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Configuring Your Neutral and Ground Connections
14
6.2 Alternate Grounding Configuration, Wye-Connected Service
This configuration must NOT be used when single-phase loads are directly connected to the UPS.
The alternate configuration is similar to that shown in 6.1 - Preferred Grounding Configuration,
Wye-Connected Service, except that the service entrance neutral is not brought into the UPS mod-
ule. In this configuration, the UPS output transformer is considered a separately derived source. The
UPS module neutral is bonded to the UPS ground, which is connected to a local grounding electrode
in accordance with NEC 250-26.
Please note that this configuration represents a price/performance trade-off. Whenever the UPS mod-
ule transfers to or from bypass, two AC sources (input and bypass) are briefly connected together and
circulating current must flow. In the previous configuration, the current flows through the neutral
conductor. In this configuration, the current flows through the ground path, possibly tripping ground
fault interrupters (GFIs) and distorting the bypass waveform reference.
Proper adjustment of ground fault interrupters is necessary to avoid unwanted tripping.
Figure 2 Alternate grounding configuration, wye-connected service
This configuration is reserved for applications that meet all the following criteria:
• The facility has wye-connected service.
• The module rectifier input and bypass input are fed from the same source.
• The connected load is strictly 3-wire (such as one or more PDUs) and does not require a neutral
from the UPS.
• Special precautions are taken to prevent tripping the ground fault interrupters. The time delay
should be set to at least 0.2 seconds to prevent tripping when the UPS performs a transfer or
retransfer operation.
!
CAUTION
Failure to properly set the ground fault interrupters could cause loss of power to the critical
load.