hipot testing

[vegasbaby]

I need to hipot test a 15kv cable that was shorted by a subcontractor and is 1.5 miles long. I want to check the cable on either side of the short at one of the 22 vaults. The owner wants to test it at the distribution station and replace all of the cable even thou the cable is spliced at every vault. I want to only replace what falls below standards and feel that only the area that was shorted should be effected

 

[hurk27]

I would not get in between a battle of what the owner wants and the liability of the damage the subcontractor may or may not have caused, it's really not your call unless your the sub contractor who did the damage, if the owner wants the test of the whole run then thats what I would do, and if he is willing to pay to have the whole run replaced instead of repaired then make the money, just be sure you put in your contract that the owner was advised that the damaged cable could be replaced between termination points, but they opted to replace the whole run.

also make sure the owner knows they are responsible for payment to you not the subcontractor who damaged the cable, it was their property that was damaged, it was them who requested the repair, it is them who must pay you for your work, it is up to them to collect from the other parties.

I have had a few GC's try to tell me to go after the party who damage something more then once, but I flat out told them is wasn't my property that was damaged and I would not have any recourse to take the offending party to court in the event of non-payment, the contract is with the owner of the property that was damaged.

Don't get caught up in this trick.

 

[mkgrady]

I need to hipot test a 15kv cable that was shorted by a subcontractor and is 1.5 miles long. I want to check the cable on either side of the short at one of the 22 vaults. The owner wants to test it at the distribution station and replace all of the cable even thou the cable is spliced at every vault. I want to only replace what falls below standards and feel that only the area that was shorted should be effected

Can you tell us more of what happened? If the run has a "short" you already know it will fail a hipot test?

Was a loop feed crossed phase and turned on? Did the run just fail a hipot test after installation? Was it ever hipotted? What are the splices? Load break, deadbreak elbows, hand tape?

 

[K8MHZ]

A hi-pot for locating shorts????

Wrong tool for the job.

1) Hi-pot testing is considered destructive testing. It is entirely possible to damage the cable under the test.

2) Destructive testing is falling by the wayside. Non destructive tests are taking it's place. Time domain reflectivity and ELF tests are now dominating the avante guard testing market.

3) Hi-pot testing is not a one shot deal. Since the test is designed to measure insulation integrity, not as a troubleshooting means, each test consists of varying voltages and measuring the leakage over time in order to create a graph that is illustrative of the integrity of the insulation. The first test after installation is the base for further tests as time goes by to monitor the degradation of the insulation.

4) Hi-pot testing is dangerous. A 15kV cable should be tested at around 75kV with the exact test voltage dictated by the engineering department of the cable manufacturer. For instance, I tested an 18kv iso-phase conductor at 63.5kV based upon the engineering standards. Rule of thumb would have been too high, but still, when the tester is doing it's thing, hundreds of feet of conductor is charged to several times it's nominal rating with a possibly fatal amount of electricity.

5) Hi-pot tests leave residual charges sometimes lasting for several minutes, even when a drain is connected. We had one long run of high voltage DC cable that still had several hundred volts in it after a 15 minute drain. Remember, shielded cable is not just wire, it is a HUGE capacitor.

That being said, you should use something else to locate the short. A megger would be better and I have a DVM that measures down to 2 meghoms that works great and is over half as accurate as a hi-pot and uses far less voltage, reducing the chances of damaging the insulation.

Once the short is located and repaired, THEN it's time to use the hi-pot if and only if the maker of the cable is in favor of it and gives you the proper test voltage and plotting parameters. The biggest problem would be that if you don't have access to the original test results, the results you get may have little or no value. If all you are looking for is a pass or fail, a megger may indeed be a better tool.

 

[vegasbaby]

hi-pot

hi-pot

I didn't explain my problem very well, sorry. The cable was damaged by a subcontactor using a auger for a fence post. The wire was later dug up to reveal the damage across two phases. I would like to test the wire at the vaults close to the damage not just on either end of the whole run. My plan is to reuse most of the wire, the owner wants all new wire.

 

[mkgrady]

I didn't explain my problem very well, sorry. The cable was damaged by a subcontactor using a auger for a fence post. The wire was later dug up to reveal the damage across two phases. I would like to test the wire at the vaults close to the damage not just on either end of the whole run. My plan is to reuse most of the wire, the owner wants all new wire.

Replacing the whole run seems like a total waste. Do they have some theory as to why that is necessary?

On the other hand I wish I were the contractor replacing the whole run. Heck, why not replace the distribution switchgear and all the downstream transformers while you are at it?

 

[zog]

I didn't explain my problem very well, sorry. The cable was damaged by a subcontactor using a auger for a fence post. The wire was later dug up to reveal the damage across two phases. I would like to test the wire at the vaults close to the damage not just on either end of the whole run. My plan is to reuse most of the wire, the owner wants all new wire.

Still, a DC hipot is considered a destructive test for service aged MV cables by ANSI, NETA, IEEE, and ICEA. Recommended tests include one of the following
-VLF
-Tan Delta
-Partial Discharge

I recommend you hire a qualified testing company before you damage the rest of the cable. There are a few in Las Vegas, if you want to PM me I can recommened someone for you.

 

[K8MHZ]

Still, a DC hipot is considered a destructive test for service aged MV cables by ANSI, NETA, IEEE, and ICEA. Recommended tests include one of the following
-VLF
-Tan Delta
-Partial Discharge

I recommend you hire a qualified testing company before you damage the rest of the cable. There are a few in Las Vegas, if you want to PM me I can recommened someone for you.

Our POCO no longer uses destructive testing, even on new cables. VLF was used to test the last run of MV's I did and that was several years ago. Pooh! All my Hi-pot training and experience is now pretty much useless.....

 

[K8MHZ]

The owner wants to test it at the distribution station and replace all of the cable even thou the cable is spliced at every vault.

If that is an error, it is one on the side of safety.

Why would you want to talk him out of it?

Sounds like a pretty good job to get, and jobs aren't all that easy to come by right now.

 

[zog]

Our POCO no longer uses destructive testing, even on new cables. VLF was used to test the last run of MV's I did and that was several years ago. Pooh! All my Hi-pot training and experience is now pretty much useless.....

Time for new training, ya have to change with the times. Really VLF is just a low frequency AC hipot, Tan Delta can be done with an accesory attached to your VLF test set, and is more or less the same principles as a PF test. PD, that is a little more complex.

 

[ceb58]

I didn't explain my problem very well, sorry. The cable was damaged by a subcontactor using a auger for a fence post. The wire was later dug up to reveal the damage across two phases. I would like to test the wire at the vaults close to the damage not just on either end of the whole run. My plan is to reuse most of the wire, the owner wants all new wire.

If that is an error, it is one on the side of safety.

Why would you want to talk him out of it?

Sounds like a pretty good job to get, and jobs aren't all that easy to come by right now.

Sounds like the owner wants a lot of free cable on the back of the fence contractor. If he turns it into his insurance co. they will probably send in an elect. engineer to look the situation over. He may recommend testing the complete run but if it only tests bad between the vaults in the damaged run they will pay for that section only.
Same as when one of my guys broke a mirror on trim out. GC was screaming it was a $150.00 mirror. No problem. I took it to a glass shop and $20.00 later he had his mirror back. My guy broke it. I was responsibly for it. I fixed it. But there was no way I was buying a whole new mirror.

 

[G._S._Ohm]

TDR testing is nondestructive and will clearly show you what segments are significantly different [damaged] than the others.

More generally, paraphrasing what an attorney told me, the owner is entitled to a used cable that works, not a new cable.

 

[hurk27]

More generally, paraphrasing what an attorney told me, the owner is entitled to a used cable that works, not a new cable.

But it is not the place of the contractor to get in between what the owner wants and what the fence guy is going to pay for, this is a very dangerous position to be in as I tried to point out in post 2, I have seen this to many times, and the contractor gets stuck in between and often winds up in court fighting as to why he replaced the whole thing, this is why it is important to make the contract with the owner and spell out all the ifs, And's, and butt's in the contract.

And if the owner wants the whole cable replaced then he pays for it, and it is up to him to go after any other party's for the damage

 

[vegasbaby]

update

update

Cable was DC hipot tested at the substation. All 9,000 linear feet were pulled out before I got there. No test data recorded, only sworn statements.
Any comments on 27,000' of 15kv 600 mcm cable shorting out completely.
There were vaults every 1000'

 

[mkgrady]

Cable was DC hipot tested at the substation. All 9,000 linear feet were pulled out before I got there. No test data recorded, only sworn statements.
Any comments on 27,000' of 15kv 600 mcm cable shorting out completely.
There were vaults every 1000'

I don't see how the cable can fail in multiple locations. Cables typically fail at splices and terminations (if done improperly) or where it may have been damaged during installation, and that could be anywhere in the run. If you are asking if the whole run could have been bad we don't know because it has already been removed and it is too late to test it.

I don't understand why there was no attempt to find the actual problem. Why wasn't a thumper used to locate the fault? Why didn't just the faulted cable get replaced?

 

[eric9822]

Who paid the bill?

 

[vegasbaby]

hi-pot

hi-pot

No one paid the bill yet but the insurence company is on the hook. Still trying to see if it was justified to pull all the cable out.
Cable looked pretty rough when it was removed

To what extent would the cable have to be damaged for it to fail ?

 

[mkgrady]

No one paid the bill yet but the insurence company is on the hook. Still trying to see if it was justified to pull all the cable out.
Cable looked pretty rough when it was removed

To what extent would the cable have to be damaged for it to fail ?

If it failed a hipot test it would be no good. There is no set amount of leakage (you are measuring amps of leakage from the conductor to the shield) that would make a cable fail. It is a function of cable length, voltage rating, humidity, and the amount of corona taking place during the test.

No good reason has been given for replacing any more than the section of cable that failed a test.

 

[zog]

If it failed a hipot test it would be no good. .

On the other hand, just because it passes a DC hipot test does not necessarily mean the cable it good either. A DC hipot will only reveal gross installation errors.

 

[76nemo]

See the last sentence of his post again Zog.