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  1. #27
    Join Date
    Apr 2009
    Location
    Mobile, AL.
    Posts
    82
    Post Likes
    A nother thing to be aware of in an "ungrounded" delta is the leakage currents of loads connect acn "ground" a leg of the delta unintentionally. We often see ungrounded deltas that are in effect grounded because of this and run just fine until there is another ground on another phase.
    Some plants ground one phase intentionally and then the voltages to ground can be"funny" but be OK phase to phase.
    If the plant has connected one phase to ground hard then a ground fault on that phase woud do nothing, but on the other two would cause a trip.
    I re-read the first post about the phase reversal on the chiller and I wonder if a motor was replaced and then the phase was changes to make it run correctly? There could be offsetting errors that cause the board to read the phase reversal. This could have been a problem when the unit was first installed or created later.

  2. #28
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
    Post Likes
    I have a little trick that I like to use when getting unusual readings on a volt meter. The new high impedance volt meters will read most anything... and as most of you know you wind up reading some noise sometimes. I have had several occasions to be confused by a "noise" reading. So I put a real load in place of the meter.... say a light bulb or other load to verify that you are really seeing a voltage reading that has power potential and not some bs noise reading. Then you can make decisions based on real data... as an old guy used to tell me... qualify or condemn it... lets move on. Then if you determine that you really do have a voltage reading to ground on each leg... then you know for sure that the delta is truly NOT isolated... if it is not isolated there is a unsafe condition that could manifest itself in some ugly ways.... all of them involve smoke and damage to equipment and possible people. We used to have all our 480 volt systems that were high resistance grounded. We were in the petrochemical business and it was not good to have an unscheduled shutdown of the process... so if we got a ground fault we would get an alarm and go and figure out where the short was.... in the "figuring out" process one day we had a very experienced man get between the energized steel and the actual ground... so he had basically bypassed the grounding resistor with his body... it lit him up big time... 277 across his entire body... it ruined him... he lived but never worked again. SO be VERY careful with problems on isolated systems and If your not 100% sure of what is going on... get some qualified help.

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