Quantcast Transient suppression for lines in metal conduit

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entrance or exterior equipment termination, with low-energy transient suppression included as an integral part
of the equipment as shown in Figure 1-43. However, in many cases, end equipment connected to coaxial lines
cannot tolerate added capacitance imposed by capacitor Cl.  Also, in most cases, the added resistance of
resistor R2 cannot be tolerated. Because most end equipment connected to coaxial and twinaxial lines has a
relatively low withstand level, the configuration shown in Figure 1-43c, without resistor R2 and capacitor Cl,
should be used for transient suppression. The silicon avalanche diode suppressor TSl should always be bipolar.
The configuration shown by Figure 1-43c should be used for protection of equipment that directly interfaces
externally exposed twinaxial lines. In most cases, it is necessary to use a bipolar SAS since the twinaxial lines
normally conduct both dc and low-level audio signals. Specific design criteria is provided in paragraphs (1) and
(2) below.
(1) Facility entrance suppression. The high-energy transient suppression specified for location at
facility entrance or exterior equipment termination should be designed for in-line installation on applicable
lines.  The lines should be terminated at a metal connector plate located in a junction box at the facility
entrance or exterior equipment termination. Transient suppression components should be enclosed in a sealed,
metal enclosure with appropriate connectors to facilitate in-line installation. The ground side of suppressor(s)
in the sealed package must be connected as directly as possible with No. 12 AWG copper wire (minimum) to a
ground point located on the exterior of the sealed package to facilitate connection to a ground bus or tie point
in the junction box. The package for a twinaxial line must include two suppression circuits, one for each of the
two center conductors. Also, when a coaxial cable shield is not directly grounded at interfaced equipment, the
enclosure for in-line installation must also contain two transient suppression circuits, one for the cable center
conductor and one for the cable shield. Circuit configurations for each type of line are depicted in Figures 1-44
and 1-45.  Primarily because of the grounding configuration, MOV or equivalent devices should be used at
facility entrance.
(2) Equipment entrance suppression. Equipment entrance suppression is shown in Figure 1-44 for
coaxial line-equipment interfaces. The transient suppression should be enclosed in shielded, compartmentalized
areas to prevent cross-coupling of transient energy to other equipment circuitry. The transient suppression
must be located so that transients are attenuated prior to entering any susceptible equipment components,
including EMI filters. Because of the normally low withstand levels for end equipment, only bipolar avalanche
diode suppressors should be used at equipment entrance. However, MOV suppressors may be used when the
protected equipment can safely withstand the clamp voltages that will appear across protected equipment. For
the most effective protection, the ground side of transient suppressors should be bonded directly to equipment
case. When direct bonding is not possible, short, direct connections to equipment case must be used.
Transient suppression for lines in metal conduit. When externally exposed lines are enclosed end-to-
end in ferrous metal conduit, the amplitude of transients projected to be conducted to equipment will be
attenuated a minimum of 90%. The number of transients that occur will not change. Therefore, the number of
transients listed in Table 1-19 will still occur, but amplitudes will be only 10% of the amplitudes listed in
Table 1-19. When the equipment manufacturer is absolutely certain that all externally exposed equipment lines
will be enclosed in ferrous metal conduit, total transient suppression should be designed as an integral part of
the equipment. The total transient suppression should consist of a 5-watt resistor in series with the landline
input, and an MOV or SAS connected line-to-ground on the equipment side of the 5-watt resistor.


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