MIL-HDBK-1004/5
Adding a zener-type suppressor can limit some of the transient
spikes which exist on the 118-volt system. Recordings of tests show that
spikes of approximately 260- to 270-volt crest were recorded. These spikes
could be reduced by the use of zener-type suppressors.
These observed spikes of 1.55 to 1.62 per unit of system peak
voltage pose no danger to the distribution equipment. Most of the 118-volt
equipment such as cables, rectifiers, etc., have an insulation capability of
at least 2.5 per unit. This applies similarly to all the 4,160-volt
equipment.
If the apparatus used in the 400-Hz system is not able to withstand
these 1.6-per-unit spikes, it is more cost-effective to provide extra surge
protection at the terminals of the apparatus than it is to add surge
suppressors at all utilization service assemblies. With zener-type
suppressors, the voltage could be clamped to approximately 1.38 per unit.
Slightly higher voltages could be expected if the discharge current is above
10 amperes. This assumes that all the zener-type suppressors have the same
clamping voltages. Usually the tolerances are between 5 and 15 percent. If a
15-percent tolerance increases the clamp voltage to 1.58 per unit, then the
zener-type suppressors are not effective in limiting spikes with a magnitude
of 1.6 per unit.
For this reason, varistors shall not be used to limit the voltage
for protection at the 120-volt level. For 400-Hz equipment which is sensitive
to voltage spikes of approximately 1.5 times normal voltage, zener-type
suppressors (with very low tolerance) shall be installed on the terminals of
that equipment.
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