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 MIL-HDBK-419A
1.3.3.5.16 Transient Protection for Externally Exposed Equipment Lines. In order to effectively protect
equipment against damage from lightning generated transients on externally exposed (outside) equipment lines,
the following must have some definition which is provided in subsequent paragraphs.
Frequency of Transient Occurrence.
a.
Amplitude and Waveform of Occurring Transients.
b.
Equipment Withstand Levels.
c.
d.
Protection Methods Against Transients.
1.3.3.5.17 Frequency of Transient Occurrence. There is no existing method for precise calculation of the
number of lightning generated transients that will occur at a specific location in a given period of time.
However, by using the best available data listed in Section 1.3.3.5.4, projections are that 1750 transients will
occur in a lo-year period at a facility located in a high-lightning incident area with an average of 100
thunderstorm days per year, and only 175 transients will occur in a lo-year period at a facility in a low-incident
lightning area with an average of 10 thunderstorm days per year. Note that the number of transients is
decreased by one order of magnitude for the low-lightning incident area. Therefore, by using Figure 1-34 to
determine the average number of thunderstorm days per year in a specific location, and decreasing 1750 by 10%
for each 10 decrease in the average number of thunderstorm days per year, the number of transients projected
to occur at any location in the United States can be determined.
Table 1-16. Transients Projected to Occur on Externally Exposed Line in
High-Lightning Incident Area Over 10-Year Period
No. of Transients
Percentage
Peak Voltage (Volts)
Peak Current (Amperes)
2
0.1
750
to 1,000
750
to 1,000
15
0.9
500
to
749
500
to
749
18
1
400
to
499
400
to
499
3
300
to
399
to
399
300
53
8
200
to
299
140
200
to
299
100
to
199
19
100
to
199
332
50
to
99
50
to
99
420
24
25
to
49
25
to
49
23
403
5
to
24
21
5
to
24
367
Note: The source impedance for design purposes is assumed to be 1 ohm.
1-73
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