megger question.

[zog]

we used a high freq tester on cables to check for pinholes

You cant use high freq for cable testing, I mean you could if you had a huge power supply but even a standard 60hZ AC hipot is difficult to use for cable testing, most testing companies use VLF (.1-0.1 Hz) for cable testing.

cvn-68 nimitz

Have you been watching the series "Carrier" on PBS? It is fantastic, unlike any navy documentary I have seen, they are following the Nimitz on a westpac.

 

[zog]

well high potting is a common test we do on every job , we use high voltage ac or dc test sets .Example of test we start at 1000volts then we go up in 1000 volt steps each step is a 5 minute test , we look for micro amps charging or discharge time it tells us all about that cable and whats wrong with it , we look at voltage holding time and leakage during the 5 minute test ramping up in voltage can tell us if the cable or conductor is cut or just a pinhole , if its got a bad termination or boot or if its a leak in the loadbreak or internally shorted . but the best tester is the high freq tester at 20,000 cps 45,000 cps at high voltage say 50,000 vac it could find a pinhole in a conductor so small you could never find ever with any common tester.Sometimes its very important to test cables or conductors in a area of say a nuke plant or a area of high degree of you can not fail ever !! and the test is engineered & spec on this stuff is most important . Best to ya Bill i just learned this at work and in the military if i find a link ill try and post it .

You do know that DC hipot is not allowed on in service MV cables right? IEEE, ANSI, NETA, and ICEA all agree that DC hipot can cause premature failure of MV cables and is considered a destructive test. If you are in Nuke plants you are probally following the EPRI cable testing procedures which have been revised to reflect this. I was part of the EPRI cable testing development commitee on this exact topic. EPRI is leaning towards tan delta testing for all nuke plants but will also allow VLF or PD for certian cable types.

 

[zog]

Here is that thread, long but should be referenced here.

http://forums.mikeholt.com/showthread.php?t=101447&highlight=hipot

 

[wptski]

Well we rap or seal the opposite end of cable under test we use just your standard plastic handy wrap that you put your sandwich in everyday its rated for high voltage if left open it will pass high voltage to ground thur the air ,you can hear it hissing . The other end the part that the anode high voltage test end has a special cap over the conductor end or terminal sealing it airtight. Then we test it in steps this can take on the average 45 minutes to one hour on each cable or conductor lots of fun, The milliamp current meter is the key you must watch it at all times you can actually blow out a cable thats good if you do not test correctly . Thats why we ramp up voltage in slow steps so we dont blow out a cable , milliamps running up high fast dead short to ground . milliamps leaking down slowlly holding and then we increase voltage slightly and if it bleeds down again its a cut , nick ,pinhole in jacket 90% of the time ,we pull cable out of raceway and run it thur a water tank off a roll to roll you use a standard megger at 1000 volts and guess what you fine nick, cut , pinhole . once a cable is pulled out it is trashed hv cables are not repaired its real expensive ,so i get paid well by my company . best to ya

I like that detailed procedure as it verifies what I said about the cut end of a cable. Yes, I understand "any" nick or pinhole will show up if dunked in water.

I read some instruction for a Biddle/Megger high pot last night and it also mentioned finding pinholes too. It also mentioned actual arcing through the insulation but these are 4/5KV AC/DC and DC units.

 

[wptski]

Have you been watching the series "Carrier" on PBS? It is fantastic, unlike any navy documentary I have seen, they are following the Nimitz on a westpac.

They showed that a couple of months ago. It was very good. Home comings always bring tears to my eyes!

 

[ohmhead]

High freq is really low freq what!!

High freq is really low freq what!!

Well Zog , I stand corrected , When we say high frequency we mean just that but low freq ultrasonic testing we use it to pin point and pinhole or leakage in a cable or termination ,which can point directly to the trouble ,we test with full power on or use with the pd test set to check our work . my 20,000cps 40,000 cps was sound wave freq not ac freq of testing voltage sorry . We had these in the navy years ago now its new tech stuff ! but we today still test the old dc way . I have not worked nuke since navy days we just do small plant work and industrial or commercial , we mostly test laminated insulation types but cross link gets pd tested we follow what engineer specs or what ever my boss says to use . best to ya .

 

[electricalperson]

You do know that DC hipot is not allowed on in service MV cables right? IEEE, ANSI, NETA, and ICEA all agree that DC hipot can cause premature failure of MV cables and is considered a destructive test. If you are in Nuke plants you are probally following the EPRI cable testing procedures which have been revised to reflect this. I was part of the EPRI cable testing development commitee on this exact topic. EPRI is leaning towards tan delta testing for all nuke plants but will also allow VLF or PD for certian cable types.

i never known a DC hi pot test is destructive. thats a good bit of information. i dont do much hi pot testing anyway. very rarely usually only do it when we pull in MV feeds on private property for the power company. theres also not many factories or other large industrial places near me that need MV cable for motors or whatnot

 

[ohmhead]

manufacture hv cables thinner is more sales

manufacture hv cables thinner is more sales

Well for years no one would like it when it was a old cable exsiting ,we only test new stuff thank goodness , but yes mv cable old no one wants to test that its a blow out waiting to happen ,so now they have pd & tan delta testers to help out . new cables are thinner skinned insulation is cut down so thin it can not take a real test of the old dc hi pot , sorry cable manufactures thats my thinking . If you design a mv cable it should hold up laminated cables hold up ? comments take care

 

[zog]

Well for years no one would like it when it was a old cable exsiting ,we only test new stuff thank goodness , but yes mv cable old no one wants to test that its a blow out waiting to happen ,so now they have pd & tan delta testers to help out . new cables are thinner skinned insulation is cut down so thin it can not take a real test of the old dc hi pot , sorry cable manufactures thats my thinking . If you design a mv cable it should hold up laminated cables hold up ? comments take care

It is a matter of the materials used, not thickness of insulation. XLPE cables are cheap, thats why people buy them, they are the ones that have issues with DC hipotting and water trees, even the XLPE-TR is not much better. EPR is a moe expensive option but lasts longer and take take more abuse from the environment, spikes, or testing. .