Issue:
APC 3 Phase UPS; Installation of dual mains or two sources input to UPS; where does the neutral conductor get terminated?

Product line:
APC 3 Phase UPS

Environment:
All models, all serial numbers

Cause:
Installation of dual mains or two sources input to UPS; where does the neutral conductor get terminated?

Resolution:
In the USA when we have two sources we feed the rectifier input (mains1) as 3-phase +PE and the bypass (mains2) as 3-phase +N+PE. We do this because the neutral bus in the UPS is common to both inputs and output and the battery system mid-point.

Our National Electrical Code forbids connecting the two source neutrals together. Also with the common neutral bus in the UPS you can get circulating currents flowing between the two transformers and also ground fault current from upstream system loads flowing in the UPS neutral bus. Therefore it is not recommend having input neutral on the rectifier feeder (mains1) for dual feed systems.”