MIL-HDBK-1004/5
Section 7:
SURGE SUPPRESSION
7.1
Surge Protection at the Medium-Voltage Level. Surge arresters are
commercially available for both 60- and 400-Hz voltages. The insulation
capability of the equipment must be coordinated with the sparkover values of
the arresters.
Conventional silicon-carbide arresters have a spark gap in series
with the silicon-carbide blocks. Therefore, the application at 400 Hz should
not be a problem, since no current is conducted until the arrester sparks
over. The lowest rating available is 3 kilovolt, RMS, with a corresponding
switching surge sparkover voltage of 8.25 kV, 1.95 per unit of rated arrester,
crest voltage. The next higher rating is 4.5 KV RMS with a sparkover voltage
of 12.4 kV (1.95 per unit).
Metal-oxide arresters have similar characteristics. The smallest
arrester has a rating of 2.7 kV RMS, and a protective level of 5.6 kV. A 4.5-
kV RMS rated arrester has a protective level of 9.2 kV. These arresters have
been tested for 60-Hz application. No tests have been performed, and no
information is available for 400-Hz application. For a 4,160-volt system, the
nominal line-ground peak voltage is 3.39 kV, and therefore, the arrester
sparkover voltage of 9.2 kV is 2.71 times the nominal voltage.
7.2
Protection at the 120-Volt Level. The MIL-SPEC-704 requirement for
a 400-Hz system limits the maximum voltage to less than 180 volts RMS or 1.5
per unit of the nominal 120-volt rating. As discussed previously, the
protective levels of silicon-carbide or metal-oxide arresters on the 4,160-
volt systems are significantly higher, and therefore, they could not limit
voltage to the 1.5 per-unit level as required. For this reason, 4,160-volt
surge protection shall not be used to protect the load circuits on the 120-
volt level.
Protection of the 120-volt system can be accomplished with either
varistors or zener-type suppressors. The lowest rating of varistors for
industrial use is 130-volt RMS. With a 10-ampere current through the
varistor, a typical clamping voltage is 1.7 per unit of rated peak voltage.
The clamping voltage is the voltage where the limit occurs. For a varistor
rated 130 volts, the clamping voltage is 312 volts. Criteria require that the
voltage is limited to 180 volts times the square root of 2 or 255 volts.
Varistors are not suited for this application since their clamping voltage is
312 volts.
The catalogs for zener-type suppressors give limited information on
the capability of the devices. Only at the maximum values of current is the
voltage given, and that voltage is approximately 1.56 per unit of nominal peak
voltage. One manufacturer has indicated that at 10-ampere current, a clamping
voltage of approximately 1.35 per unit of nominal peak voltage can be
accomplished.
78
Previous Page
