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 MIL-HDBK-419A
(2) Other external line interfaces (dc to 3 MHz). The most effective method for providing
transient suppression is to design low-energy level transient suppression as an integral part of the equipment
and specify that high-energy level transient suppression, of a design provided by the manufacturer, be installed
on applicable lines in cable demarcation junction boxes at building penetration or exterior equipment
termination. Total transient suppression may be designed as an integral part of the equipment but caution must
be exercised to ensure that a separate, dedicated path to earth ground be provided for the high-energy level
dissipation section of the transient suppression.
(3) External line interfaces (above 3 MHz). All transient suppression must be designed as an
integral part of the applicable equipment. This is necessary because effective suppression devices/circuits are
not currently available for in-line installation on rf lines above 3 MHz, primarily because of high insertion
losses. If useable, effective high-energy level suppression becomes available in the future, the most effective
transient protection can be realized by installing high-energy level suppression on applicable lines at a metal
bulkhead connector plate at building penetration and including low-energy transient suppression as a part of the
equipment.
Existing equipment. The most effective transient protection can be provided as described in a(l),
b.
(2), and (3) above. When room is not available in the existing equipment to add required transient suppression
components, the components can be installed in a small enclosure affixed to the chassis or cabinet rack for all
except rf lines that carry rf signals above 3 MHz.
1.3.3.7.6 AC Power Input. The clamp voltage, appearing across protected equipment by the secondary ac
surge arrester installed at the facility main service disconnect means, when dissipating a transient surge, may
be higher than the withstand level for the equipment. Therefore, effective transient suppression must be
designed as an integral part of the equipment.
a.
Transient suppression design. To provide effective protection, equal suppression must be installed
line-to-ground on each service conductor input and the neutral input. For floating (ungrounded) line-to-line
power inputs, line-to-ground suppression must be installed and line-to-line suppression is optional. Suppressors
installed at the equipment power input should have a slightly lower turn-on voltage and a slightly faster
response time than suppressors of the secondary ac surge arrester at the main service disconnect means. This
permits the suppressors integral to the equipment to clamp short-duration overshoot voltage that occurs before
the secondary ac surge arrester can turn on and clamp in response to a transient. Also, with a lower turn-on
voltage, the suppressors at the equipment will have a lower clamp voltage for a given transient surge than the
secondary arrester and thus provides optimum equipment protection. However, with the specified character-
istics, the surge suppressors at the equipment will tend to dissipate the occurring transient before the secondary
arrester turns on. Therefore, it is imperative to have an inductor or a minimum 10 foot cable added in series
with the input line. If the inductor is properly chosen, the secondary surge arrester may then turn on very
rapidly after the equipment suppressor(s) turn on because of the voltage increase across the inductor. The
voltage increase is caused by current drain through the equipment suppressors to ground. Figure 1-41 depicts a
typical suppression circuit for use at the equipment level on ac inputs with a neutral. Figure l-42 depicts a
typical suppression circuit for use on ungrounded (line-to-line) inputs.
1-84
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