TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
As the sparkover voltage of a gap varies with the
(2) The ground lead is securely fastened to the
atmospheric pressure, the protective characteristics
arrester terminal and ground.
of arresters are affected by the altitude at which
(3) The arrester housing is clean and free from
they are installed. Standard arresters are consid-
cracks, chips, or evidence of external flashover.
ered suitable for altitudes up to 6,000 feet.
(4) The arrester is not located in such a man-
c. Selection. Both types do the same job, but the
ner as to be subject to:
need for selection of higher voltage levels for the
(a) Damaging fumes or vapors.
silicon-carbide type means the protection is slightly
(b) Excessive dirt or other current-con-
less. When gapped type arrestors fail, consider re-
placing them with the metal-oxide type.
(c) Excessive humidity, moisture, dripping
d. Equivalence. ANSI/IEEE C62.11 provides an
MCOV rating and a corresponding duty-cycle volt-
water, steam, or salt spray.
age rating for MOSA units. The duty-cycle rating is
(d) Abnormal vibrations or shocks.
based on the familiar voltage ratings of ANSI/IEEE
(e) Ambient temperatures in excess of 40 de-
C62.1, long used for the silicon-carbide design.
(5) Any external gaps are free from foreign ob-
9-7. Classification of surge arresters.
jects and set at proper spacings.
ANSI/IEE C62.1 classifies arresters as station, in-
b. Electrical tests. Visual inspection will not al-
termediate, distribution, and secondary types. The
ways detect a damaged arrester. Interior damage
best (lowest) available protective level and energy-
may result from a broken element, presence of mois-
discharging capability is provided by the station
ture, a severe direct lightning stroke, or the use of
type with successively poorer (higher) protection
an arrester with an incorrect rating. Sometimes
levels for the other classifications. For distribution
these conditions will cause radio interference. Elec-
arresters, ANSI/IEEE C62.11 defines a normal-duty
trical tests, to detect inferior arrester units, may be
and a heavy-duty type, dependent upon the test
made either in the field or shop. Tests must be made
severity. Heavy-duty arresters are more durable
strictly in accordance with manufacturer's recom-
and generally have lower protective characteristics.
mendations, and the results interpreted in line with
9-8. Maintenance of surge arresters.
(1) Power factor tests. Each type and class of
Modern surge arresters require little operational
lightning arrester has a specific power factor when
maintenance and the degree to which such mainte-
new. Periodic testing of a unit will show little devia-
nance can be done is normally limited by lack of
tion from the original (when new) power factor, so
adequate test equipment. This limits surge arrester
long as it remains in good operating condition. A
maintenance to visual inspection and simple electri-
major deviation from the original value indicates
cal tests. It is recommended that units found to be
that the arrester has been mechanically damaged or
defective be replaced rather than repaired: Where
an arrester is composed of two or more individually
(2) Megger tests. A megger test can be made to
complete units, each unit should be tested sepa-
provide additional information on the condition of
rately. Thus, a bad unit may readily be replaced and
an arrester. Such a test may indicate shorted valve
the good units retained. Surge arresters are almost
elements in valve-type arresters.
always applied with one terminal connected to an
(3) Operation tests. Electrical tests to deter-
electrically energized source and one terminal to
ground. No work should be done, or contact made
mine 60-hertz breakdown and leakage current may
with surge arresters, when connected to the ener-
be made in the field or shop, but must be made
cautiously so as to avoid damage to the arrester. It
a. Visual inspections. Visual inspection should be
is questionable whether these tests can be justified
made periodically to ensure that:
for military installations, where the number of ar-
(1) The line lead is securely fastened to the line
resters potentially subject to such tests is relatively
conductor and the arrester.
Section III-OTHER DEVICES
9-9. Surge shielding devices.
strokes from structures and equipment where more
than surge arrester protection is justified.
Shielding devices, such as lightning rods (air termi-
a. Maintenance. Maintenance should ensure
nals), lightning masts, and overhead ground wires,
proper ground connections having minimal resis-
are installed for the purpose of diverting lightning
tance. All supports and device clamps used in the