Emerson NX Manuel D’Utilisation

It is important that the operator check the instrument readings if abnormal equipment performance 

is suspected. Any metered value that differs appreciably from normal could mean an impending 

malfunction and should be investigated.

• If the UPS has not operated on battery power during the last 10 hours, the batteries should 

require little charging current. Battery mimic should indicate normal DC voltage with the battery 

charge current no more than 1% of maximum discharge current.

• Input current on each phase should be within 10% of the average input current.
• Alarm messages indicate malfunction or impending malfunction. A daily check of the Display 

Screen will help to provide an early detection of problems.

• Tracing a problem to a particular section is facilitated by alarm messages and the metered 

parameter indications.

Risk of recurring problem. Can cause degraded performance and equipment damage.
If the UPS system has an open fuse, the cause should be determined before replacing the fuse. 

If the cause is not corrected, it could recur. Contact Liebert Services for assistance.

If a problem occurs within the UPS, review all alarm messages along with other pertinent data. 

Contact Liebert Services at 1-800-LIEBERT to report a problem or to request assistance.

During normal UPS operations, short-term overload current demand from the bypass source may 

reach 10 times the UPS output current rating. This overload current demand may be caused by the 

magnetizing inrush current of one or more downstream transformers (i.e., power distribution units) 

or faults on downstream branch circuits. The instantaneous trip point(s) of the upstream bypass 

feeder breaker(s) must be set to support these temporary overloads. The magnitude of short-term 

overload bypass current demand is typically six to eight times the UPS current rating, but must be 

determined by analysis on a per-site basis. This analysis, generally known as an End-to-End Fault 

Coordination Study, must be done by a registered professional engineer experienced in this activity 

and familiar with local codes and related requirements. 
Liebert strongly recommends periodic inspections of the bypass feeder breaker instantaneous trip 

settings, as well as the module input (rectifier) feeder breaker trip settings, to ensure that they are 

correct. For a variety of reasons, although typically during circuit breaker maintenance procedures by 

others, trip settings have been known to be inadvertently left improperly set. Correct trip setting of 

these circuit breakers is most important to achieving high-availability from your Liebert UPS system.
For further information regarding proper trip settings for your feeder breakers, call 1-800-LIEBERT.

NOTE
The instantaneous trip setting of the breaker feeding the UPS bypass input should be high 

NOTE
While Emerson can provide typical guidelines, the responsibility for the proper breaker trip