Liebert installation manual Liebert Series 600T UPS, 65-225 kVA, Single Module

Figure 2. Alternative Grounding Configuration, 480 or 600 VAC input and output

7.2Alternative Grounding Configuration, 480 or 600 VAC Input and Output, Isolated Power Distribution Units, Wye-Connected Service

This configuration is similar to that shown in Section 7.1, except that the service entrance neutral is not brought into the UPS module. In this configuration, 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 the previous configuration, the current flows through the neutral conductor. In this configuration, the current flows through the ground path, possibly tripping ground fault interruptors (GFIs) and distorting the bypass waveform reference. Proper adjustment of ground fault interrupters is necessary to avoid unwanted tripping.

This configuration is reserved for those applications which meet all the following criteria:

•The facility has Wye-connected service

•The module rectifier input and bypass input are fed from the same source

•The connected load is strictly 3-wire (such as one or more PDUs) and does not require a neutral from the UPS

•Special precautions are taken to prevent trip- ping the ground fault interruptors. The time delay should be set to at least 30 cycles to pre- vent tripping when the UPS performs a transfer or retransfer operation.

CAUTION

Failure to properly set the ground fault interruptors could cause loss of power to the critical load.

Liebert Series 600T UPS, 65-225 kVA, Single Module 9