been successfully packaged for in-line installation on critical rf lines, but unit cost is excessive. Also, gas-filled
spark gaps do not always provide satisfactory protection because of high sparkover (turn-on) voltage, slow
turn-on time, and low arcing voltage. Therefore, the best alternative at present is to include transient
suppression design as an integral part of new equipment.
(1) Transient suppression design. Potential sources of effective transient suppression are gas-
filled spark gaps, MOV in series with rf chokes, and surge-rated, low capacitance silicon avalanche diodes
paralleled with selected rf chokes. All of the suppression devices and components are for line-to-ground
connection at the line-equipment interface.
(2) Transient suppression grounding. The total transient suppression is included as an integral part
of the equipments, and may have to dissipate the transient currents listed in Table 1-19. However, in most
cases, these lines will be enclosed in ferrous metal conduit, and the amplitude of occurring transients will
therefore be only 10% of the values listed in Table 1-19. In either case, the transient suppression should be
grounded directly to equipment case ground using the shortest and most direct method possible. The equipment
case must, in turn, be effectively connected to the earth grounding system via the equipment rack and the
equipment grounding conductor, when applicable.
(3) Packaging design. The transient suppression should be located in a shielded, compartmen-
talized section of the equipment and located so that conducted transients are attenuated prior to entering any
susceptible circuit component.
18.104.22.168 Corrosion Control.
The materials of which lightning protection subsystems are made must be highly corrosion resistant.
Junctions or contact between dissimilar metals must be avoided; where such unions are unavoidable, moisture
must be permanently excluded from the contacting surfaces.
Where any part of a copper protective system is exposed to the direct action of chimney or other
corrosive gases, the exposed copper elements are to be protected by a continuous hot dip coating of lead. The
coating should extend at least 0.6 meters (2 feet) below the top of the chimney or past the vent or flue opening.
Where aluminum down conductors are used, do not permit them to come in contact with the soil.
(1) Connections between aluminum down conductors and copper ground electrode risers are not to
be made lower than one foot above grade level; use UL-approved bimetallic connectors for these connections.
(2) Aluminum parts, including fasteners and anchors, should be protected from direct contact with
concrete or mortar wherever such concrete or mortar is wet or damp or may become intermittently wet or
Aluminum parts also must be protected from contact with alkaline-based paints.