Dale Power Solutions Ltd
User Manual
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Figure 2 Power cable wiring configuration for a single UPS
Units in parallel
A group of paralleled units behave as if it were one large UPS. To ensure units share the load equally and
the system complies with relevant wiring regulations, follow these requirements:
■ Use units of the same rating only and connect them to the same bypass source.
■ Reference the Bypass and the Main input sources to the same neutral potential.
■ Ensure that any RCDs (residual current devices) are appropriately set and located (see Earth leakage
current below).
■ Connect the outputs of all units to a common output bus.
■ Ensure the length and specification of power cables including the bypass input cables and UPS
output cables of each unit is the same. This ensures load sharing when operating in bypass mode.
Note:
Optional isolation transformers are available for applications where sources do not share the
same neutral reference or where the neutral is not available.
Earth leakage current
The residual earth current introduced by the RFI suppression filter inside the UPS is between 3.5 mA and
1000 mA. We recommend that you verify the selectivity of the residual current detector (RCD) with all
other differential devices both upstream of the input distribution board and downstream (towards the
load).
Caution:
To avoid false alarms, locate earth leakage monitoring devices upstream of the
common neutral point when used in systems with split bypass input or when used in paralleled
UPS configurations. Alternatively use the device to monitor the combined 4-wire rectifier and
split bypass input currents.
Ensure that an RCD upstream of the UPS has these characteristics:
■ Sensitive to d.c. unidirectional (class A) pulses
■ Insensitive to transient current pulses
■ Average sensitivity adjustable between 0.3 and 1A
Such residual current circuit breakers (RCCB) are identified by the symbols:
