Configuring Your Neutral and Ground Connections

Models: 610

6.0CONFIGURING 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 anything other than a grounded wye system (corner grounded delta or impedance grounded wye), contact your Liebert representative for details about the Isolated Neutral kits for the System Control Cabinet and UPS modules.

! WARNING

If the building service feeding the UPS is any configuration other than those mentioned above, contact your Liebert representative or Liebert Global Services immediately.

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.

•What are the UPS input and output voltages? Systems with 480 VAC input and output have sig- nificantly different needs from systems with 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 4 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.