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 MIL-HDBK-419A
1.3.3.5.18 Amplitudes and Waveforms of Occurring Transients. Transients occurring on landlines have been
defined as 10-by-1000 microsecond, 1000-volt peak pulses where 10 microseconds is the time from the start of
the transient to peak voltage, and 1000 microseconds is the time from the start of the transient until the
amplitude exponentially decays to 50% of peak value. Source impedance cannot be precisely defined but for
design purposes is assumed to be 1 ohm. Therefore, for design purposes, a typical worse case lightning-induced
transient can be defined as 10-by-1000 microseconds, 1000 volts peak with a peak surge current of 1,000
amperes. Using Table 1-8, the 1750 transient pulses defined in Section 1.3.3.5.17 and the worst case transient
pulse defined above, the number of transients of varying amplitude would be as listed in Table 1-16 over a
lo-year period for an externally exposed line in a high-incident lightning area (average of 100 thunderstorm
days per year).
1.3.3.5.19 Equipment Withstand Levels. Equipment withstand levels were generally defined in Section
1.3.3.5.3. Nothing of substance can be added. However, manufacturers generally do not specify equipment or
component withstand levels against lightning generated transient surges. It is imperative that the withstand
level be analyzed and determined for each item of equipment to be protected. The withstand level should be
10% below both the damage threshold level and operational upset level for the equipment. The damage
threshold level is defined as the level where immediate component destruction occurs or the repeated
application energy level that decreases useful operating lifetime of equipment components, whichever is lower.
The operational upset level is defined as the transient voltage that causes an intolerable change in equipment
operation. It is imperative that an accurate withstand level be established. Otherwise, designed transient
suppression may not be effective, or conversely, costly transient protection may be designed when not required.
1.3.3.5.20 Protection Methods Against Transients. Methods listed below are effective, when properly
implemented, in providing equipment protection against lightning generated transients appearing on externally-
exposed equipment signal, status, control and ac and dc intrafacility lines. Subsequent paragraphs delineate
proper implementation techniques for the listed methods.
Completely enclosing buried lines end-to-end in ferrous metal, watertight conduit.
a.
Installation of buried guard wire above buried cable runs not in metal conduit.
b.
c.
Connecting transient suppressors line-to-ground on both ends of externally exposed equipment lines
as soon as feasible after building penetration or at point of termination at exterior equipment.
d.
Including transient suppressors or transient suppression circuits as an integral part of protected
equipment at all external line-equipment interfaces.
e.
Peripherally bonding the shields of rf coaxial lines to building entry plates by use of bulkhead
connector plates.
1.3.3.5.21 Enclosing Cable Runs in Ferrous Metal Conduit. Transients are induced on external lines by
electromagnetic waves created by lightning current flow, and by cloud-to-cloud lightning discharges. There-
fore, completely enclosing buried external cable runs in ferrous metal, watertight, electrically continuous
conduit provides an effective protection level against lightning-generated transients.
1-74
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