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Configuring Your Ground and Neutral Connections 13
7.1 Preferred Grounding Configuration, 480 or 600 VAC Input and Output, Isolated
Power Distribution Units, Wye-Connected Service
Figure 1 Preferred Grounding Configuration, 480 or 600 VAC input and output
One of the most-common configurations of the Series 600T UPS is the Single Module System 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 cus-
tomers, the UPS modules usually have 600 VAC input and output. The same principles apply if
the connected load is an isolation transformer feeding various loads. Figure 1 above shows a typi-
cal installation.
Notice that the UPS module main input and bypass input are connected to a grounded-wye ser-
vice. 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 ground-
ing electrode in compliance with NEC 250-26.
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 nomi-
nal) occurs.
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.
GROUNDING
ELECTRODE
PER NEC 250-24