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When To Conduct Resistance Testing on Utility Feeders?

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  1. Jrmcritical's Avatar
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    When To Conduct Resistance Testing on Utility Feeders?

    Hey Guys,

    Just curious on what the rest of the electrical testing community thinks about conducting Insulation Resistance ("Megger") testing on isolated utility feeders or in some cases buss.

    For Example: You are conducting preventative maintenance on a 4,000A service switchboard (less than or = to 480V) in Manhattan, NYC. Since the customer wants maintenance performed on the service switch, ConEd (the local utility) has been hired to "disconnect" the service in the street. Upon completion of maintenance, DLRO contact resistance testing, and an insulation resistance test of the isolated switchboard, would you megger the feeders going out to the street (assuming you confirmed they are deenergized and utility personnel are clear.)?

    Coworker 1: "Yes, I see the feeders (or buss) as an extension of the switchboard."
    Coworker 2: Never, that is strictly the responsibility of the local utility.

    Personally, I have mixed feelings. I've done some testing on the side (haven't we all...) and my normal procedures are for me to see how the local utility has disconnected the service. If buss plates were removed, and there is a visible gap then yes, I'll conduct an insulation resistance test. If the utility workers had to use shims or cable boots, then I most likely would not conduct an insulation resistance test.

    What would you do.....

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    Quote Originally Posted by Jrmcritical View Post
    Hey Guys,

    Just curious on what the rest of the electrical testing community thinks about conducting Insulation Resistance ("Megger") testing on isolated utility feeders or in some cases buss.

    For Example: You are conducting preventative maintenance on a 4,000A service switchboard (less than or = to 480V) in Manhattan, NYC. Since the customer wants maintenance performed on the service switch, ConEd (the local utility) has been hired to "disconnect" the service in the street. Upon completion of maintenance, DLRO contact resistance testing, and an insulation resistance test of the isolated switchboard, would you megger the feeders going out to the street (assuming you confirmed they are deenergized and utility personnel are clear.)?

    Coworker 1: "Yes, I see the feeders (or buss) as an extension of the switchboard."
    Coworker 2: Never, that is strictly the responsibility of the local utility.

    Personally, I have mixed feelings. I've done some testing on the side (haven't we all...) and my normal procedures are for me to see how the local utility has disconnected the service. If buss plates were removed, and there is a visible gap then yes, I'll conduct an insulation resistance test. If the utility workers had to use shims or cable boots, then I most likely would not conduct an insulation resistance test.

    What would you do.....

    Who owns the cables, who gets the bill? It's up to them to decide whether to test.

    As for testing, what's on the other end? Assuming the cables are still connected to the transformer, the low side would be included in the test, making phase-to-phase readings on the individual cables impossible.

    Not sure what the procedures are in your local jurisdiction but normal procedure around here is for the utility to come in and physically remove their buss in the metering cabinet with feeders still hot. The cabinet is LOTO and no testing is done past customer owned equipment.

    Sometimes you get lucky with and the utility will offer to de-energize the cables as a safety measure.

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    Quote Originally Posted by SecondGen View Post
    Who owns the cables, who gets the bill? It's up to them to decide whether to test.

    As for testing, what's on the other end? Assuming the cables are still connected to the transformer, the low side would be included in the test, making phase-to-phase readings on the individual cables impossible.

    Not sure what the procedures are in your local jurisdiction but normal procedure around here is for the utility to come in and physically remove their buss in the metering cabinet with feeders still hot. The cabinet is LOTO and no testing is done past customer owned equipment.

    Sometimes you get lucky with and the utility will offer to de-energize the cables as a safety measure.
    It definitely depends on the location, but I'm Manhattan, the local utility owns the feeders up to the CT compartment. Very rearly do they disconnect at the CT compartment, as ots typically done in the vault.

    Assuming the customer requests that the utility feeder be tested, would you perform an insulation resistance test on shimmed bus or booted cable lugs, or would you want to have a complete removal of the links?

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    Quote Originally Posted by Jrmcritical View Post
    It definitely depends on the location, but I'm Manhattan, the local utility owns the feeders up to the CT compartment. Very rearly do they disconnect at the CT compartment, as ots typically done in the vault.

    Assuming the customer requests that the utility feeder be tested, would you perform an insulation resistance test on shimmed bus or booted cable lugs, or would you want to have a complete removal of the links?
    Complete removal of the links is preferred, the more separation the better. Second option would be to boot up the cables best as possible, anything with a direct connection (whether it be a shim, boot, tape, etc.) will affect the reading.

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    Quote Originally Posted by Jrmcritical View Post
    Hey Guys,

    Just curious on what the rest of the electrical testing community thinks about conducting Insulation Resistance ("Megger") testing on isolated utility feeders or in some cases buss.

    For Example: You are conducting preventative maintenance on a 4,000A service switchboard (less than or = to 480V) in Manhattan, NYC. Since the customer wants maintenance performed on the service switch, ConEd (the local utility) has been hired to "disconnect" the service in the street. Upon completion of maintenance, DLRO contact resistance testing, and an insulation resistance test of the isolated switchboard, would you megger the feeders going out to the street (assuming you confirmed they are deenergized and utility personnel are clear.)?

    Coworker 1: "Yes, I see the feeders (or buss) as an extension of the switchboard."
    Coworker 2: Never, that is strictly the responsibility of the local utility.

    Personally, I have mixed feelings. I've done some testing on the side (haven't we all...) and my normal procedures are for me to see how the local utility has disconnected the service. If buss plates were removed, and there is a visible gap then yes, I'll conduct an insulation resistance test. If the utility workers had to use shims or cable boots, then I most likely would not conduct an insulation resistance test.

    What would you do.....
    It all depends on the customer and the location. I am in Florida and the customer owns from the transformer to the building. That said most customers do not think of testing the feeders unless they have had issues in the past.
    I just had a project where the customer scope included testing the main feeders, because they lost a set a couple of years ago, from the utility transformers to the building main. The utility company, after pulling the bayonet fuses, removed each phase blade at the transformer secondary. I discussed with the customer that we would test each phase to ground and phase to phase still connected to the paddles and the main breaker (open). It was greed that if any of the readings were low then we would separate the parallel conductors to locate the problem set. My readings were low after locating and removing all of the sensing fuses and lightning arrestors. When I went back out to the utility transformer to assess the cause of the low readings I noticed a smaller set (2/0) of conductors among the other six (350MCM) conductors on the blades at the transformer. The fire pump controller circuit! Once the fire pump controller was isolated the readings were good and I didn't have to do six separate tests per phase. Next time I will locate and isolate the fire pump first. Live and learn I guess.

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