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3.5.1 Overloads (Without Transfer)
The UPS system is capable of sustaining full output voltage (±2% of the nominal voltage) for overload
conditions that remain within (under) the current versus time curve of system overload capacity and
the capacity of modules on-line (Figure 69). Note that the time scale is not linear. Load On UPS is
illustrated in Figure 54.
For high current demands of short duration (momentary overloads) the critical load is supplied simul-
taneously by both the UPS system and the bypass line. Whenever the critical load requires more than
155% of the capacity of the modules on-line, the bypass line will supply up to 1000% rated system cur-
rent through the solid state static switch for 40 milliseconds. Refer to 3.3.3 - Momentary Over-
loads.
Whenever an overload condition occurs, you should determine the cause of the overload.
If an overload condition exceeds the UPS system overload capacity or the capacity of the modules on-
line, the UPS system initiates an automatic load transfer to the bypass line.
Figure 69 Current-versus-time curves of module overload capacity
3.5.2 Automatic Transfers to Bypass
The UPS system will initiate an automatic load transfer to the bypass line if an overload condition
exceeds the current-versus-time curve of system overload capacity or capacity of the modules on-line
or if specified UPS system faults occur. Note that when the UPS system is operating in the non-
redundant mode, an automatic transfer to bypass will be initiated when any UPS module is taken off
line, either automatically or manually. Load On Bypass is illustrated in Figure 53.
The Overload Transfer and Output Undervoltage alarm messages will initiate an automatic transfer
to bypass and the Load On Bypass message will be displayed. The status report screens will include
the Automatic Transfer to Bypass message. Other UPS system faults will initiate an automatic trans-
fer to bypass followed immediately by the shutdown and isolation of the UPS modules. Refer to 3.5.5 -
Automatic Emergency Modules Off.
In an automatic transfer to bypass the UPS Output and System Bypass circuit breakers do not over-
lap as they do in a manual transfer. The static bypass switch is fired (closed), the UPS Output circuit
breaker is opened and the System Bypass circuit breaker is closed.
Bypass power is supplied to the critical load through the static bypass switch during the short time
while both circuit breakers are open.
NOTE
A load transfer to the bypass line will be completed whenever an automatic transfer to bypass
is initiated. If the OK to Transfer condition is present, the load transfer will be uninterrupted.
If the Static Switch Unable alarm message is present for any reason, the automatic transfer
will be interrupted for 40 to 120 milliseconds. Because of the reliability of the UPS components,
an interrupted load transfer is a very unlikely occurrence.
150%
125%
104%
150%
100%
% Load
1000%
100%
40ms 30s 10m 40ms
PULSED-PARALLEL
(FAULT-CLEARING)
INVERTER OVERLOAD
% Load