Liebert 1000kVA Operation & Maintenance Manual page 23

Theory of Operation
Static Switch Isolation
The motor-operated system bypass circuit breaker (SBB), wired in parallel with the static switch,
automatically closes in approximately 200 milliseconds after the load is transferred to the bypass
power source, removing the static switch from the power flow.
Also, if required for maintenance, the static switch can be isolated from the bypass line by opening (to
the OFF position) the Static Switch Disconnects. For maintenance, this should only be done with the
UPS system critical load on bypass power.
Pulsed Parallel Operation
When an overload condition such as magnetic inrush current or a branch load circuit fault exceeds the
overload capacity of the connected modules, the static bypass switch pulses on for 40 milliseconds.
This allows up to 1000% of the rated full load current from the bypass line to clear the overload with-
out closing the bypass circuit breaker. The bypass source is briefly in parallel with the UPS system,
permitting the bypass source to carry the initial overload current. If the overload clears before 40 mil-
liseconds, a load transfer to bypass is not made. If the overload condition continues, the automatic
transfer is made (maintaining the load voltage within the specified limits).
This pulsed static switch operation reduces nuisance operation of motor-operated circuit breakers for
such short-term conditions and serves, under some circumstances, as a backup in the event that an
external bypass feeder breaker trips open during this pulse-paralleling period.
Load Transfers
Transfers to (transfer) or from (retransfer) the bypass may be performed automatically or manually in
a make-before-break sequence. This is accomplished through the overlapping operation of the UPS
output and the system bypass power switching devices.
Manual load transfers and retransfers are initiated by the operator from the System Control Cabinet
(SCC).
In a manual operation or an automatic retransfer, the two motorized circuit breakers—UPS output
and system bypass—are both closed simultaneously for a short period of time (overlap).
Automatic transfers are initiated by the SCC system control logic when an overload is beyond the
specified capabilities of the UPS system or when a fault occurs within a non-redundant UPS module.
An automatic retransfer is initiated if this function is enabled and if system conditions for a retrans-
fer are present.
In an automatic transfer, the circuit breakers do not overlap, but, during the short time gap, bypass
power is supplied to the critical load through the solid state static switch. In an SCCC control cabinet
with a continuous duty static switch, if for some reason the Static Bypass Breaker (SBB) fails to close,
the static switch will carry the load continuously.
17