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3.3.3 Momentary Overloads
An overload in the critical load will continue to be supplied by the UPS module if the overload condi-
tion does not exceed the current versus time curve. The overload curve includes 150% of rated current
for 30 seconds, 125% for 10 minutes and 104% continuously.
Momentary overloads from 155% to 1000% of rated current is supplied by pulsing the solid state
static bypass switch for 40 milliseconds. This provides current from the bypass line in parallel with
current from the UPS system output. By providing two power sources simultaneously, high current
can be supplied to the critical load with full output voltage regulation. The critical load can be sus-
tained through inrush currents and momentary faults without operating the motorized circuit break-
ers. Pulse paralleling also serves, under some circumstances, as a backup in the event that an
external bypass feeder breaker (e.g., BFB) trips open during this pulse-paralleling period, retaining
the UPS inverter connection with the critical load.
If an overload condition exceeds 40 milliseconds, the UPS module continues to supply power to the
critical load if it remains within the current-versus-time curve of overload capacity. If the system
overload capacity is exceeded, the UPS control logic completes an automatic transfer to the bypass
line by operating the motorized system circuit breakers (output and bypass). Refer to 3.5 - Auto-
matic Operations.
Figure 44 Momentary overload (through pulse-parallel static switch)
OUTPUT
CB
Controls
Battery
INPUT
CB
RIB
BFB
Critical
Load
AC Output
MBD
THREE BREAKER MAINTENANCE BYPASS SWITCHBOARD
CONTROL POWER
UPS MODULE
MAIN INPUT
SWITCHGEAR
Rectifier/
Charger
Inverter
Bypass CB
Static
Bypass
Switch
Static
Switch
Disconnects
BIB
MBB
MIB