[A Little Short]

I had a service call for flickering/dim lights and some receptacles not working. It was in a barn about 200' from the house main.

First I thought he had a loose or open neutral, but the voltage measurements didn't show that. I only showed around 190V to the sub panel at the barn but there wasn't any high voltage on one of the legs.
I had around 72V on one leg-neutral and around 119V -neutral on the other.

So I decided to meg the conductors. This is where I'm confused.

Readings from: Line A to neutral - 11G
Line A to ground - 11G
Same with Line B to neutral/ground

From line to line the reading was 0.0 M

I did determine from the voltage readings that the A leg conductor was bad. Probably underground somewhere since this was an underground feed. I just can't figure out why the reading was the same on both legs to neutral and ground.
I would have thought the bad conductor would have had a different reading.

I'm guessing the low reading on line to line is due to insulation damage along with damage to the wire its self. But why wouldn't it show on line to neutral and ground testing?

 

[Spunk#7]

Time to thump the leg with low voltage.

 

[Celtic]

Time to thump the leg with low voltage.

How?

 

[Cletis]

How?

For 1 thing 11g is low for a meg reading

 

[A Little Short]

For 1 thing 11g is low for a meg reading

:blink::no:

 

[dronai]

What make and model megger do you have ?

 

[Cletis]

Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect

 

[bobelectric]

We are talking meg ohms,not gigabits,right ?

 

[bobelectric]

Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect

20M I would say OK.

 

[Cletis]

20M I would say OK.

I've generally noticed micro-shorts or intermittent shorts happen between say 5000 ohms and 1 M Ohms on the 500 V pulse. Seems like dead shorts read from 0-5000 ohms

 

[varmit]

Questions and comments:

A. Did you have the "main" breaker off in the barn - to avoid continuity trough any loads?

B. Is this wire aluminum? Aluminum wire is prone to dissolving if the insulation gets even a small pin hole in the insulation and the soil ph is acidic. Copper is susceptible to this also, it only takes a little longer for copper to break down.

The cause of this conductor breakdown is the inductive heating between the conductors caused by the relatively low resistance between conductors and the inductive current caused by the downstream load.

C. 11 meg would be below the normal threshold for use. I would usually like to see around 50 meg ohm.

D. If the ground is rocky, it is almost certain to cause problems for direct burial conductors. Of course, even if the conductors are installed in conduit, which will almost always fill with water, and the conductor insulation is damaged during installation, there will be problems.

 

[RobRoy]

For 1 thing 11g is low for a meg reading

Well Cletis, 1 giga ohm, is 1000 mega ohms. So, 11gohm is 11000mohm. Seems like a misunderstood interpretation on your part.:whistling2:

 

[brian john]

11 gigohms =11,000 megohms

Direct burial?

It would be very unusal to have 11G Line to N and have dead short phase to phase

Did you isolate both ends of the circuit?

You have a load on the circuit? MY BET for now

 

[Cow]

Is it aluminum wire? Direct buried? What are the voltage readings when the barn main is shut off? Is the wire properly sized for voltage drop and the load at the barn?

 

[gold]

Did you open the ground and neutral conductors and meg them with the breaker open with respect to each other and each hot?

What kind of loads are on it? Wire size etc

 

[A Little Short]

What make and model megger do you have ?

Fluke 1507

Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect

G= gigaohms = a sack full of megaohms:laughing:

Questions and comments:

A. Did you have the "main" breaker off in the barn - to avoid continuity trough any loads?

B. Is this wire aluminum? Aluminum wire is prone to dissolving if the insulation gets even a small pin hole in the insulation and the soil ph is acidic. Copper is susceptible to this also, it only takes a little longer for copper to break down.

The cause of this conductor breakdown is the inductive heating between the conductors caused by the relatively low resistance between conductors and the inductive current caused by the downstream load.

C. 11 meg would be below the normal threshold for use. I would usually like to see around 50 meg ohm.

D. If the ground is rocky, it is almost certain to cause problems for direct burial conductors. Of course, even if the conductors are installed in conduit, which will almost always fill with water, and the conductor insulation is damaged during installation, there will be problems.

11 gigohms =11,000 megohms

Direct burial?

It would be very unusal to have 11G Line to N and have dead short phase to phase

Did you isolate both ends of the circuit?

You have a load on the circuit? MY BET for now

Is it aluminum wire? Direct buried? What are the voltage readings when the barn main is shut off? Is the wire properly sized for voltage drop and the load at the barn?

Did you open the ground and neutral conductors and meg them with the breaker open with respect to each other and each hot?

What kind of loads are on it? Wire size etc

**Some answers in red above**

It is direct burial XLPE 2-2-4-6 AL

The readings (Meg) were taken with both ends of all 4 conductors disconnected.

As far as the voltage readings they were both with the main on and off.
There was a slight (2 volt) drop when the few loads that were on that were on the good leg.

I don't know if the ground was rocky or not. The feed goes through the edge of the horse pasture and then under a paved drive.

As far as loads, mainly just lights. There is a small water tank heater 120V that is used to keep the water trough from freezing.
There is also a 240V water heater, but obviously it wasn't working with one leg bad.

I do know there is some voltage drop because of the distance, but everything has been working ok until they lost one of the legs.

I'm not even going to try and locate the break in the conductor. I'm just going to refeed the barn.

I just don't understand why I didn't get a low reading on the bad leg to ground and or neutral, only line-line.

 

[frenchelectrican]

I was reading this and the megaohms level you have is too low to be ok for be used as it is and you allready have a direct short or blowen conductor one of the two.

The biggest killer will be near the building where the conductours come out of the ground and paved driveway if buried less than .6 meter deep and paved driveway is good place for failure.

If this conductor been in the ground for more than 15 to 20 years then it will be a good time to replace it anyway due over the time you will get a pinhole leak on the conductour and it will eat away the alum conductour pretty good.

Merci.

 

[nrp3]

I have pulled out a section of 500 AL that was gone four inches of aluminum just from one pinhole.

 

[A Little Short]

I was reading this and the megaohms level you have is too low to be ok for be used as it is and you allready have a direct short or blowen conductor one of the two.

The biggest killer will be near the building where the conductours come out of the ground and paved driveway if buried less than .6 meter deep and paved driveway is good place for failure.

If this conductor been in the ground for more than 15 to 20 years then it will be a good time to replace it anyway due over the time you will get a pinhole leak on the conductour and it will eat away the alum conductour pretty good.

Merci.

I don't know how long the paved driveway has been there but it was definitely put in after the conductors. But everything has been working with the driveway in place for a while.
So I suspect the driveway and ground may have settled and has just now affected the conductors.

That is if that is where the break is. But is for sure a leading suspect!

 

[brian john]

I have pulled out a section of 500 AL that was gone four inches of aluminum just from one pinhole.

YEP aaaaaaaa