Noninterruptible power is not a requirement set forth by any military
standard but is an equipment specification levied by the procuring agency. The
two most commonly specified interrupt times, through which the equipment is
required to continue operating, are a short-term interrupt (10 to 300
microseconds) and a long-term interrupt (20 to 150 milliseconds).
Operation for a short-term interrupt of prime power is intended to provide
adequate time, after recognition of a power interrupt, to perform a "put-away"
routine to store data, such that an automatic recovery occurs upon restoration
of power. Noninterruptible power is somewhat of a misnomer since it is really
not intended to provide performance through a short-term power transient as the
name might imply.
The short-term interrupt capability is usually inherent in the input
capacitor storage capability of the power supply along with proper selection of
the input power detection level. Accordingly, the impact on the power supply is
minor. Operation through a long-term interrupt is intended to accommodate
transfer from one power bus to another as implemented by a break-before-make
transfer of power buses on an aircraft or a shipboard automatic bus
One method of operating through a bus transfer is to add capacitance within
each power supply. For example, adding a capacitance of 6500 microfarads after
the rectifiers of a 3-phase, 60 Hz, 115 VAC, 500 W shipboard power supply would
allow 55 milliseconds of hold-up time at 20% low line. This capacitor would have
a volume of 67 cubic inches and a weight of 3.3 pounds. The capacitor would
occupy 40% of the volume of a 3 watt/cubic inch power supply and require the
rest of the circuitry to operate at a 5 watts/cubic inch output power density.
This is not unreasonable, but as output power density increases, the impact on
the power supply becomes substantial.
One alternative to adding capacitors to each power supply for storing energy
is to provide for auctioneering or diode OR'ing more than one DC bus together
and providing an appropriate amount of centralized or distributed capacitance
for energy storage to control the common bus voltage during the automatic