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 MIL-HDBK-419A
provided in Table 1-6, and there is agreement among authoritative sources that the peak current for a large
percentage of strikes is in the 10 to 30 kiloampere range. Note that in Table 1-7, 1818 of the 2721 current
amplitudes or 66.8% were in the range of 1 to 20 kiloamperes. Also note that only 14% were greater than
40,000 amperes, and it follows directly that 86% of the peak amplitudes were 40 kiloamperes or less. Only 45
of the 2721 measured amplitudes, or 1.65%, were above the 100-kiloampere level. Also, it is emphasized that
the peak current amplitudes noted in the foregoing resulted from direct strikes to metal towers for primary
Induced transient amplitude. After installation of appropriate transient suppression, induced
b.
transients will still occur as a result of close proximity, high-intensity strikes, and some transient energy will be
coupled through the service transformer onto the incoming ac service lines. The amplitude of those coupled and
induced transients will be reduced a minimum of 50% of direct strike amplitudes due to earth resistance,
attenuation of electromagnetic fields due to propagation through air, and coupling losses imposed by the service
transformer winding. Therefore, 86% of the transient current surges appearing at a facility main service
disconnect means will be 20 kiloamperes or less, and the greatest percentage, 68%, of the surges will be in the
500 ampere to 10,000 ampere range. Only 1% of the surges will be above 50 kiloamperes, and only 0.25% will
be above 75 kiloamperes. Table 1-8 provides a tabulation of transient amplitudes and the percentage of
transients on incoming ac lines that will as a maximum be of the amplitude listed.
Transient waveforms, ac lines. Waveshapes for transients will vary depending on the proximity of
c.
the strike, intensity of the strike, and length and inductance of the incoming ac service lines. Table 1-6 lists
the typical time to peak current as 1.5 to 2 microseconds and 40 to 50 microseconds as the typical time from
the start of the pulse until the current decays to 50% of peak value. Thus, a typical waveform for current
surges generated by a direct strike is 2-by-40 microseconds. Transients measured at main service disconnects
(amplitudes in excess of 3,000 volts) have had rise times of, 1 to 2 microseconds and decay times of 20 to 40
microseconds. However, the inductance of some incoming ac service lines will slow down the rise time slightly.
Most manufacturers of secondary ac surge arresters use either 8-by-20 or lo-by-20 microsecond current
waveforms for testing and specification purposes, primarily because the waveform is relatively easy to generate
while a 2-by-40 microsecond waveform is quite difficult to generate. The 8-by-20 or 10-by-20 microsecond
waveforms are considered suitable for testing. However, the user of the arrester should be aware of the
following:
(1) Transients with rise times faster than 8 microseconds may appear across the arrester
terminals resulting in a higher sparkover or turn-on voltage for the arrester than specified.
(2) Transients with decay times up to 40 microseconds may appear across the arrester terminals
which will require the arrester to dissipate considerably more transient energy than would be required for a
20 microsecond decay time.
1-52
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