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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
shielding installation should be checked for rigidity,
ings. The spacing between the electrodes is based on
as insecure mountings may cause a mechanical fail-
coordination of the protective characteristics of the
ure. Periodically inspect the structures to ascertain
gaps with other protective devices in a station. On
that shielding devices have not been seriously dam-
some installations, gaps may serve as the primary
aged by previous discharges.
surge protective device.
b. Construction. Copper, copper-clad steel, galva-
b. Maintenance. Protective gaps are installed
nized steel, or a corrosion-resistant metal alloy are
with a specific separation between the electrodes.
all materials used in the construction of shielding
Severe electrode burning, resulting from lightning
or system overvoltage discharges, may require a
devices.
readjustment of gap spacing. Maintain proper spac-
9-10. Surge capacitors.
ing and keep electrodes free from burrs and sharp
protrusions.
Surge capacitors are wave-shaping devices which
9-12. Surge bypass resistors.
after it is imposed on an electrical system.
a. Use. In general, a surge capacitor is connected
A bypass resistor is a device containing nonlinear
resistance material, in which the current varies as a
in parallel with a surge arrester for protection of the
turn-to-turn insulation of rotating machinery, such
power of the applied voltage. When a surge voltage
as motors and generators.
occurs, the resistance of this device is decreased to
b. Maintenance. See chapter 13 for capacitor
divert the surge current around the protected wind-
maintenance.
ing.
a. Application. Bypass resistors are normally ap-
9-11. Surge protective gaps.
plied for the protection of turn-to-turn insulation of
Protective gaps (sometimes referred to as rod gaps)
series-connected windings in such apparatus as
regulators, autotransformers, and reactors. Bypass
normally consist of two electrodes, spaced in the air
at a specific distance, with one electrode connected
resistors are also useful to reduce switching surges
on transmission lines, where the substantial surge
to ground and the second electrode to the line poten-
reduction provided outweighs their cost.
tial. Electrodes may be of various sizes and shapes
b. Maintenance. If bypass resistors are mounted
and are generally made of conventional lightning
rod material.
in air, any accumulation of dust particles between
a. Application. Protective gaps may be installed
resistance elements should be removed periodically
on substation structures in conjunction with line
using dry compressed air. Proper clearances must
be maintained to allow for free air circulation.
switches, or as an integral part of equipment bush-
9-3
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