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Thread: Ungrounded controll transformers

  1. #1
    Join Date
    Jun 2011
    Location
    NYC, NY
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    2

    Ungrounded controll transformers

    I am working in NYC on an old building and mostly every motor controller has its own controll transformer. 480/120. The x1 and x2 terminals on the secondary side read 128 volts when tested to each other. There is no grounded conductor to the x2 neutral terminal (some older transformers have x1 as their neutral).


    Taking readings from the fused secondary terminal (usually x1, but sometimes on older transformers its x2) to ground gives inconsistant readings. I believe there could also be a situation where a ground fault on the secondary side would energize the metal components in contact with the ground fault, but would not blow the fuse due to the ungrounded neutral secondary.

    Should I install a grounded conductor to the controll (secondary) transformer neutral ??

    If I install a grounded conductor to the neutral terminal will the controll circuit operate correctly, or will there be some ill effects ??


    I believe it would operate fine, unless there is a fault to ground, and these controll transformers were installed prior to the NEC becoming aware of the importance of grounding and bonding.

  2. #2
    Join Date
    Feb 2003
    Location
    Illinois
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    18,586
    There is no safety issue with the use of ungrounded control power transformers for motor control circuits. A single ground fault creates a grounded system, it does not energize the metal that it is faulted to.
    I didn't look, but I seem to recall an exception that would permit these control circuits to be ungrounded.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    Jun 2006
    Location
    Ohio
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    11,612
    Quote Originally Posted by don_resqcapt19 View Post
    ...
    I didn't look, but I seem to recall an exception that would permit these control circuits to be ungrounded.
    I didn't look as well, but as I recall, the transformer (secondary being an SDS) has to be rated 1kW or less.
    I'll never get there. No matter where I go, I'm always here.

  4. #4
    Join Date
    Jun 2011
    Location
    NYC, NY
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    Thanks Dudes,

    Would there be any advantage in grounding the neutral terminal ??

    Was this wired/installed this way for any specific reason, or just because of the age of the installation ??

    Thanks for your help and insight

  5. #5
    Join Date
    Jun 2006
    Location
    Ohio
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    11,612
    Quote Originally Posted by Moparmyway View Post
    Thanks Dudes,

    Would there be any advantage in grounding the neutral terminal ??

    Was this wired/installed this way for any specific reason, or just because of the age of the installation ??

    Thanks for your help and insight
    The advantage in grounding the secondary would be personnel safety... so you have to ask just how much safer personnel would be by grounding. The disadvantage would be possible loss of control circuit on first fault.

    Age probably contributes in the control circuit being ungrounded... but stating how much is better left to others
    I'll never get there. No matter where I go, I'm always here.

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