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 MIL-HDBB-419A
manufacturer is not conclusively certain that external landlines will be enclosed in metal conduit, designed
transient protection must ensure that the equipment will be adequately protected against the transient levels of
Table 1-19. Subsequent paragraphs provide design guidelines for transient suppression for all types of landlines.
Coaxial and twinaxial lines are treated separately. Also, externally-exposed landlines that carry signals of
3 MHz to 400 MHz are treated separately.
Control, status, intrafacility power, and audio landlines. Control, status, intrafacility power, and
a.
audio lines, other than coaxial or 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. Effective design is shown in Figure 1-43.
(1) Suppression design and component selection. Transient suppression will effectively protect
equipment only when proper components are selected so that the components operate in conjunction to provide
the desired function. This is necessary so that the clamped output of the suppression components/circuits can
provide optimum equipment protection. Actual suppression components are shown in Figure 1-43 as GT1, RV1,
RV2, and TS1. The suppression component at the equipment entrance should be chosen so that it has a lower
turn-on and clamping voltage than the suppression component at the facility entrance. Therefore, resistor R1
must provide a voltage to turn on the suppression component at the facility entrance and limit current flow
through the suppressor at equipment entrance. Otherwise, the suppression component at the facility entrance
may not turn on when a transient occurs. The component will not normally turn on when a transient of less than
400 volts peak amplitude occurs and the component is a gas-filled spark gap (GT1). However, when a transient
of greater amplitude occurs, the suppression component at the facility entrance must turn on. Otherwise, the
suppression component at the equipment entrance will attempt to dissipate the entire transient to ground. As a
result, the suppression component at the equipment entrance will attain a higher clamp voltage as it dissipates
additional transient current. The higher clamp voltage is reflected across protected equipment. In addition,
the suppression component is likely to fail.
(a) Gas-filled spark gap GT1. A gas-filled spark gap is suitable for use as a transient
suppressor at the building/facility entrance in some cases. The device has a relatively high sparkover (turn-on)
voltage and a relatively slow turn-on time when compared with a metal oxide varistor (MOV) or silicon
avalanche diode suppressor (SAS). For typical lightning-induced transients on landlines, turn-on voltage is a
nominal 500 volts with an associated turn-on time of 5 microseconds. These characteristics are satisfactory as
long as the value of resistor R1 is 10 ohms or more, and the peak pulse current rating for the suppression
component at the equipment entrance is not exceeded. When R1 is 10 ohms, a peak current of 50 amperes is
required to provide a voltage of 500 volts across R1 which is the nominal turn-on voltage for GT1. Since GT1
turns on after a nominal 5 microseconds, the peak pulse current rating for most MOV and SAS devices will not
be exceeded. After the spark gap turns on, arc voltage across the device is a nominal 20 volts. This may be
sufficiently below the normal line voltage to create operational upset of the protected equipment, which in
some cases cannot be tolerated. If normal line voltage is greater than 20 volts, difficulty may be encountered
in turning off the device, depending on available current. The arc mode of operation may be sustained by
current greater than 1 ampere for some devices. When the value of R1 is less than 10 ohms, an MOV or other
equivalent suppressor must be used at the facility entrance because a spark gap will not turn on before the
suppressor at the equipment entrance is damaged by overcurrent, particularly when the suppressor at equipment
entrance is an SAS.
1-90
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