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Configuring Your Neutral and Ground Connections
13
6.1 Preferred Grounding Configuration, Wye-Connected Service
One of the most common configurations of Series 610 UPS Single-Module Systems is with 480 VAC
input, 480 VAC output and a connected load consisting of multiple Power Distribution Units (PDUs)
with isolation transformers in the PDUs to produce 208 VAC. For Canadian customers, the UPS mod-
ules usually have 600 VAC input and output. The same principles apply if the connected load is an
isolation transformer feeding various loads. Figure 1 shows a typical installation.
Notice that the UPS module main input and bypass input are connected to a grounded-wye service. In
this configuration, the UPS module is not considered a separately derived source. The UPS module
output neutral is solidly connected to the building service neutral, which is bonded to the grounding
conductor at the service entrance equipment.
The isolation transformers in the PDUs are considered a separately derived source. Therefore the
PDU neutral should be bonded to the PDU grounding conductor and connected to a local grounding
electrode in compliance with NEC 250-26.
Figure 1 Preferred grounding configuration, wye-connected service
Advantages of this configuration include:
• A measure of common-mode noise attenuation, since the isolation (common-mode rejection) occurs
as close to the load as practical (i.e., at the PDU).
• The UPS module can be located remotely from the PDU without compromising common-mode
noise performance.
• By using UPS modules with 480 VAC input and output and creating 208 VAC at the PDU,
smaller and less costly power feeders can be used and less voltage drop (as a percent of nominal)
occurs.
NOTE
Impedance-grounded wye sources must be grounded in accordance with 6.5 - Grounding
Configuration, Corner-Grounded Delta or Impedance-Grounded Wye.
NOTE
Some UPS modules are equipped with input isolation transformers. However, these
transformers have no effect upon any system grounding considerations. These modules will be
grounded exactly as shown in these pages.
NOTE
If there is a 4-pole Automatic Transfer Switch (ATS) between the service entrance and the UPS,
this configuration cannot be used. Refer to 6.2 - Alternate Grounding Configuration, Wye-
Connected Service or 6.3 - Preferred Grounding Configuration With Isolated Bypass
to determine a suitable configuration.
GROUNDING
ELECTRODE
PER NE
C
250-24