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20 Theory of Operation
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.
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 without 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 milliseconds, 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.
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 sys-
tem 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 capa-
bilities 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 retransfer are present.
In an automatic transfer, the circuit breakers do not overlap, but during the short time gap bypass power is sup-
plied to the critical load through the solid state static switch.