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Electrical Connections—UPS
17
3.2.1 Rectifier and Bypass Input Supply of the UPS
Overcurrents
Install suitable protective devices in the distribution unit of the incoming mains supply, considering
the power cable current-carrying capacity and overload capacity of the system. Generally, the
magnetic circuit breaker with IEC60947-2 tripping curve C (normal) at the 125% of the nominal
current listed in Table 41 is recommended.
Split bypass: In case a split bypass is used, separate protective devices should be installed for the
rectifier input and bypass input in the incoming mains distribution panel.
Earth Leakage, RCD Devices
Any residual current detector (RCD) installed upstream of the UPS input supply:
• Must be sensitive to DC unidirectional pulses (Class A)
• Must be insensitive to transient current pulses, and
• Must have an average sensitivity, adjustable between 0.3 and 1A.
To avoid false alarms, earth leakage monitoring devices when used in systems with split bypass input
or when used in paralleled UPS configurations, must be located upstream of the common neutral
sinking point. Alternatively, the device must monitor the combined four-wire rectifier and split
bypass input currents.
The residual earth current introduced by the RFI suppression filter inside the UPS is greater than
3.5mA and less than 300mA. Liebert recommends verifying the selectivity with all other differential
devices both upstream of the input distribution board and downstream (toward the load).
3.2.2 External Battery
The DC-compatible circuit breaker provides overcurrent protection for UPS system and battery,
which is provided by the external battery cabinet.
3.2.3 UPS Output
If an external distribution panel is used for load distribution, the selection of protective devices must
provide discrimination with those that are used at the input to the UPS (see Table 47).
NOTE
The rectifier input and bypass input must use the same neutral line.
For an IT power network system, a 4-pole protective device must be installed on the external
input distribution and external output distribution of the UPS.