Filter
Top Products
Configuring Your Neutral and Ground Connections
12
6.0 CONFIGURING YOUR NEUTRAL AND GROUND CONNECTIONS
Improper grounding is the largest single cause of UPS installation and start-up problems. This is not
an easy subject, since grounding techniques vary significantly from site to site, depending on several
factors. The questions you should ask are:
• What is the configuration of the input power source? Most of the recommended schemes for UPS
grounding require grounded-wye service. The UPS system requires a bypass neutral for sensing
and monitoring the quality of the bypass input. If the building service is ungrounded delta or cor-
ner-grounded delta, contact your Liebert representative for details about Isolated Neutral kits.
A Power-Tie or distributed redundant system has different grounding requirements from stand-
alone UPS modules. If using one of those systems, refer to Liebert’s Power-Tie configuration user
manual, SL-30030.
• Does the system have a bypass isolation transformer? Systems with 480 VAC (or 600 VAC) input
and 208 VAC output require a bypass isolation transformer. In some cases, it may be necessary to
treat that transformer as a “separately derived source” with its own grounding electrode.
• What are the UPS input and output voltages? Systems with 480 VAC input and output have sig-
nificantly different needs from systems with 480/208 VAC or 208/208 VAC.
• What is the connected load? Does the critical load consist of one or more Power Distribution Units
(PDUs)? Do the PDUs have isolation transformers?
Proper grounding should be based on NEC Section 250, but safe and proper equipment operation
requires further enhancements. The following pages detail Liebert’s recommendations for grounding
various system configurations to ensure optimal UPS system performance.
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 Figures 1 through 7.
!
CAUTION
The UPS ground lug must be solidly connected to the service entrance ground by an
appropriately sized wire conductor per NEC Article 250. Each conduit or raceway containing
phase conductors must also contain a ground wire, both for UPS input and output, which are
solidly connected to the ground terminal at each termination point. Conduit-based grounding
systems tend to degrade over time. Therefore, using conduit as a grounding conductor for UPS
applications may degrade UPS performance and cause improper UPS operation.