MIL-HDBK-1004/5
60 Hz. There is also decreased electromagnetic pull on magnetic-trip
elements. Because all current ratings of devices are affected to different
degrees, consider applicable derating factors during design phase.
Check with the appropriate manufacturers to determine ratings
appropriate to the equipment. Also, specially calibrate thermal and magnetic
characteristics of protective devices for use on 400-Hz systems.
3.1.4.2
Surge Arresters. Provide Surge arresters for 60-Hz system
protection where necessary (see MIL-HDBK-1004/2, Power Distribution Systems).
In general, the only exposed lines will be those of the 60-Hz distribution
system. Therefore, provide 400-Hz protection only for devices whose
insulation capability is below that provided by the 60-Hz surge protection,
which will normally protect the medium-voltage 400-Hz devices. Varistors
available for use with the low-voltage 400-Hz system can limit surges to about
1.7 times the peak voltage; provide where required. The using agency will
furnish details of any equipment requiring other than varistor protection.
For 400-Hz electronic equipment sensitive to voltage spikes as low as 1.5
times the nominal voltage, zener-type suppressors (silicon-avalanche diodes)
can limit the voltage to 1.38 per unit. Provide these zener-type suppressors
normally on the equipment terminals.
3.1.4.3
Bus and Cable Material. Because of its lower resistance, use
copper, except where such use is clearly impracticable. Fully justify the use
of anything other than copper in the design analysis (see Section 4).
3.1.4.4
Conduit. The presence of magnetic materials in the vicinity of
electric conductors increases the flux density thereby increasing resistance
and inductance. Therefore, use nonmagnetic materials, such as aluminum or
plastic, for all raceways. Use nonmagnetic materials, such as aluminum,
bronze, or plastic, as appropriate, for cable terminations, cable clamps, and
other equipment.
3.2
Medium-Voltage Distribution System Design. Because the 15,000-foot
maximum feeder length dictates the number and location of acceptable central
plant sites, make the layout of the medium-voltage feeder lines first.
Type of Distribution. Generally, use raceway systems for
3.2.1
distribution of 400-Hz circuits. Bare, aerial 400-Hz systems are precluded
because of the excessive inductance of such circuits. Overhead distribution
systems using preassembled, messenger-supported, insulated cable are
acceptable in areas where lightning storms are few and where aircraft
clearance criteria do not apply. In areas where protection against
lightning-induced surges is required, use surge arresters specifically
designed for use at 400 Hz for protection of underground-to-aerial risers.
The use of 60-Hz arresters is ineffective and hazardous because of the
capacitive elements of arresters. The change in frequency changes the
capacitance and, therefore, disturbs the even-voltage gradients which prevent
premature sparkover.
17
Previous Page
