isolation of one UPS module (including its bypass) from the other module is
inherent in the "cold" standby redundant configuration (see Figure 3).
c) The parallel redundant UPS contains a built-in manual bypass
circuit in the control cabinet which will serve to isolate the UPS modules
during emergency conditions or maintenance. This manual bypass circuit
provides make-before-break switching and contains synchronizing provisions.
Where frequency conversion is provided by the UPS, a solid state frequency
conversion system or a motor-generator set will be required to bypass the UPS
module.
4.7
UPS Distribution System. The UPS system serves critical loads
only. Noncritical loads are served by separate distribution systems supplied
from either the electronic or utility bus as appropriate.
4.7.1
Critical Load Protection. Provide critical load panels with
current-limiting circuit breakers or fast acting fuses to shorten the
transient effects of undervoltages caused by load faults.
4.7.2
Critical Motor Loads. Due to the energy losses and the starting
current problem inherent in motor-generator sets, their connection to a UPS
bus should be limited to frequency-conversion applications only. Generally,
motor-generator sets shall be started on the UPS bypass circuit. However,
motor-generator sets may be started on the rectifier mode of operation under
the following conditions:
a) When rating of the motor-generator set is less than 5 percent
of the UPS rating.
b)
When reduced voltage starters are utilized for each motor load.
c) When more than one motor-generator set is connected to the
critical bus, each set must be energized sequentially rather than
simultaneously.
Solid state frequency conversion systems may be considered in lieu of
motor-generator sets.
4.8
Emergency Electric Power Source Requirements. The UPS system can
serve the critical electronic load continously during periods of normal
source electric power outage only if one or more emergency engine generators
are provided and sized to supply not only UPS equipment, but all necessary
auxiliary equipment such as lighting, ventilation, and air conditioning. The
emergency electric power source shall meet the criteria given in paras. 4.8.1
through 4.8.5.
4.8.1
Automatic Operation. The emergency generator(s) shall be automatic
start, automatic transfer-to-load upon normal electric power failure and
shall include provisions for automatic load shedding and load restoration
where required.
4.8.2
Paralleling. At a minimum, manual synchronizing of the emergency
generator(s) with the normal electric power source shall be provided. As an
alternative, automatic synchronizing of the emergency generator with the
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