Emerson 50 and 60 Hz Manuel D’Utilisation

Figure 63 illustrates the Liebert NX UPS in what is known as the split-bypass configuration 

wherein a separate power switch to a dedicated bypass power source that also feeds the maintenance 

bypass line connects the static bypass line. Where a separate power source is not available, the bypass 

and rectifier input supply connections are linked.

The circuit blocks labeled Static Switch in Figure 63 contain electronically controlled switching cir-

cuits that enable the critical load to be connected to either the inverter output or to a bypass power 

source via the static bypass line. During normal system operation the load is connected to the 

inverter; but in the event of a UPS overload or inverter failure, the load is automatically transferred 

to the static bypass line.
To provide a clean (no-break) load transfer between the inverter output and static bypass line, the 

static switch activates, connecting the load to bypass. To achieve this, the inverter output and bypass 

supply must be fully synchronized during normal operating conditions. This is achieved through the 

inverter control electronics, which make the inverter frequency track that of the static bypass supply, 

provided that the bypass remains within an acceptable frequency window.
A manually controlled, maintenance bypass supply is incorporated into the UPS design. It enables the 

critical load to be powered from the utility (bypass) supply while the UPS is shut down for routine 

maintenance.

Any external battery is connected to UPS through a circuit breaker fitted inside the battery cabinet –

or located adjacent to the batteries where a battery cabinet is not used. This circuit breaker is closed 

manually, but it contains an undervoltage release coil, which enables it to be tripped from the UPS 

control electronics following certain detected faults. It also has a magnetic trip facility for overload 

protection. The undervoltage release coil control is replaced by a battery contactor located inside the 

UPS fitted with either internal batteries or with battery start option or both

For 30-40kVA UPS with internal batteries, a standard temperature probe is installed to measure the 

internal battery temperature to optimize battery management. The measured temperature can be 

displayed from the UPS front panel.
For UPS with external batteries, an optional battery temperature interface equally optimises the 

external battery management by connecting up to four external temperature sensors from the battery 

cabinet(s) to a control unit inside the UPS.
For details, refer to Figure 27.)

The UPS is equipped with two identical and fully redundant control power supply boards. Each of 

them takes inputs from the AC and DC sources. When one of the sources or even if one of the control 

power boards fails, the UPS system can still operate normally. This feature further enhances the reli-

ability of the system. 

One single-phase Shuko-type universal outlet of 3A current handling capability provides nominal 

UPS output voltage of up to 3A current capacity for the ease of testing, commissioning & servicing of 

the UPS.

NOTE
When the UPS is operating in bypass mode or on maintenance bypass, the connected