I would like to know too, perhaps when capacitor can store extra energy of the fault/lightning strike current flow through then surge arrester can dispitate energy safely.
Basic characteristic of a capacitor, voltage across the capacitor can't change instantaneously. Provides very short duration protection while arrestors begin to conduct.
What David says. Another way of looking at it is that a fast transient consists of high-frequency components and that a capacitor is a short-circuit to high frequencies.
And, a third way is to think smoothing. That is also what capacitors do. They remove fast changes from the voltage.
All three views are actually one and the same thing and makes use of the fundamental property: i = C*du/dt. The larger the du/dt, the more current through the capacitor.
I assume as below:Transient surges consists of voltages of different harmonics. At higher harmonics (frequency) the impedance offered by capacitor is less (z=1/CW where W=2 pi f) so that more current flows resulting in voltage drop of high frequency component of surge. In effect, the fast front of the surge wave is smoothened out along with reduction in peak value. Equipment has higher withstand capability for such a smoothened wave compared to a fast front wave for the same peak value.
Surge capacitors are normally applied to generators and motors. Dry type insulation is particularly vulnerable to damage from voltage surges with high rates-of-rise. The net effect is, with surge capacitors, the rate-of-rise is significantly reduced.
You still need surge arresters to divert the surge to ground, but the duty on the arrester becomes less if surge capacitors are used.
