104 Operation
3.4.2 Automatic Transfers to Bypass
TheUPS system will initiate an automatic load transferto the bypass line if an overload condition exceedsthe
currentversus time curve of system overload capacity, or capacityof the modules on-line, or if specified UPS
systemfaults occur. Note that when the UPS system isoperating in the non-redundant mode, an automatic trans-
ferto bypass will be initiated when any UPS moduleis taken off line, either automaticallyor manually. Load On
Bypassis illustrated in Figure 4 8.
TheOverload Transfer and OutputUndervoltage alarm messages will initiatean automatic transfer to bypass and
theLoad On Bypass message will be displayed. The statusreport screens will include the AutomaticTransfer to
Bypassmessage. Other UPS system faults will initiate an automatic transfer to bypass followed immediatelyby
theshutdown and isolation of the UPS modules. Refer to 3.4.5 - Automatic Emergency Modules Off.
Inan automatic transfer to bypass the UPS Output and System Bypass circuitbreakers do not overlap as they do
ina manual transfer. The static bypassswitch is fired (closed), the UPS Output circuitbreaker is opened, and the
System Bypass circuit breaker is cl osed.
Bypasspower is supplied to the critical load throughthe static bypass switch during the short time while both cir-
cuitbreakers are open.
3.4.3 AutomaticRetransfers to UPS
AutomaticRetransfer to UPS is an option that you can selectfrom the SCC System Configuration screen. If you
donot want the UPS system to initiate any automatic retransfers, set Max Auto-RexferAttempts to zero (0).
Inan automatic retransfer, the two motorized circuit breakers (System Bypassand UPS Output)are both closed
simultaneouslyfor a short period of time (overlap).
Thefollowing critical bus conditions must be present toi nitiatean automatic retransfer of the critical loadfrom
thebypass source to the UPS system:
1. Thenumber of Auto-Rexfer Attemptsselected must be greater than zero (0). If zero (0) is selected, no
automatic retransfer s will occur.
2. Criticalloadwas initially transferred to the bypass source due to a system overload only. (A manually
initiatedretransfer from bypass is required if the criticalload exceeded the capacity of the modules on-line,
orif a UPSfault occurred.).
3. Overloadhas since droppedbelow 100% of the rated SCC current.
4. EnoughUPS modules are on-lineto supply the critical load.
5. OKto Transfersignal received from the control logic for at least 10 seconds, within 5 minutes of the system
overloadtransfer. (A manually initiatedretransfer from bypass is required for overloads lasting 5 minutes or
more.)
6. Cyclic-typeoverloads,which occur up to five (select range is 0 to 5) times in 60 minutes, are automatically
returnedto the UPS system for each event including the Nth overload. A manually initiatedretransfer from
bypassis required for the N + 1 overload.
NOTE
Aload transfer to the bypass line will be completed whenever an automatictransfer
tobypass is initiated If the OK to Transfer conditionis present, the load transfer will
beuninterrupted. If the Static Switch Unablealarm message is present for any
reason,the automatic transfer willbe interrupted for 40 to 120 milliseconds. Because
ofthe reliability of the UPS components, an interruptedload transfer is a very
unlikelyoccurrence.
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PROD U C T