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MIL-HDBK-419A Terminal Protection Devices. Terminal Protection Devices (TPDs) and electromagnetic shields
provide additional means of lightning and/or EMP protection (Section The TPDs are fast-response
protection devices installed on exposed circuits such as power lines, signal, and control cables. Lead lengths of
each TPD shall be kept to an absolute minimum. They are installed on lines for the purpose of shunting
extraneous pulses to ground and are typically installed on signal and power lines at their point-of-entry into a
shelter's signal entry panel. Examples of commonly used terminal protection devices are carbon blocks, gas-
filled spark gaps, zener diodes, and EMI power and signal line filters. Surge arresters used to protect a system
against lightning may serve to protect it against certain types of EMP given the response time of the arrester is
properly designed (see Volume I, Section Signal Reference Subsystem. Grounding techniques used in the signal reference subsystem are a
function of operating frequencies. Lower frequency circuits (30 kHz and below) shall be single point grounded.
Higher frequency circuits (above 30 kHz) shall employ an equipotential plane which may, in the case of a
metallic van or shelter, be the skin of the housing. All equipment cases or cabinets must be bonded to the
equipotential planes by the shortest and most direct route. In fixed site facilities an equipotential plane will be
installed in accordance with Section 1.5.1. If a combination of both higher and lower frequencies circuits exist,
use the higher frequency signal ground technique. All digital circuits are considered higher frequency signal
circuits.  Signal reference subsystems, in a multishelter configuration shall be bonded to a common earth
electrode subsystem at one point only to minimize inter-shelter interference on signal cabling. Fault Protection Subsystem. The fault protection subsystem (grounding/green conductor) shall be
designed to carry current only in the event of equipment or system faults. The fault protection subsystem
includes equipment racks, cabinets, conduit, junction boxes, raceways, ductwork, pipes and other normally non-
current carrying metal elements. For shelterized equipments, the fault protection subsystem is connected to
the earth electrode subsystem via the power entrance panel.  Care should be taken to ensure the fault
protection subsystem and the signal reference subsystem are not connected to the earth electrode subsystem at
the same point. Measuring Ground Resistance in Tactical Environments.  The resistance to earth of the earth
electrode subsystem shall be measured by the fall of potential technique (see MIL-STD-188-124A para
or Section 2.7.2 of Volume I). If the tactical situation does not permit this method to be used, the three-point
or triangulation method is an adequate substitute. Section 2.7.3 of Volume I refers.  Reducing Ground Resistance in Tactical Environments. Three basic methods should be considered for
grounding tactical equipments and systems; (1) utilization of earth electrode subsystems of existing/permanent
facilities, (2) utilization of recently configured earth electrode, ground rod/ground rod configurations, and (3)
utilization of antenna counterpoises (including radial grounds). These methods and means of reducing related
resistance are described below. Existing Facilities.  Wherever feasible, installation of earth electrode subsystems should take
advantage of the low-resistance properties of existing facilities such as water pipes, water well casings,
plumbing, and other metals embedded in and in contact with the earth. Resistance of the facilities should be
measured prior to use to determine if the 10 ohm or less resistance criteria is met. Use of existing facilities as
grounding means is especially desirable in permafrost situations.


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