TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
9-1. Overvoltage protection.
ground fault may increase the line-to-ground volt-
age of the unfaulted phases. An overvoltage results
This chapter describes the maintenance and repair
when resonance occurs from single-pole switching of
of protective devices installed to limit transient
three-phase circuits. Accidental contact with a
over-voltages on lines. Abnormal voltages are caused
higher-voltage system may cause an overvoltage.
most frequently by lightning, but system distur-
Forced-current zero interruptions, improperly ap-
bances can also cause damaging voltage surges.
plied, may cause a high transient voltage. The pro-
9-2. Lightning-induced voltage surges.
tective devices discussed for lightning-induced
surges will also protect the system from these
Overhead lines are extremely vulnerable to direct
strokes or to induced voltage influences. Under-
ground systems derived from aerial lines may also
9-4. Surge limiting protective device require-
a. Causes. Lightning results from the potential
difference between clouds or between a cloud and
A surge limiting protective device must limit tran-
earth. A lightning stroke may be in direct contact
sient over-voltages or surge voltages that could dam-
with an electric line and equipment. The charged
age apparatus. The device must bypass the surge to
clouds of a passing lightning storm may also cause
ground and discharge severe surge currents of high
an electrostatically induced voltage.
magnitude and long duration without injury. The
b. Protection. The high voltage of a lightning
device must continuously withstand the rated
surge, imposed on lines and devices without surge
power voltage for which it is designed. The device's
protection, will flash over the insulation in the ma-
protective ratio is the maximum surge voltage it
will discharge, compared to the maximum crest
jority of cases. Where flashover occurs, through air
or on insulators, it rarely causes permanent dam-
power voltage it will withstand following discharge.
Surge arresters provide the most accepted method
age, but flashover occurring through the solid insu-
of surge limiting protection, since they provide the
lation on equipment or cable can result in perma-
highest degree of surge elimination. Other methods
include shielding lines and equipment from direct
9-3. System operating voltage disturbances.
lightning strokes; and providing devices designed to
divert or change the wave form of the surge, such as
Transferring a system from one operating condition
to another may generate a short-time transient
protective gaps, surge capacitors, and bypass resis-
overvoltage, known as a switching surge. A line-to-
Section II-SURGE (LIGHTNING) ARRESTERS
9-5. Definition of a surge arrester.
a. Metal-oxide type. A metal-oxide surge-arrester
(MOSA) utilizing zinc-oxide blocks provides the best
A surge arrester is a protective device for limiting
performance, as surge voltage conduction starts and
surge voltages on equipment by discharging or by-
stops promptly at a precise voltage level, thereby
passing surge current. Surge arresters allow only
improving system protection. Failure is reduced, as
minimal flow of the 60-hertz-power current to
there is no air gap contamination possibility; but
there is always a small value of leakage current
current has been discharged, a surge arrester, cor-
present at power frequencies. Therefore, the arrest-
rectly applied, will be capable of repeating its pro-
tective function until another surge voltage must be
er's maximum power-frequency continuous operat-
ing voltage (MCOV) can not be exceeded.
b. Gapped silicon-carbide type. Silicon-carbide
9-6. Types of surge arresters.
has more nonlinearity than zinc-oxide. Without a
gap the increase in leakage current, because of this
Surge arresters used for protection of exterior elec-
trical distribution lines will be either of the metal-
nonlinearity, would soon burn out the arrester. A
oxide or gapped silicon-carbide type. Expulsion-type
gap prevents burnout, but it does mean that the
arrester will not operate until the gap sparks over.
units are no longer used.