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Thread: Ungrounded electrical systems

  1. #1
    Join Date
    Sep 2007
    Location
    Colorado Springs
    Posts
    134

    Ungrounded electrical systems

    I've been reading and trying to understand ungrounded systems, pros and cons, and some history on the grounded system we use in this country and got to the part about undgrounded systems in dairy farms and hospital operating rooms. So i go to work and there in the hospital I been helping remodel for the last 3 months is working isolation system that i can stick my meter in and try to learn something.

    So on the regular hospital grade duplex recepticle I get 120 volts between the hot and the normally neutral side, I get 60 volts from the hot side to the ground pin, and 60 from the normally neutral side to the ground pin, also 60 volts each side to the chassis. I'm understanding that the ground is not tied to the neutral and is an earth ground, but what I'm not understanding is how this system prevents electrical shocks, (60 volts), in a wet procedure location? Thanks for all the help..

  2. #2
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Try checking the voltages again, but with a 120v bulb in parallel with the voltmeter. Let us know what you get, and then we'll explain why.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
    Join Date
    Sep 2007
    Location
    Colorado Springs
    Posts
    134
    Thanks Larry. I won't be back out there for at least a week while they are trying to make some decisions etc. but i'll check it out ... I was also wondering why the safety people didn't make a big stink about pulling temporary construction power off these units as they were obviously not gfci'd. They were very watchful on our normal temp power..??

  4. #4
    Join Date
    Aug 2006
    Location
    Miami Fla.
    Posts
    6,887
    Could you be working on a system like this?

    647.3 General.

    Use of a separately derived 120-volt single-phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor shall be permitted for the purpose of reducing objectionable noise in sensitive electronic equipment locations, provided the following conditions apply:
    (1) The system is installed only in commercial or industrial occupancies.
    (2) The system’s use is restricted to areas under close supervision by qualified personnel.
    (3) All of the requirements in 647.4 through 647.8 are met.
    Or this?

    647.5 Three-Phase Systems.

    Where 3-phase power is supplied, a separately derived 6-phase “wye” system with 60 volts to ground installed under this article shall be configured as three separately derived 120-volt single-phase systems having a combined total of no more than six disconnects.

  5. #5
    Join Date
    Sep 2007
    Location
    Colorado Springs
    Posts
    134
    I'm thinking I'm dealing with 250.22 (2). which includes 517.61 and 517.160. Also 517.20, Wet procedure area. Also covered in this article http://www.iaei.org/magazine/?p=1057 among others?????

  6. #6
    Join Date
    Sep 2007
    Location
    Colorado Springs
    Posts
    134
    I think I found my system in this article with paragraph 8 and 9 explaining my 60 volts to ground. Thanks .. http://www.ashe.org/ashe/facilities/...0Baretich,.pdf

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