which the critical load is transferred to supply the inverter output. UPS
module No. 1 can then be completely isolated for maintenance or repair.
g) Parallel Redundant UPS Configuration. A parallel redundant UPS
configuration consists of two or more UPS modules with a system control
cabinet and a common battery. UPS modules operate in parallel, and the
system is capable of supplying the rated critical load upon failure of any
one UPS module. The failed UPS module will be disconnected from other UPS
modules without an interruption to the critical load (see Figure 4).
4.5
Redundancy. For facilities requiring UPS systems, a nonredundant
UPS system is satisfactory for most operations. The justification for the
added expense of either redundant system is given in paras 4.5.1 and 4.5.2.
4.5.1
"Cold" Standby Redundant UPS System. Provide a "cold" standby
redundant UPS system when the installation site is isolated and logistic
support is poor or when the primary electric power supply exhibits poor
reliability by experiencing more than four power interruptions per month or
when the primary electric power supply provides voltage or frequency
fluctuations which are beyond the limits permitted in MIL-HDBK-411, Long Haul
Communications (DCA) Power and Environmental Control for Physical Plant.
4.5.2
Parallel Redundant UPS System. Provide a parallel redundant UPS
system when the frequency of the primary electric power supply must be
converted to serve the critical load (that is, 50 Hz to 60 Hz, or 60 Hz to
400 Hz).
4.6
Electric Service and Bypass Connectors. Two separate electric
services, one to the UPS rectifier circuit and the other to the UPS bypass
circuit shall be provided. Where possible, they shall emanate from two
separate buses with the UPS bypass connected to the electronic (noncyclic)
bus and the rectifier connected to the utility (cyclic) bus. This connection
provides isolation of sensitive electronic loads from the effects of UPS
rectifier harmonic distortion and motor startup current inrush.
4.6.1
Electric Service Size. A UPS system is considered to be a
continuous type load, and service to both the rectifier and bypass circuits
should be sized in accordance with article 220 of the National Electric Code.
The required current for the rectifier circuit is calculated based on the
UPS output rating divided by the UPS efficiency and multiplied by a load
factor of 125 percent to take into account battery charging. The UPS bypass
circuit may be rated for less current since both UPS losses and battery
charging requirements need not be supplied.
4.6.2
Maintenance Bypass Provisions. To provide for maintenance of
equipment, bypass provisions are provided as follows:
a) For the non-redundant UPS unit, install an external bypass
switch in order to completely isolate the UPS unit (see Figure 2). This is
to enable testing of the UPS unit, including the static switch, with dummy
load without interruption to the critical bus.
b) Each module of the "cold" standby redundant UPS has the same
built-in bypass circuit provisions as the nonredundant UPS. Complete
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