GROUNDING |
ELECTRODE |
PER NEC 250-24 |
Figure 1. Preferred Grounding Configuration, 480 or 600 VAC input and output
7.1.Preferred Grounding Configuration, 480 or 600 VAC Input and Output, Isolated Power Distribution Units, Wye-Connected Service
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 customers, 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 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.
Advantages of this configuration include:
•A measure of common-mode noise attenua- tion, since the isolation (common-mode rejec- tion) 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 com- mon-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 Section 7.5.
8Installation Manual
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