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Thread: Three Phase Ground Fault Protection

  1. #1

    Three Phase Ground Fault Protection

    I have a question concerning ground fault protection on aerator motors in a lagoon at a water treatment facility. These are 3 phase 480V motors, floating throughout the lagoon, connected via a submersed 4 Wire cord. Of course, no one actually enters the lagoons, but maintenance is often performed by boating out to the motors. GF protection is not required for the equipment. Should (or is it) required for personnel safety? I cannot find any thing to say it is.
    Thanks

  2. #2
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    Re: Three Phase Ground Fault Protection

    I don't feel gfci protection is needed if safe working practices are performed, like deenergizing the equipment before entering the lagoon.
    Bryan P. Holland, MCP

  3. #3
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    Re: Three Phase Ground Fault Protection

    Although it may not be required, it is worth careful consideration. Specifically, it is worth considering the following two "proposed design criteria":
    </font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">They will be turning off a given motor before working on that motor. But the other motors will continue running.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Somebody is going to fall out of the boat and into the water.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif">
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  4. #4

    Re: Three Phase Ground Fault Protection

    All the aerators are not shut off to go work on one. Although that does sound like the right thing to do, the process cannot support this (IOW, shutting down all the aeration for a lenghty time would upset the lagoon conditions).

  5. #5
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    Re: Three Phase Ground Fault Protection

    Many motor starters can be purchased with GF protection. This will offer equipment protection but not personnel protection, as the setting is too high.
    Boating out to the motor in a metallic boat will place the worker at the same potential as the water, which may offer a level of safety when the motor under service is turned off and disconnected from the starter (no ground continuity out to the motor).

    There was an interesting program on the discovery channel showing a helicopter lifting a lineman to some high voltage conductors to perform maintenance. All was OK, because the helicopter was at the same potential as the conductors.

    [ January 31, 2006, 04:08 PM: Message edited by: ron ]
    Ron

  6. #6
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    Re: Three Phase Ground Fault Protection

    You would never get a 4-6ma people protection GF to work as it would be prone to nuisance tripping. You may be able to apply a 30ma equipment protection rated GF device.

  7. #7
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    Re: Three Phase Ground Fault Protection

    Did this very thing at a wastewater treatment plant where I worked. Worked from an aluminum boat to lubricate aerator motors. We only shut off the one we were working on. Electrical service to the plant was 480V 3 wire ungrounded system monitored by a ground fault indicator. As I remember no one ever considered shutting off all 4 motors at once.
    John, Chair City, NC
    Technology: Mans best efforts to make things as good as they used to be

  8. #8

    Re: Three Phase Ground Fault Protection

    This is turning into an interesting debate. I called the manufacturer, and they said "ground fault protection is not neccessary for the equipment, check your local codes. if there is a short to ground, the breaker will trip anyway right?" I then read these 2 article from this web site and it makes me think even more.

    http://www.mikeholt.com/mojonewsarch...s~20040826.php

    http://www.mikeholt.com/mojonewsarch...s~20031117.htm

  9. #9
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    Re: Three Phase Ground Fault Protection

    GFCI protection does not protect workers from being injured or killed, safe working practices do. The GFPE protection the manufacturer is suggesting will not protect persons, only the equipment.
    Bryan P. Holland, MCP

  10. #10
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    Re: Three Phase Ground Fault Protection

    Originally posted by gabany:. . . they said ". . . if there is a short to ground, the breaker will trip anyway right?"
    True, but irrelevant.

    Please take note that a current of far less than one amp can be fatal. However, adding one more amp of current to the load seen by a breaker is not going to cause the breaker to trip. A GFCI is not there to protect against a "short to ground." It is there to protect against a small amount of current flowing through a person's body.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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