Vertiv | Liebert
®
NXL
™
500-750kVA UPS Installation Manual |
21
2.6
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.
2.6.1
Four-Wire Input connections
The UPS module bypass input is connected to a solidly grounded source. In this configuration,
the UPS module is not considered a separately derived source. The UPS module output neutral is
solidly connected to the bypass source neutral, which is bonded to the grounding conductor at
the source equipment.
2.6.2 Three-Wire Input connections
This configuration must NOT be used when single-phase loads are directly connected to the
UPS. The UPS output is considered a separately derived source. The UPS module neutral is
bonded to the UPS ground and connected to a local grounding electrode in accordance with NEC
250-26. Please note that whenever the UPS module transfers to or from bypass, two AC sources
(UPS output 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 (GFI’s) and distorting the output voltage waveform. 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.
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.
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
through
.
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.
