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Cable hi-pot withstanding time, why 15 minutes?

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  1. Join Date
    Feb 2015
    Quote Originally Posted by moon1958moon View Post
    Hi everyone, in my instruction of cable hi-pot testing, the shield cable withstanding time is 15 minutes, un-shielded cable is 5 minutes.

    It is super boring to hipot a high voltage cable, (step up time plus 15 minutes withstanding time, about half hour per cable. could we reduce the withstanding time), could somebody please explain why it needs 15 minutes?

    Thank you.
    Man if you think that is boring you better hope you never have to test cables for the Federal Government! Spec at "No Such Agency" for 15kv cable is 4 steps at 5 minutes each for a TD reading and then 60 minute at full voltage for withstand.. It really gets to be an issue with staying awake at that point lol.

    EDIT: I forgot to mention that this is PER cable.. No tying them together. Bring a book!

  2. #32
  3. SuspectK is offline
    Junior Member
    Join Date
    Dec 2014
    ...sigh... Clarksville, TN
    Quote Originally Posted by robertellis87 View Post
    You can definitely tell the difference in the caliber of technicians with posts like that. Why does it matter that tech "A" has a level 4 NETA cert and and tech "B" has a level 2 NETA cert if the technicians take shortcuts like that? These certifications are a joke, especially when your pay/raises are dependent on them. What good does it do to obtain the certification if you're going to offer sub-par testing to your customer? If you hi-pot the three together and get 120 micro-amps of leakage current, but independently you would have seen 2 micro, 3 micro, and 115 micro respectively, you don't see that as a reason to investigate a little? Don't give me that "it didn't trip the test set so it passes" crap either. We all know you can spot a deteriorated cable/stress cone.
    IEEE standard specifies the monitored withstand as a go/no go test. It either holds or it doesn't. Period. DC hipotting is dead. The results are meaningless from a VLF withstand test, which is the industry standard now.

    Tan-Delta, regardless that there aren't acceptance standards from IEEE for it, is seen more and more as a requirement by commissioning agents these days.
    But to reply to OP's first post...

    Back when I did DC testing, it was the general gambit outlined down below and 60 minutes total on DC withstand...first 15 minutes or so were incremental steps of voltage, but I still tested all cables at once if they all passed insulation resistance tests, unless the customer wanted something else that made sense...no testing cables without terminations on them. I can't believe how many times I've ran into that.

    All VLF tests, the customer generally wants 60 minutes also. I thought IEEE 400.2 had an outline of the fault detection percentages broken down in withstand testing duration, 60 minutes being essentially 100% of all possible faults within the cable itself being discovered. I saw one user has had a similar thing laid out, but I'm certain this use to be in IEEE 400.2, IEC, or similar.

    I remember it being something along the lines of (these are rough estimates from memory):
    30 minutes- 72% Faults detected
    45 minutes- 84% Faults detected
    60 minutes- 99% Faults detected

    I've never been requested or taught a 15 minute monitored withstand test. I've only seemed to be involved with 60 minute dielectric withstand tests for cables...most of the time from customers/clients, we're allowed to test all conductors together if they pass tan delta(which apparently is pointless with acceptance testing according to the recent Power Test discussions).

    Generally, after DC stopped being a go-to method, I've done this:

    Cabinet inspection- ensure LOTO before touching, check for clearances for cables during testing, and ensure no sharp edges are there to damage stress cones.
    Phasing/Ring out and Shield cont- Ring out cables with shields or a jumper to check shield resistance and phasing and set numbers at the same time. (or confirm phase to phase with shield resistance measured by one end of shields being grounded.)
    Ensure cables are clear, and check again for sharp edges that might be in the cabinet when I'll be fumbling around with cables later on.

    I only start the tests below if the cables under test aren't manipulated after insulation resistance. Insulation resistance is to confirm the cable isn't completely screwed, but it's also to cover your ass in case it's laying up against a conductor for some odd reason.
    Insulation resistance- Phase to phase and phase to ground(I've noticed no difference in measurements with shields grounded or ungrounded on the same cables, but generally prefer to have them grounded by jumpers on both ends...in case partial discharge testing is required.

    Tan-Delta- generally three steps for acceptance, four for maintenance...the site I basically live on right now(for the foreseeable future) requires dielectric withstand for fourth step on accceptance tests, you can tell which techs are good on their setup and environmental control when reviewing reports with this added step. A lot of them hike up immediately on the 21/44 kV steps that we're using here....but what do you do about it? There's no absolutely no criteria regarding the fourth step.

    Monitored withstand- 60 minutes, with ~25% incremental steps with all cables at once if passed Ins. Res. and/or Tan-Delta.

    If any issues occur, offline partial discharge testing is performed with cables that are >260 feet(EPR) when it's determined what cables are the issues. If less than 260 feet, cables get new terminations, and rarely is it ever not just a sloppy term job. I haven't had to review or personally test a cable that was partial discharged. I don't believe it could work.... the PD signals from the far end would be overlapped by hellish noise if you tied them in series to get it long enough to catch measure the reflections.. if you didn't tie them in series, there would be no reflection.

    I love partial discharge testing.... I kinda get like a dog seeing a bone when there's a hint of it on site. It's really neat and intuitive test(if you know how to interpret it).

    edit- holy hell. I didn't realize I typed a novel..

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