Liebert Series 600T UPS, 65-225 kVA, Single Module 9
Figure 2. Alternative Grounding Configuration, 480 or 600 VAC input and output
7.2 Alternative Grounding Configuration, 480 or 600 VAC Input and Output, Isolated Power Distribution Units, Wye-Connected Service
This configuration is similar to that shown in
Section 7.1, except that the service entrance
neutral is not brought into the UPS module. 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
module 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 interruptors (GFIs) and
distorting the bypass waveform reference. Proper
adjustment of ground fault interrupters is
necessary to avoid unwanted tripping.
This configuration is reserved for those
applications which 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 trip-
ping the ground fault interruptors. The time
delay should be set to at least 30 cycles to pre-
vent tripping when the UPS performs a
transfer or retransfer operation.
CAUTION
Failure to properly set the ground fault
interruptors could cause loss of power to
the critical load.