Direct burial Underground feeder going bad?

[Sierrasparky]

I don't have much experience with DB Aluminum conductors of the 1960's.
I got a call to look at a Direct Burial (DB) feeder. It was individual AL conductors 120/240 service DB. The breaker has tripped on occasion on only one leg. I decided to megger the run as I was told it was DB and it was working when I arrived and I was assured that no shorts or faults were present. All breakers were on and functioning. So de energized and I removed the conductors from the terminals and Meggered them with a Biddle Digital Megger. I measured only 16M.
The underground run is roughly 240' 1/0 AL. I have never seen a megger reading so low in a functioning feeder before. I had read that 1 megohm is good for this voltage. This seems odd as I have Megged new conductors of similar length and show many times greater.

My question are these conductors on the way to failing completely and maybe tripped the breaker due to a fault that blasted clear for the time being?

 

[LEO2854]

I don't have much experience with DB Aluminum conductors of the 1960's.
I got a call to look at a Direct Burial (DB) feeder. It was individual AL conductors 120/240 service DB. The breaker has tripped on occasion on only one leg. I decided to megger the run as I was told it was DB and it was working when I arrived and I was assured that no shorts or faults were present. All breakers were on and functioning. So de energized and I removed the conductors from the terminals and Meggered them with a Biddle Digital Megger. I measured only 16M.
The underground run is roughly 240' 1/0 AL. I have never seen a megger reading so low in a functioning feeder before. I had read that 1 megohm is good for this voltage. This seems odd as I have Megged new conductors of similar length and show many times greater.

My question are these conductors on the way to failing completely and maybe tripped the breaker due to a fault that blasted clear for the time being?

I think that may be the case.

 

[RLyons]

The breaker actually tripped? I've done a few underground service break locating and repair and never had a breaker trip???? I've had underground light feeder/UF trip breakers but never a service? Most times the culprit is within 5' of the meter or the hand hole...and a couple landscaping incidents.

 

[Sierrasparky]

This is a feeder from the service to another building. One leg was dead ,tested at the breaker. None of the branch breakers were tripped in the remote building.

I could not tell if it was tripped or not completely reset from being off in the winter. The breaker may have been cycled by the home owner who knows.

I meggered it because there have been UG failures at this site before. Usually no point in that case because the wire blasted clear apart. All we needed to do was locate the where the wire went bad and splice or replace complete.

 

[kwired]

This is a feeder from the service to another building. One leg was dead ,tested at the breaker. None of the branch breakers were tripped in the remote building.

I could not tell if it was tripped or not completely reset from being off in the winter. The breaker may have been cycled by the home owner who knows.

I meggered it because there have been UG failures at this site before. Usually no point in that case because the wire blasted clear apart. All we needed to do was locate the where the wire went bad and splice or replace complete.

If your meg reading is high then you are looking at the wrong place for reason for breaker tripping. If conductors are faulted to earth they are not going to trip breaker anyway, fault current may be only a few amps at most. I'd be looking at either a malfunctioning breaker, hot breaker termination, or a fault in other connected equipment.

 

[ptonsparky]

16 Meg is not great but also not likely to trip a breaker. We have had them fault and burn enough that they eventually go phase to phase an blew POCO transformer fuse but that was 480.

A small hole in the insulation while the conductor itself is almost completely gone is a possibility.
What is the load side voltage like while under load?

 

[kwired]

16 Meg is not great but also not likely to trip a breaker. We have had them fault and burn enough that they eventually go phase to phase an blew POCO transformer fuse but that was 480.

A small hole in the insulation while the conductor itself is almost completely gone is a possibility.
What is the load side voltage like while under load?

120v / 16,000,000Ω = .0000075A, I think I have the decimal in the right place, this may not trip the breaker.

 

[ptonsparky]

120v / 16,000,000Ω = .0000075A, I think I have the decimal in the right place, this may not trip the breaker.

"not likely" sounds somewhat better than "get your head out of your..."

 

[kwired]

"not likely" sounds somewhat better than "get your head out of your..."

US Army basic training in 1986, "get your head out of your...", was most likely to be heard, and was quite effective:happyyes:

 

[Sierrasparky]

The conductors currently meg out at 16 Megohm. This is in dry soil conditions.

Who is to say what the reading is with saturated soil! This is not a residence and the breaker could have been tripped for months as the area was no used until now.

 

[kwired]

The conductors currently meg out at 16 Megohm. This is in dry soil conditions.

Who is to say what the reading is with saturated soil! This is not a residence and the breaker could have been tripped for months as the area was no used until now.

Even if you got down to 5 ohms with 120 volts to ground you only have 24 amps of current flow. 15 and 20 amp branch circuits will trip but 30 amp circuits or larger will never see any overcurrent (with no other load).

 

[Sierrasparky]

Even if you got down to 5 ohms with 120 volts to ground you only have 24 amps of current flow. 15 and 20 amp branch circuits will trip but 30 amp circuits or larger will never see any overcurrent (with no other load).

You are forgetting one thing. This is dry well drained soil right now. When the breaker possibly tripped the soil was very wet. The conductors run buried across a seasonal creek. I would bet they are no more than 18 or less inches. In saturated conditions I would bet the readings would be substantially less.
My concern is that is this feeder considered to be failing?
Should it be replaced?

 

[ptonsparky]

Get your locator out, find and fix the fault. We find, cut, test again and repeat as needed before splicing.

 

[Sierrasparky]

Unfortunately I know of no one in my area that have such a device to locate a breech. There could be several areas that have been damaged over the years.

 

[ptonsparky]

Unfortunately I know of no one in my area that have such a device to locate a breech. There could be several areas that have been damaged over the years.

Then I see a good opportunity for you to fill a need. I never regretted that purchase.

 

[kwired]

Then I see a good opportunity for you to fill a need. I never regretted that purchase.

Same here.

I will also say the only times I have ever found a cable that needed complete replacement was either because it was extremely short or there was a lot of rodent damage the entire length or run. Maybe a rare case of lightning damaging insulation at times, but lightning usually just finds a weak point in the insulation and causes further damage there.


If it is underground aluminum it doesn't develop a low enough resistance to trip circuit breakers. I have never seen a case where it was even noticed there is a problem until the conductor is deteriorated enough that it is completely open circuited.

My suspicion is that even if there is any underground fault activity, unless the fault is between conductors, and with only 120 volts to ground that any breaker tripping is not associated with the fault.