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the circuit parameters and produces voltage surges. Those
arising out of switching operations are attributed to internal
causes. In this chapter we limit our discussions to the
phenomenon of voltage surges, as related to internal causes
and particularly as a result of switching. The requirement
and the type of protection remain the same for external
or internal causes of system disturbances.

17.6 Definitions

In the following text we will use a few new terms. For
the sake of more clarity on the subject these are defined
as follows.

17.6.1 An impulse Time (ps)-
f, = Virtual front time
An impulse is an intentionally applied voltage or current = 1.67t
in a laboratory. It is in the form of an aperiodic and uni- t' = 0.3t~ = 0.5t
directional waveform (Figure 17.2). It rises rapidly without
appreciable oscillations to a maximum value and then Figure 17.3 Defining a voltage or a current impulse waveform
falls, usually less rapidly, to zero, with small, if any,
loops of opposite polarity (Figure 17.4). The parameters

which define a voltage or a current impulse are its polarity,
peak value, rate of rise (front time) and time to half its
value on the tail, as noted later.
A transient of an external or internal nature, as discussed
above, is then related to one such type of impulse, for
laboratory testing a particular equipment or system, to
establish its suitability.

Identifying an impulse waveform
The distinction between a lightning impulse and a
switching impulse may be made on the basis of the
duration of the wave front (shape), rather than of its
origin. A voltage impulse with a wave front duration of
Front time (ps) --t less than 1 ys, up to some tens of ps, is generally
A 12/50 ps waveform considered as a lightning impulse (Figure 17.2(a)) whereas
(a) Lightning impulse waveform an impulse with a front duration of some tens of thousands
of ps is a switching impulse (Figure 17.2(b)) according
to IEC 60060- 1. The type of wave is generally designated
as tl/t2 (Figure 17.3), where

1 tl = virtual front time of an impulse:
For a voltage impulse having a front duration of
less than 30 ys (normally lightning surges)
tl = 1.67 x time taken by the impulse to rise from
30% of its peak value to 90%
For a voltage impulse having a front duration of
more than 30 ys (normally switching surges)
tl = 1.05 x time taken by the impulse to rise from
0% to 95% of its peak value
For the sake of laboratory testing and analytical
studies, these surges have been represented by
250 Front time (ps) - 2500 standard impulse waveforms. According to IEC
A 250/2500 ps waveform 60060-1 a 1.2/50 ys impulse is called a standard
(b) Switching impulse waveform lightning impulse, and a 250/2500 ys impulse a
standard switching impulse.
Figure 17.2 Standard impulse waveforms For equivalent current impulses, such as 8/20 ys