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Figure 2: Back-to-back power-factor
cap strike voltage waveforms
result of a system response to an impulsive transient.
As mentioned above, many people
think transient voltage surge suppressor
devices (TVSS) eliminate any transient
problems. To some degree, that is true
because the devices can clamp the tran-
sient voltages at a predetermined level.
That leads to recommendation in IEC
61000- 4-30. For this reason, the tran-
sient current is often a better measure
of the severity of alternating current
system transients than the transient volt-
age. It’s actually better to measure both,
provided that the CTs can pass the high
frequency waveforms through them. A
metering CT with a 1-kHz bandwidth
isn’t much use for measuring such a
use. However, without proper ground-
ing, the TVSS cannot function properly.
Also, each time the clamping devices do
their job, they lose some of their ability
to protect, eventually either letting the
voltage clamp too high to be effective, or
they self-destruct. It is a good practice
to monitor for transients no matter the
protection employed, because they can
still hurt your system even though you
can’t see them.