Above my pay grade Whats acceptable and whats not

[Timboe]

I had a problem at work. I was testing a main feeder at work so I called a high voltage maint. company in. So I have these 600kcmil wires run in conduit under ground appx. 1000 feet. It is a paralle feed two conduits with 3 wires a neutral and ground in each. I had this company that does this just thing meggar the wires and send me a report.
Phase
A 45.1 G ohms PI=2.61
A 9.82 G ohms PI=1.63
Phase
B 6.62 G ohms PI=1.46
B 1.70 G ohms PI=3.41
Phase
C 3.38 M ohms PI=1.00
C 11.2 G ohms PI=1.54
So as a result of these readings we are not useing the three worst wires per there advice. I was wondering what the polarization index means and how do you tell just by the ohm readings where the cut of point would be just by the numbers and not compairing the numbers on the better wires the to worst wires. Also this is a 480v three phase feed.

 

[Cow]

Do they only test to ground? Where are the other three measurements tested between AB AC BC? I've never done PI/DAR so I can't help you there.

The C that's testing at 3.38 M is a little scary. I really don't see a problem with the others since they are all in the Gigaohm range, even though they all test different.

Hopefully Zog will come along and set you straight with your numbers. He's way more of an expert than I am.

 

[Timboe]

Yes i beleive this was tested to ground. We now longer use the other wires in the conduit with the bad mega ohm wire. So we changed the breaker to now match the system with only 3 wires but thats a whole other topic. Do you know what a PI is.

 

[Timboe]

I think they unhooked the wires from both breakers and tested each wire independently. This is all that was given to me in there report listing the readings on all six wires.

 

[cadpoint]

Put the reading of per conduit together, frankly they should have done that anyways...

If your breaking up a conduit service of circuits that is a code violation.
IE A service in conduit one and B & C in another...

 

[Timboe]

No we are using all three wires in one conduit and just abandoned the other conduit and wires in the conduit with the wire that is bad. All the wires we are now using are in the same conduit.

 

[George Stolz]

So as a result of these readings we are not useing the three worst wires per there advice. I was wondering what the polarization index means and how do you tell just by the ohm readings where the cut of point would be...

The PI is the ratio between the reading at 1 minute and ten minutes.
http://www.predictivemaintenance.co.za/polarizationindex.php

To find the bad spot, you would need an underground fault locator. If this weren't 600s a 1000' long I'd be quick to say to jerk it out and replace them. Since they are likely THHN conduit this may be your best bet. I'd love to hear a better option.

 

[Timboe]

Thanks, There is no better option than to replace them.

 

[Hv&Lv]

An underground fault locator would probably be useless since the wires are in conduit. The fault locator depends on earth return. Unless, of course, the conduit is full of water and the joints at the bad spot leak.
A TDR would be a better choice. The radar screen will show a slight upward blip where the resistance is higher. The blip would look similar to a splice signature. See if the company you used has a TDR machine.

 

[Timboe]

I dont think this will work because there are at least ten pipes side by side in one ditch. This is how it is on the building drawings.

 

[Timboe]

So is this low reading of 3.38 m ohm mean that the wire is no good. I was told many times that anything over 1 meg per 1000volt was considered acceptable.

 

[George Stolz]

See if the company you used has a TDR machine.

If they did, or even a fault locator for that matter, would it matter? How would you repair it, knowing where the break was?

So is this low reading of 3.38 m ohm mean that the wire is no good. I was told many times that anything over 1 meg per 1000volt was considered acceptable.

It is "scary." It shouldn't be that low, and it's wise to consider repairing it in a scheduled timely manner as opposed to after the conductors fail. What were the readings from neutral to ground in those conduits, did they tell you?

I guess the next question from us is, what prompted you to have the testing done? What was the original problem?

 

[zog]

I had a problem at work. I was testing a main feeder at work so I called a high voltage maint. company in. So I have these 600kcmil wires run in conduit under ground appx. 1000 feet. It is a paralle feed two conduits with 3 wires a neutral and ground in each. I had this company that does this just thing meggar the wires and send me a report.
Phase
A 45.1 G ohms PI=2.61
A 9.82 G ohms PI=1.63
Phase
B 6.62 G ohms PI=1.46
B 1.70 G ohms PI=3.41
Phase
C 3.38 M ohms PI=1.00
C 11.2 G ohms PI=1.54
So as a result of these readings we are not useing the three worst wires per there advice. I was wondering what the polarization index means and how do you tell just by the ohm readings where the cut of point would be just by the numbers and not compairing the numbers on the better wires the to worst wires. Also this is a 480v three phase feed.

600V cable should be at least 100M per ANSI/NETA standards, so there is some concern on that set of phase C that is 3M. George explained PI, very odd to see that on a LV cable test report PI is usually done on motors and transformers. This is an odd looking test report and before I go replacing this cable I would have another testing company come look at it and re-test. (And watch them this time and ask questions, us testing guys actually enjoy when the customer is present and interested)

 

[Hv&Lv]

If they did, or even a fault locator for that matter, would it matter? How would you repair it, knowing where the break was?

My point is to see where the break is and to find out why the break is there. Has the pipe been damaged in some way? Before I try to replace a run that long, I want to know if the pipe is burnt on the inside, damaged from dig ins or bore ins, or seperated at a joint.
If you are lucky, the wire was nicked at installation and can be replaced with no problem.
Personaly, I won't replace a run that long without knowing why the failure was there in the first place. Of course, not many people have to guarantee their installations forever.

 

[petersonra]

I had a problem at work. I was testing a main feeder at work so I called a high voltage maint. company in. So I have these 600kcmil wires run in conduit under ground appx. 1000 feet. It is a paralle feed two conduits with 3 wires a neutral and ground in each. I had this company that does this just thing meggar the wires and send me a report.
Phase
A 45.1 G ohms PI=2.61
A 9.82 G ohms PI=1.63
Phase
B 6.62 G ohms PI=1.46
B 1.70 G ohms PI=3.41
Phase
C 3.38 M ohms PI=1.00
C 11.2 G ohms PI=1.54
So as a result of these readings we are not useing the three worst wires per there advice. I was wondering what the polarization index means and how do you tell just by the ohm readings where the cut of point would be just by the numbers and not compairing the numbers on the better wires the to worst wires. Also this is a 480v three phase feed.

I don't see how the very low reading can be good. The other ones seem Ok to me. I think I would be inclined to pull it out and replace it. Not fun when there are three of them and 1000 foot long run. I am guessing you will end up replacing all four of them that are in that conduit.

I think Zog's idea of having another look at it first makes sense though. I can't imagine pulling this wire out and replacing it is going to be a lot of fun or cheap.

I would want to find out how it happened as well. If you have some kind of issue inside the pipe, you could well damage the replacement.

Is there by any chance a spare conduit?

 

[Timboe]

Ok heres the situation. I was called to work on Sunday to see why the lights were out. When I got there i found more than the lights were out there was also a few other panels with no power. Went to the switch gear and found the 800 amp breaker tripped. The breaker is electonic controlled went to the screen and found it tripped on a ground fault. Tested between all phases between b and c had continuaty but went to zero. Suited up reset the breaker lasted about 10 minutes tripped again. called in the experts because 4 weeks before this a 600 amp breaker tripped and blew all three wires apart. We had to install new conduit but ran the new conduit in the ceiling and pulled all new wires 1300 feet. Yes we had to contract it out very large bill. But on the current situation after we put in a temperary breaker because I only used the three better wires the ac unit one the roof had a compressor bunr up which could have been causing the ground fault condition on the electronic breaker. Alot of things have been happing and I think there is something going on under ground. We are going to have the whole system tested. The building is only 7 years old and is 500,000 sq feet. Alot of info here and is kind of hard to explain without writing two pages worth of facts as they happened.

 

[zog]

Ok heres the situation. I was called to work on Sunday to see why the lights were out. When I got there i found more than the lights were out there was also a few other panels with no power. Went to the switch gear and found the 800 amp breaker tripped. The breaker is electonic controlled went to the screen and found it tripped on a ground fault. Tested between all phases between b and c had continuaty but went to zero. Suited up reset the breaker lasted about 10 minutes tripped again. called in the experts because 4 weeks before this a 600 amp breaker tripped and blew all three wires apart. We had to install new conduit but ran the new conduit in the ceiling and pulled all new wires 1300 feet. Yes we had to contract it out very large bill. But on the current situation after we put in a temperary breaker because I only used the three better wires the ac unit one the roof had a compressor bunr up which could have been causing the ground fault condition on the electronic breaker. Alot of things have been happing and I think there is something going on under ground. We are going to have the whole system tested. The building is only 7 years old and is 500,000 sq feet. Alot of info here and is kind of hard to explain without writing two pages worth of facts as they happened.

Did you tell your testing guys all of that? If you did, and you still have issues, you need a better testing company. Even good companies can have bad techs. Where are you located? I know good companies nationwide that can help you, but this sort of thing is hard to do on a forum.

 

[Timboe]

Yes we are near pittsburgh PA and the company we used is very good but even if the AC unit on the roof caused the ground fault trip it is a good thing because we found that wire with the 3 mega ohm reading. The best thing to do is test the whole system at this point. We had to get a section of the building replaced due to sinking so this could turn into something bad altogether. All have answered my questions and I have more info today than I had yesterday.
Thank you