Table 1-18. Transient Surges, Line-to-Line, Expected to
Appear Across Equipment by Secondary
AC Surge Suppressor Over a 10-Year Period
(Ungrounded Service Only)
Surge Current Amplitude
Number of Surges
500 V, 50 A
750 V, 100 A
1 kV, 200 A
1.5 kV, 300 A
Landline transients. The number and amplitude of transients projected to be conducted to each
landline equipment interface are listed in Table 1-19. The waveform for the transients is 10-by-1000
microseconds where 10 microseconds is the risetime from zero to peak amplitude for the transient, and 1,000
microseconds is the time from the start of the transient until exponential decay to 50% of peak amplitude. The
information presented in Table 1-19 is based on data contained in Section 22.214.171.124. Since an equipment designer
will not normally know whether external lines will be enclosed in ferrous metal conduit, different transient
amplitudes are not provided in Table 1-19 for external lines enclosed in metal conduit.
126.96.36.199.3 Determination of Equipment Damage (Withstand) Levels. Manufacturers do not normally specify
withstand levels for components. Therefore, an analysis should be performed to determine the withstand level
for each item of equipment that directly interfaces any externally exposed lines including ac input lines.
Transients that are projected to be conducted to equipment are provided in Tables l-17, 1-18, and 1-19. The
analysis should be based either on results of laboratory tests or engineering analysis. Also the analysis must
include all equipment circuitry that will be exposed to transients. Three factors determine the withstand level
for the equipment as follows:
Component destruction level. The component destruction level is the transient energy level that
either causes immediate component destruction or degrades component operation to a point so that useful
operation cannot be achieved. This energy level is not usually specified or controlled by the manufacturer.
Shortened component operating life. Useful component operating life can be appreciably shortened
by repeated overstressing of components. The overstressing occurs as a result of repeated application of some
level of transient energy. This energy level may be difficult in some cases to determine, but is certainly
meaningful when designing protection against transients.