Emerson 610 Manuel D’Utilisation

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.

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.

If the building service feeding the UPS is any configuration other than those mentioned 

above, contact your Liebert representative or Liebert Global Services immediately.

NOTE
Some UPS modules are equipped with input isolation transformers. However, these 

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.