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Thread: New Installation Design and Safety Concern

  1. #1
    Join Date
    Feb 2008
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    Currently working in Lea County, NM near Jal, NM
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    14

    New Installation Design and Safety Concern

    E/I inspector on oil truck unloading station. Concern: service 21.6kv to 480v 3ph. 4w to main disconnect; 480v 3ph 4W to transfer switch; 480v 3ph 3w to MCC. Transfer switch will have emergency power supplied by portable emergency generator. Concern: transfer switch has one neutral buss shared by normal and emergency power. If emergency gen. is hooked up to transfer switch in emergency mode will an imbalanced load from the system be felt back at the service transformer through the common neutral in an emergency power situation? Would this be a problem for repair/maintenance person working in the transformer during emergency generating condition?

  2. #2
    Join Date
    Aug 2004
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    Northern illinois
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    16,900
    Quote Originally Posted by bandylb18 View Post
    E/I inspector on oil truck unloading station. Concern: service 21.6kv to 480v 3ph. 4w to main disconnect; 480v 3ph 4W to transfer switch; 480v 3ph 3w to MCC. Transfer switch will have emergency power supplied by portable emergency generator. Concern: transfer switch has one neutral buss shared by normal and emergency power. If emergency gen. is hooked up to transfer switch in emergency mode will an imbalanced load from the system be felt back at the service transformer through the common neutral in an emergency power situation? Would this be a problem for repair/maintenance person working in the transformer during emergency generating condition?
    I am not sure what you mean by "imbalanced" load.

    whomever is working on the xfmr would need to use safe electrical work practices to insure whatever is being worked on is not energized. IMO, the transfer switch is probably not adequate for this purpose unless it is a manual switch that can be locked into a safe position for working on the xfmr.

    presumably the N-G bond is being made inside the transfer switch. I think it is not a safety issue to have the grounded conductor not opened.
    Bob

  3. #3
    Join Date
    Dec 2007
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    NE Nebraska
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    I assume you are questioning neutral current here.

    That current is trying to get back to the source - which when running on the generator is trying to return to the generator. If you bring the load neutral as well as the generator supply neutral to the transfer switch - that is your primary path. Where grounding/bonding occurs will determine what paths are more likely to become an alternate path to have concerns about.
    I live for today, I'm just a day behind.

  4. #4
    Join Date
    Mar 2003
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    New York
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    No code book in front of me, it used to have the info. on solid or transfer neutral depending on if permanent or temporary generation as in separately derived service; been ~20 yrs. since I did one so little bit foggy.

  5. #5
    Join Date
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    Reading OP a little more carefully, doesn't appear there even is any neutral load. If there is no neutral connected to any loads, any unbalance current is returning via other phase conductors there is no neutral current to be concerned about.
    I live for today, I'm just a day behind.

  6. #6
    Join Date
    Jul 2005
    Location
    Tennessee
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    15,973
    agree. see no problem. 95% of the generators installed here have a solid neutral, most with neutral loads, a few without.
    There has been no problem that I know of.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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