High Voltage Amperage Leak Allowance

[ScottGibson6897]

I am on a project and one or more of the maintenance personnel and myself are not sure what the correct leakage in micro amps is allowed when HIPOT testing a 5kv cable with two splices. I am seeking as much info on this type of installation as possible.

 

[zog]

There is no spec, the spec would depend on the cable type, age, size, and length so publishing a spec would be impossible. You need to plot the values over the 15 minute test a look for the proper shape while comparing phases.

Is this an existing cable >5 years? If so you should not do a DC hipot test, it is considered destructive. A DC hipot will only tell you if you have gross installation failures, it is like going to the doctor running on a treadmill and if you dont die the doc says you are OK. A VLF AC hipot or better yet a Tan Delta test will give you a much better idea of the condition of your cable. Refer to IEEE 400 for details.

 

[ScottGibson6897]

more info

more info

Part of this feeder is an older cable and has possibly been submerged in the flood waters in south Louisiana for up to a week. The cable was cut at points where it was believed to be out of harms way and spliced on both ends. The total run is approx. 1700' with 1300' being salvaged cable. Is it true, that if the reading holds steady or drops without any rise in the micro amp loss, that the cable is satisfactory? We are within 2-3 days of energizing and I don't want to take any unnecessary risk. Is there any documentation that I can present to an individual that will explain that a DC Hi Pot test could damage an older cable?

 

[zog]

AS long as the current does not spike or rise it should be safe to energize, dosnt mean the cable is good, just that it probally wont blow up.

AS far as references for DC hipot being destructive on cables aged >5 years you can look at:

ICEA specs
IEEE 400
IEEE 576
AEIC specs
NETA specs

They all agree on the DC hipot issue. Now that said, a DC test in your situation is better than doing nothing. I would recommend finding a NETA testing company that does VLF or Tan Delta testing.

You may hear about VLF testing, great test for many situations but long cable runs with different dielectrics used confuse the results, not the best test for the cable you are describing.

 

[ScottGibson6897]

Thanks for the input...

Thanks for the input...

With your responses and a little more research...a decision was made not to further test the existing cable due to the fact that it was an older cable that has a XLPE type insulation that could be damaged by dc test voltage. The new cable that is spliced into the existing cable has a EPR type insulation and would not be damaged by dc voltage, but it was not required that this particular cable be Hi Pot tested. I only have one more question if you have time to answer. With a reading or leakage of 1.5 micro amps on our test equipment...would this be any concern for future testing? If not, at what point should there be a concern that a cable or stress cone may be damaged or not acceptable?

 

[zog]

As a very general rule of thumb, when you start seeing about 100 microamps you should be concerned and look at doing a better condition assesment test like I mentioned before.

You need to do some test on this cable, even though a DC hipot can be destructive it is still better than not doing any test at all. Worst case you hipot the cable and damage it and it fails. If you didnt test that cable it was going to fail anyways, when you energize it and it fails you may damage other equipment or injure someone.

 

[Greg Goodell]

Megging:

Megging:

Our company just had 4 sets of 750 mc cables [alumi] megged by a electrical contractor & there report said all 12 conductors megged 1000 to 1500 megaohms or more. They claimed all were good & nohing was wrong with the cables.
Within 48 hours of there finding we had the 1600 amp breaker that these cables were on trip & blow a hole in 1 of the cables. This shut down the manufacturing part of our company for hours, with los of mdse. & time.
Could you explain how this could happen if the megger job was done correctly?
They are taking no responsibilty for there work, but will be glad to install new feeders at a premium price.
Thanks for any advice.

 

[zog]

Provide more info about the cables and the test voltages and methods used to test.

 

[Designer69]

I thought readings of at least 50 Mohms or higher were acceptable for megger tests!?

 

[zog]

Depends on what you are testing. Depends on what specs you are following. As a NETA Certified guy I can never accept 50 Meg.