Configuring Neutral And Ground Connections; Four-Wire Input Connections; Three-Wire Input Connections - Liebert NXL Installation Manual

2.5

Configuring Neutral and Ground Connections

Improper grounding is the largest single cause of UPS installation and startup problems. Grounding
techniques vary significantly from site to site, depending on several factors.
Proper grounding should be based on NEC Section 250, but safe and proper equipment operation
requires further enhancements. The following pages detail 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 7 through 10.
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.
2.5.1

Four-Wire Input connections

The UPS module main input and bypass input are connected to a grounded 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.
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 480VAC input and output and creating 208VAC at the PDU, smaller
and less costly power feeders can be used and less voltage drop (as a percent of nominal) occurs.
2.5.2

Three-Wire Input Connections

This configuration must NOT be used when single-phase loads are directly connected to the UPS.
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 this configuration, the current flows through the
ground path, possibly tripping ground fault interrupters (GFIs) and distorting the bypass waveform
reference. Proper adjustment of ground fault interrupters is necessary to avoid unwanted tripping.
The time delay should be set to at least 0.2 seconds to prevent tripping when the UPS performs a
transfer or retransfer operation.
NOTICE
Risk of improperly set ground fault interrupters. Can cause equipment damage.
Failure to set the ground fault interrupters properly could cause loss of power to the critical
load.
15
UPS Electrical Installation