Lv]

Now I've heard that a megger will damage a wire that otherwise was marginally ok. Got no first hand experience to say one way or the other as to megger damaging a wire.
What would a megger tell me that I don't already know? That the existing wire is compromised.
What @Hv&Lv looks like is referencing is a Locator.

It’s a sheath fault locator and works excellent if the wire is broke in the earth.
It works by measuring resistance, sending a signal down the line and an A frame will pinpoint a fault to within 1/2 inch where the signal leaves the wire..

Yes, I said 1/2 inch.

It will locate a wire, but I prefer a pipe horn for locating.
I only use it for fault locating
That being said, I haven’t used it for two years.
Trying to quit..

 

[jim dungar]

Now I've heard that a megger will damage a wire that otherwise was marginally ok. Got no first hand experience to say one way or the other as to megger damaging a wire.
What would a megger tell me that I don't already know? That the existing wire is compromised.
What @Hv&Lv looks like is referencing is a Locator.

Don't confuse a manufacturer's name with industry slang for their products, like using Kliens instead of linesman's pliers.

Megger is a company.
One of their products is a high potential test set, a Hi-Pot, which is used to test cable insulation using thousands of volts. These have been known to cause insulation failure. These instruments are usually used by trained testing companies rather than electrical contractors.

Another product is a meg-ohmeter which is used to check the resistance similar to a VOM, but using voltages typically from 500 - 1000V. This relatively low voltage is unlikely to damage standard 600V rated cables. These instruments are commonly used by electrical contractors.

 

[ptonsparky]

1/2" tolerance is correct.
I have put some flags in the ground to check the accuracy just for S&G, then measured after the fault was unearthed. It was closer to 1/4" on one. Usually not that important once you have a backhoe on location.

 

[kwired]

With no locating equipment, three main places that seem to be higher probability for problems.

1 and 2 are at each end of the underground run, about right at the point it turns to go up to whatever it is supplied from or to whatever it is supplying. Can be abrasions at that point on say a raceway or sleeve help contribute to failure, or I think maybe transients even tend to work on insulation over time right at that point where it turns often rather abruptly.

3. you mentioned signs of excavation going on in alleged path of this underground cable/conductors.

With no locating equipment I'd likely check those locations out at very least before replacing entire run. Even if you find a problem, "megger" still is useful in determining whether remainder of underground portion is in decent condition. If you have say a rodent issue you might find the break that currently keeps it from working but it may not be all that long before another break occurs.

 

[Fred B]

It’s a sheath fault locator and works excellent if the wire is broke in the earth.
It works by measuring resistance, sending a signal down the line and an A frame will pinpoint a fault to within 1/2 inch where the signal leaves the wire..

Yes, I said 1/2 inch.

It will locate a wire, but I prefer a pipe horn for locating.
I only use it for fault locating
That being said, I haven’t used it for two years.
Trying to quit..

So how deep of underground wire will this trace out? Sounds like an interesting device.
Is this same type of device that a private locator company might employ?
Seen some that advertise 20ft below grade.

You bill for the service that in essence could save a customer a lot of money by finding and pinpointing a break?

 

[ptonsparky]

I bought an Ideal stickered one of these about 35 years ago. This is the fault locator portion.

 

[ptonsparky]

Mine was the Ideal version purchased a week or two before the fault locator.
Neither have changed that I can tell from the pictures.

 

[kwired]

So how deep of underground wire will this trace out? Sounds like an interesting device.
Is this same type of device that a private locator company might employ?
Seen some that advertise 20ft below grade.

You bill for the service that in essence could save a customer a lot of money by finding and pinpointing a break?

If you seldom deal with underground installs, maybe not worth having. I work a lot of farms and need to locate existing underground quite often when doing new excavations. One call service gets the utility providers to locate their stuff, but they don't locate private lines, which there is often plenty of on some farms. All those private lines - the ability to find a bad spot comes in pretty handy as well. If you only do residential and lighter commercial in cities, chances are you don't need such locating ability all that often. Dwellings you maybe occasionally run into a feed to an accessory building, otherwise most everything else you need located is done by the one call services, or if it goes bad is the utility companies issue.

So yes when there is a break in a 1400 foot line to one my customer's irrigation wells, I can find it and repair it for quite a bit less than it cost to replace all 1400 feet. Also if it fails in middle of growing season, can do so with less damage to crops as I only need to get to the spot effected and not so much the entire path. There is need to locate the general path but that is damages done by foot traffic, only need to get heavy machinery to the spot where the break is once it is found.

 

[ptonsparky]

How deep?
We've had holes 5' deep. Not sure what the limit is.

 

[Hv&Lv]

So how deep of underground wire will this trace out? Sounds like an interesting device.
Is this same type of device that a private locator company might employ?
Seen some that advertise 20ft below grade.

You bill for the service that in essence could save a customer a lot of money by finding and pinpointing a break?

Deep? With triangulation deeper than you could or would want to dig.
About 6-7’ deep is as far as I want to go.

Private locators are doubtful. They are interested in locating a line only. Not on pinpointing a break.

I billed $400 for the first repair after I bought it and the guy was glad to pay it. Pulled the cash out of his pocket immediately.

 

[ActionDave]

Buying a locator and fault finder was one of the best investments my old boss ever made. Once word got out that we could locate and repair underground we would get calls to repair everything from power to a whole building to yard lights.

It became an annual rite of spring to repair feeds to irrigation pivots. I think the most I ever patched was four breaks in one feed, but not all on the same trip. That was when it became clear that a megger was an essential part of the process.