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Thread: Baffled???

  1. #1
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    Baffled???

    So i go on a service call for a AAON rooftop not cooling (460volt/3 phase) hi rise residential building in nyc. Blown condensor fan contacor burnt to all hell. (they have 3 fans tied into 1 contactor). I replaced the contactor and amped it out.. It was high so i was cheking terminations and voltage.Found getting 460 across the board but to ground was getting 28 volts on one of the legs.. The 460 volt main transformer feeds all the equipment on the roof. I checked out all the other equipment voltage and it was the same (460,28,460). To make a long story short it turned out to be a shorted wire in the conduit from the MCC panel to one of the exhaust fan motors (water in the conduit). All the motors are fed from a 460v MCC panel,with starters. So there was a short but how come the fuses didnt pick it up and blow it instead it went through and backfed the transformer causing a voltage drop for everything that was tied into it?
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  2. #2
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    first thing on any service call is check power with discconect off. you would have seen low readings on L2 and maybe saved you some time. The unit did respond to low voltage, it wasnt working which is how you got the call. units can pull their control power of any line and has single phase transformers to step it down in the unit. AAON's usually have a few in them, especially if they have gfi in unit options on 3 phase. the boards could be fed from L1 and with 460 volts present, the comp will just lock out on phase protection since the controls never lost power

    matt

  3. #3
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    the comps were working believe it or not. I was just skeptical because of the blown condensor fan contator thats what thew me a red flag. But that still doenst answer my question
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  4. #4
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    ohh yea and the 460v building transformer was feeding ALL the roof exhaust fanns and the cooling tower fan with freq drive. and all the L2 on all the sarters for all the motors were giving me a 40 volt reading
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  5. #5
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    the fan motors are single phase on those, only the comps are 3 phase. I think you may have overlooked something

    matt

  6. #6
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    Quote Originally Posted by WheresMyGages View Post
    So i go on a service call for a AAON rooftop not cooling (460volt/3 phase) hi rise residential building in nyc. Blown condensor fan contacor burnt to all hell. (they have 3 fans tied into 1 contactor). I replaced the contactor and amped it out.. It was high so i was cheking terminations and voltage.Found getting 460 across the board but to ground was getting 28 volts on one of the legs.. The 460 volt main transformer feeds all the equipment on the roof. I checked out all the other equipment voltage and it was the same (460,28,460). To make a long story short it turned out to be a shorted wire in the conduit from the MCC panel to one of the exhaust fan motors (water in the conduit). All the motors are fed from a 460v MCC panel,with starters. So there was a short but how come the fuses didnt pick it up and blow it instead it went through and backfed the transformer causing a voltage drop for everything that was tied into it?

    Are you saying that you got 460 to ground on two legs and 28 to ground on the third leg. Or AB at 460, BC at 28, and AC at 460.

    On a building with a delta buss and a consentric ground it is possable to get a low voltage from one leg to ground. That is how ground phase lights work.
    Old snipes don't die they just loose their steam

  7. #7
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    Dec 2001
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    Southern Tier, NY
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    Delta service .....???
    Carefull ....
    one corner is grounded ...
    it still has potential ...

    never seen a 480v Delta, tho ....

  8. #8
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    L1 and L3 to ground was 460 and L2 to ground was around 28 volts.. (when the shorted exhaust fan was running) when the fan was off it was 460 all around.
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  9. #9
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    Quote Originally Posted by WheresMyGages View Post
    L1 and L3 to ground was 460 and L2 to ground was around 28 volts.. (when the shorted exhaust fan was running) when the fan was off it was 460 all around.

    I will bet this is an older building. Sounds like you are dealing with a delta wound transformer feeding the building.
    Old snipes don't die they just loose their steam

  10. #10
    Join Date
    Feb 2007
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    NE Pennsylvania
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    yea it sounds like a delta. I just installed a new 35 ton liebert unit in scranton, pa and had a hell of a time with the freq drive on the condensing unit. It just kept shutting the drive down on over voltage. The dc buss voltage was supposed to be 1.5 times the avg if the incoming line voltage. I had something like 980v dc. it would just knock the drive out and the liebert unit would just alarm on hp. it happens that neither liebert or control techniques(freq drive mfg) had ever seen this problem. And the solution was that the filter in the drive just had to be removed. I guess the e. engineers put it there just in case. well got it runnin and now the customer is happy. and i just learned a bunch about electronics and how freq drives work.
    happy trails.

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