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More fun with generators

Discussion in 'Safety' started by porkchop, Sep 2, 2008.

  1. porkchop

    porkchop Well-Known Member

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    Ok so first off we had to tie into three different generators today all of them in different places and none of them easy to get to. Ok punching bag time over. A little information about the hook up. All three generators work well, are CAT generators, outputing 210ish volts, 3 phase, first genny rated for 700A, other two are 1000A gennies. They are all well grounded to the same grounding rod (not all at the same time but one cable moved to the gennerator being used at the time). I have two runs of feeder cable going into the venue down the stairs and into my power distro which has metering readouts (did meter with a multi meter before hooking it up though)

    So here's were things get fun. Hook up the first generator tie the ground and neutral together do the run things work great, but that's not our generator so we need to move to another one. Same cables, same tails, same generator tech, etc... go into the new generator, the legs meter out fine, the phase ok light is on at the distro, but the internal meters are so confused they aren't even giving a readout. Split the ground and neutral and things work like a champ. Move to the last generator for rediculous reasons that are irrelivant and this generator is already hooked up to a set of tails but has no neutral tail so we hook up with the neutral and the ground all going into the grounded tail. Same no readout nonsense. Split the ground and neutral, tie in a neutral leg and lifes great.

    So what caused this? I could see something not working because there was no return line along the neutral leg, but then why did it work in the first place with the first generator. Anyone have any ideas?
     
  2. SHARYNF

    SHARYNF Well-Known Member

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    Large generators can be set up two ways:

    If the output is going to only be feed outlets from the generaor, then the neutral and ground and frame are all connected at the generaor.

    If the generator is setup to be used where it is a standby for building wiring, where neutral and ground are already connected then the neutral and ground are not connected at the generator

    It comes down to separately derived and switched neutral set up.

    (d) Separately Derived Systems. A premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system...

    An alternate alternating-current power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.

    Thus, a separately derived system has no direct electrical connection to another system supplying power at the jobsite, while a non-separately derived system would be directly connected to another system supplying power to the site. A non-separately derived system at the jobsite would not need to have its neutral grounded, because the service-supplied system would already provide this function. Portable and vehicle-mounted generators are typically used to provide primary power for utilization equipment on construction sites and are normally separately derived systems for the purposes of OSHA's electrical standards for construction.

    Grounding non-separately derived generator systems.
    Generator Neutral [Archive] - Mike Holt's Forum

    This would be my take on your situation

    Sharyn
     
  3. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    This is contrary to CATPower's standard operating procedures when providing power for televised events. They run all five wires from the generator AND house power to a "transfer box." During the broadcast, the generator is primary, and house power is back-up.
     
  4. SHARYNF

    SHARYNF Well-Known Member

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    THAT IS CORRECT if the generator has been properly set up with a transfer box. The OP makes no mention of a transfer box. Again in my limited experience, I have found that on occasion, which could cause the problems that were seen here, if the gen were set up for standalone power then the ground/neutral are tied, if on the other hand it used a transer box then the individually derived method is used, and then neutral and ground are NOT tied at the generator but at the transfer box.
    From what I have seen in the case of a transfer box it is specifically set up so that when connected to the building system it uses the building ground to neutral connection, and when it is running independantly it then connects the neutral frame and ground at the gen. Other wise you create two ground/neutral connections and can have current running over the ground/neutral feeder wire connection between the building and the gen

    IMHO this
    is what probably caused the problem the generator should have had the transfer box (probably it was still at some other site)

    Steve T would be able to give more explanation on this

    Sharyn
     
    Last edited: Sep 2, 2008

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