Capacitors: 1.5 times dc working voltage unless transient dielectric punch-through voltage known.
DC power supplies with step-down transformer and diode bridge: 1.5 times diode peak inverse
voltage (PIV) rating times the transformer secondary to primary voltage ratio.
Small motors, small transformers and light machinery: 10 times normal operating voltage.
Large motors, large transformers and heavy machinery: 20 times normal operating voltage.
220.127.116.11.4 Frequency of Transient Occurrence. Precise calculation of the number of lightning-generated
transients that will occur at a specific location in a specified time interval is not possible. However, enough
observations have been made to permit statistical evaluation of the number of lightning flashes that are likely
to occur in an area with a known average number of thunderstorm days per year. Some flashes may not produce
any transients while others will produce several transients. The available data, after considerable averaging
and rounding, is provided in Table 1-5. The table lists a typical number of transients that might be expected to
occur from lightning strikes at facilities located in high-and low-incident lightning areas. When used in
conjunction with Figure 1-34, the table will permit calculation of the number of lightning surges that will occur
anywhere in the United States in a 10-year period. Decrease 1750 by 10% for each 10 decrease in the number
of thunderstorm days per year.
Table 1-5. Frequency of Transient Occurrences
Number of Lightning Surges
In 10 Years at One Facility
Low Incident Area
High Incident Area
Days Per Year)
Days Per Year)
18.104.22.168.5 Transient Definition, AC Service Conductors. Prediction of the exact amplitude, waveforms, and
number of transients that will occur at a particular facility over a specific time interval is not possible.
However, current amplitudes generated by many direct lightning strikes have been measured, and the
waveforms for the current have been measured and recorded. Also, sufficient data has been recorded to permit
statistical calculation of waveforms and amplitudes that are likely to occur. This data is provided in subsequent
paragraphs. Frequency of occurrence is provided in paragraph 22.214.171.124.4.
Transient amplitudes from direct strikes. Measured current amplitudes resulting from direct
lightning strikes have varied from 1,000 amperes to 250,000 amperes. Results of several thousand measure-
ments have been reduced and are provided in Table 1-6. As shown in Table l-6, typical peak current is 10 to 20
kiloamperes. Table 1-7 tabulates the peak current amplitudes measured for 2721 flashes. The median peak
value for the peak currents was approximately 15 kiloamperes. This is in agreement with the typical values