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Transformer Oil Sample Safety

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    Transformer Oil Sample Safety

    We recently had an incident at my shop where a technician was shocked while taking an oil sample from a pad-mount transformer. From what I understand, he was leaning on the enclosure when he grabbed the sample valve and there was a difference in potential between the two points which caused him to get hit.

    Sounds like the voltage difference was pretty high considering he had trouble breaking free, this typically happens when you get above the 120V range. Based on this information my first guess is that the enclosure was not properly grounded.

    Just wanted to share this incident in case others had similar stories. I remember seeing a question in NETA world a while back that addressed an issue like this, it wen't along the lines of "what's the first thing you do when you walk up to inspect an energized transformer?" The answer was inspect the tank ground.

    Seems like this is one of those things that is hard to prevent, even if you inspect the ground and it looks good, you don't know what the condition is underground. Obviously the safest thing to do is not sample from an energized transformer but we all know this isn't always possible.

    What kind of checks do you guys do in the field to prevent such an issue? It's not very practical to do a 3-point test on every transformer you want to sample but one solution would be to carry a ground rod with a lead that you can drive into the ground and connect to the tank while you sample.

    I'd love to get any other stories or opinions from the community on this one. If I get any more details about the incident I'll be sure to share them here.

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    Quote Originally Posted by SecondGen View Post
    I remember seeing a question in NETA world a while back that addressed an issue like this, it wen't along the lines of "what's the first thing you do when you walk up to inspect an energized transformer?" The answer was inspect the tank ground.
    After doing some research I found that excerpt about transformer tank grounds, it comes from NFPA 70B Article 21.2.7.2: "The case of the transformer should be regarded as energized until the tank ground connection is inspected and found to be adequate."

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    Quote Originally Posted by SecondGen View Post
    After doing some research I found that excerpt about transformer tank grounds, it comes from NFPA 70B Article 21.2.7.2: "The case of the transformer should be regarded as energized until the tank ground connection is inspected and found to be adequate."

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    11.11.8.1.3 Samples should never be taken from energized
    transformers except by means of an external sampling valve. If
    the transformer has no external sampling valve, the unit should
    first be deenergized and a sample taken internally. (See ASTM
    D923, Standard Practices for Sampling Electrical Insulating Liquids.)

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    Quote Originally Posted by Kalbi_Rob View Post
    11.11.8.1.3 Samples should never be taken from energized
    transformers except by means of an external sampling valve. If
    the transformer has no external sampling valve, the unit should
    first be deenergized and a sample taken internally. (See ASTM
    D923, Standard Practices for Sampling Electrical Insulating Liquids.)
    Thanks for posting that info, Kalbi_Rob. It should be made clear to anyone that you should never take samples from an energized transformer that requires you to open a door of any kind and reach inside the enclosure.

    More details have surfaced on the incident since my original post. The transformer did have an external sample valve and the transformer was later found to be inadequately grounded.

    They did a measurement on site and found nearly 80V between the transformer tank and the sample valve. It was a hot day and the technician was most likely sweating after sampling multiple units which helped contribute to the shock as he leaned on the tank.

    The transformer has since been repaired and sampled. Let this be a good lesson for anyone conducting oil samples to be mindful of such an incident and to always perform the proper checks before taking a sample!

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