MIL-HNBK-419A
Summary of Sections
Volume II
1987
Paragraph 1.3.3.7.1 Transient Protection Requirement: Individual items of electrical and electronic equipment that directly interface
any externally exposed equipment lines, including commercial ac, may require transient protection that is designed as an integral part
of the equipment. Whether or not protection is required is dependent on the damage susceptibility of the equipment of interest, the
level of transient suppression provided on externally exposed lines at building penetration or external equipment termination and the
level of transient energy that is projected to be conducted to the equipment. For use herein, externally exposed lines are defined as
lines exposed to outside weather elements and environmental conditions. The lines may run overhead, run along grade surface, or be buried
in earth. Included are ac power input lines and signal, control, status, and intrafacility powerlines. The lines are commonly referred to
as landlines. Transient protection is not required in equipment when an interfaced landline if fiber optic in lieu of a metallic line. In
order to provide effective transient protection, the damage (withstand) level for the equipment must be determined, and the amplitude and
number of transients that will be conducted to the equipment must be known. (See 1.3.3.7.3 below) Three areas of equipment circuitry normally
require transient protection and are listed: 1) the ac power input 2) where other externally exposed lines interface with the equipment
3) rectifier outputs of 5 to 48 vdc power supplies that operate form commercial ac power and supply operating power for solid-state
equipment.
Paragraph 1.3.3.7.3 Determination of Equipment Damage (Withstand) Levels - Manufacturers do not normally specify withstand levels
for components. Therefore, an analysis should be performed to determine the withstand level for each item of equipment that directly
interfaces any externally exposed lines including ac input lines.
Paragraph 1.3.3.7.4 Determination of Need for Transient Protection - Power supplies (5 to 48 v) operating from ac inputs and supply
operating power for solid-state equipment always require internal transient protection. Other equipment that directly interfaces externally
lines, including commercial ac inputs, may or may not require transient protection designed as an integral part of the equipment. ….. When
the transient amplitudes are above the equipment withstand level, equipment level transient protection is required, either at the ac input,
other externally exposed line equipment interfaces or both.
Paragraph 1.3.3.7.5 Minimizing Transient Damage - When equipment requires protection against lightning generated transient damage,
transient suppression design must ensure that transients are attenuated to the equipment withstand level prior to entering any equipment
component. Therefore, the transient suppression must be effective at the external line equipment interface.
Paragraph 1.3.3.7.6 Transient Suppression Design - To provide effective protection, equal suppression must be installed line-to-ground
on each service conductor input and the neutral input. ….. 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 arrestor 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
arrestor 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 that the secondary arrestor and thus provide optimum equipment protection.
Paragraph 1.3.3.7.8 Landline Transient Suppression - When the equipment withstand level is below the transient energy level projected
to occur at direct landline equipment interfaces, transient suppression must be provided by equipment design. Generally, all direct landline
equipment interfaces will require transient suppression.
Paragraph 1.3.3.7.8a Control, status, intrafacility power, and audio landlines - Control, status, intrafacility power and audio lines,
other than coaxial and twinaxial lines, are most effectively protected by transient suppression designed as an integral part of the equipment,
and specified transient suppression installed at building penetration or exterior equipment termination.
Paragraph 1.3.3.7.8 (3)b Coaxial - In general, the same transient protection describe in 1.3.3.7.8a will provide effective transient
protection for equipment that directly interfaces the coaxial lines. That is, the most effective transient protection is provided by installing
a high-energy transient suppressor at the facility entrance or exterior equipment termination.
Additional information and site survey information can be obtained by contacting
POC: Mike Helms mike@lightningmike.com
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