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Thread: Zero Potential Testing

  1. #1

    Zero Potential Testing

    Question 1: Pump ABC has been isolated at the Motor Control Center (MCC) and the circuit is locked out. Workers at Pump ABC tested the local on/off switch and the pump did not operate. Is this a satisfactory test prior to doing electrical and mechanical work on the pump? if not please explain why.

    Question 2: If Pump ACB has been isolate at the Motor Control Center (MCC) and the circuit is locked out. Is it also necessary to lock out the local on/off switch where pump ABC is located? If so why?

    Appreciate your assistance.
    Regards,
    TVH

  2. #2
    Join Date
    Sep 2004
    Location
    Norfolk, VA
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    151
    Quote Originally Posted by TVH View Post
    Question 1: Pump ABC has been isolated at the Motor Control Center (MCC) and the circuit is locked out. Workers at Pump ABC tested the local on/off switch and the pump did not operate. Is this a satisfactory test prior to doing electrical and mechanical work on the pump? if not please explain why.

    Question 2: If Pump ACB has been isolate at the Motor Control Center (MCC) and the circuit is locked out. Is it also necessary to lock out the local on/off switch where pump ABC is located? If so why?

    Appreciate your assistance.
    Regards,
    TVH
    1: It wasn't stated that the circuit at the MCC had been locked out by the same workers as those at pump abc. If not then the answer is no. If yes then the answer may be yes depending on unstated variables.
    2: See 1 above. Also "local on/off switch" may not be a legitimate lock out device.

    Variables might include whether a company approved loto procedure specific to pump abc was been followed. Another variable might be stored energy other than electrical when performing certain work on pump abc.

  3. #3
    Join Date
    Feb 2003
    Location
    Illinois
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    19,139
    For Q1, if you have tested the operation of the equipment from the local control switch and the equipment works, and then you lockout the power source and re-test the equipment from the local control switch, and the equipment does not work, it is ok for a mechanical work lockout. An electrical work lockout always requires voltage testing using correct PPE to verify de-energization before doing electrical work.

    Q2, most, if not all, of the lockout procedures I have seen require that the energy source be locked out and all local control switches be tagged out.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  4. #4
    Join Date
    Dec 2007
    Location
    NE Nebraska
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    18,626
    If you are doing electrical work, and one pole failed to open when you opened the disconnect, in a typical three phase motor that will still allow for voltage to be present via feed thru of the windings between all three lines and ground even though the motor will not mechanically operate. This introduces an electrical hazard to the people performing the work. If you check for voltage you will discover the failure and take care of that problem before proceeding to the other work.

    Depending on how this is arranged you may need to check voltage at the disconnect or motor controller and not just at the motor to ensure there is no possible way to energize the motor conductors.

  5. #5
    Join Date
    Aug 2009
    Location
    Tacoma, Wa
    Posts
    316
    You still need to accomplish a live-dead-live test with a volt meter as per OSHA--and LOTO ALL operating Switches/Stations--as has been stated if this is piece of equipment that does not require a neutral then you may have only opened one phase and pushing the start button could give a false dead indication.

  6. #6
    Join Date
    Sep 2013
    Location
    Brisbane, Australia
    Posts
    26
    Some other things to think about...

    Q1. For electrical work - no for regulatory and welded phase contact reasons. For non electrical work - perhaps. Possible issues include jammed pump shaft drive, therefore motor won't rotate. See below:
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    This tends to be more of a problem with bogged screw conveyors, mixers, bucket elevators etc. It also relies on proper operation of the control circuit which may have failed, perhaps intermittently, since the initial test. Because of these reasons some companies are moving away from try start for verification prior to mechanical work.

    Q2. I assume you mean ABC isolator and ABC control switch as opposed to ACB isolator??? If isolation verification is performed correctly ie the load side of the isolation point(s) that you plan to lock has no volts, then no other locks are necessary for safety reasons but may be necessary from a regulatory perspective in your jurisdiction.
    Last edited by cdevine; 11-07-13 at 11:51 PM.

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