20 Theoryof Operation
Also,if required for maintenance, the static switchc an beisolated from the bypass line by opening (to the OFF
position)the Static Switch Disconnects.
Pulsed Parallel Operation
Whenan overload condition such as magnetic inrush current or a branch load circuit faultexceeds the overload
capacityof the connected modules, the staticbypass switch pulses on for 40 milliseconds.This allows up to
1000%of the rated full load current from the bypass line to clear theove rloadwithout closing the bypass circuit
breaker.The bypass source is briefly in parallel withthe UPSsystem, permitting the bypass source to carry the
initialoverload current. If the overload clears before 40 milliseconds,a load transfer to bypass is not made. If the
overloadcondition continues, the automatic transferis made (maintaining the load voltage within the specified
limits).
Thispulsed static switch operation reduces nuisance operation of motor-operatedcircuit breakers for such short-
termconditions.
Load Transfers
Transfersto (transfer) or from (retransfer) the bypass may be performedautomatically or manually in a make-
before-breaksequence. This is accomplished through the overlapping operation of theUPS outputand the sys-
tembypass power switching devices.
Manualload transfers and retransfers are initiated by the operator from the System ControlCabinet (SCC).
Ina manual operation, or an automatic retransfer, the two motorized circuitbreakers—UPS output and system
bypass—areboth closed simultaneously for a short period of time (overlap).
Automatictransfers are initiated by the SCC system control logic when an overload isbeyond the specified capa-
bilitiesof the UPS system or when a fault occurs within a non-redundant UPS module. An automatic retransfer is
initiatedif this function is enabled and if system conditionsfor a retransfer are present.
Inan automatic transfer, the circuit breakers do not overlap, butduring the short time gap bypass power is sup-
pliedto the critical load through the solid state static switch.
DISCONTINUED
PROD U C T