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 MIL-HDBK-419A
(e) Resistor R2 and capacitor C1. Resistor R2 attenuates current flow to protected
equipment resulting from clamp voltage of the transient suppressor at the equipment entrance. The resistor
also speeds up, and in some cases, generates turn-on of the transient suppressor at the equipment entrance. In
addition, the resistor limits current drain from protected equipment when a transient with polarity opposite
that of the equipment power supply occurs. A power rating of 1 watt is sufficient for the resistor. The
resistance value should be as high as can be tolerated by applicable equipment, taking into consideration the
value of resistor Rl and the impedance of the associated landline. The purpose of capacitor C1 is to filter out
some high-frequency transient components, and the value of C1 should be selected accordingly. In some cases,
equipment operating characteristics and line length may preclude the use of resistor R2 and capacitor C1.
(2) Grounding for suppression components/circuits. The high-energy transient suppressors, shown
at the facility entrance in Figure 1-43 must be grounded to earth ground by means of the shortest path. This
will minimize the large voltage spikes, caused by L di/dt effects when high-amplitude transient currents flow
through the high-energy transient suppressor onto the ground, which in turn may damage protected equipment
or the low-energy transient suppressors at the equipment entrance.
(a) Grounding of transient suppressor at facility entrance. The high-energy transient
suppressors installed at the facility entrance should be located in a junction box or the main (first) service
disconnect where incoming lines are first terminated. The most effective ground for the suppressors can be
provided by a ground bus bar located in the first service disconnect or the junction box. The transient
protection devices (TPD's) must be bonded to the TPD box and grounded by the shortest means. It is important
that the ground wire has no sharp turns or bends, and is as short as feasible. The ground bus bar should be
located to permit short, direct connection of suppressors between landline terminations and earth ground.
(b) Grounding of transient suppressor at equipment entrance. The low-energy transient
suppressor at the equipment entrance should be directly bonded to the equipment case when possible. The
ground side of the suppressor at the equipment entrance must be connected with a short, straight, direct
connection to equipment case to be effective. Connection of the suppressor to equipment case references both
the suppressor and equipment circuits to the same ground potential, thus providing optimum equipment
protection.
(3) Packaging design. Transient suppression components/circuits included as an integral part of
equipment design should be enclosed in a shielded, compartmentalized section of the equipment. This is
necessary to preclude cross-coupling of transient energy to other equipment circuits. The suppression
components must be located so that transients are attenuated prior to entering any equipment component
susceptible to damage, including EMI filters. Packaging design for transient suppression specified for
installation at facility entrance is not critical. However, the design should provide for short, direct connection
of transient suppressors between the line termination and ground.
Coaxial and twinaxial lines (dc to 3 MHz). The same transients are projected to occur on externally
b.
exposed coaxial and twinaxial lines as on the control and status lines discussed in paragraph 1.3.3.7.8a. In
general, the same transient protection described in paragraph 1.3.3.7.8a will provide effective transient
protection for equipment that directly interfaces the coaxial and twinaxial lines. That is, the most effective
transient protection is provided by installing a high-energy transient suppressor and resistor at the facility
1-93
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