4-wire secondary feeds 3- wire UPS

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brian john said:
Some single UPS's have three sources, UPS input, UPS Bypass, wrap around bypass. Very common installation, and very common for there to be grounding issues with these systems.
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Echoing others, absolutely. And many of the common configuration errors can be spotted in two minutes by just drawing a two line diagram with the UPS in each mode, including (maybe especially) maintenance bypass.

The contributing factor that trips up many wannabes is the behavior of the supply neutral, which in normal UPS mode is irrelevant, but in any bypass mode has current on it.
 
I was able to get back into the u.p.s. room to follow up on this unit. The unit is a single feed with a bypass switch,no aux circuit. I took a few voltage readings and found 8.25 volts between neutral and ground from the xfrmr through the ups and also into the panel. The voltage between each phase and neutral varied from 112 on phase a,to 125 volts on c phase. I installed a temporary jumper and the voltage leveled out and the voltage between neutral and ground went to zero. next is to schedule a shut down and install a permanent system bonding jumper and a gec to the building steel.
Rick
 
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