For a 50 Hz supply, the sub-cycle surge current rating of a thyristor is given by:

Surge current rating

Surge current rating specifies the maximum non-repetitive or surge current that the SCR can withstand for a limited number of times during its life span.

This rating is specified in terms of the number of surge cycles with corresponding surge current peaks.

The surge current is always calculated for a half cycle of the total surge period.

The surge current in one complete cycle is given by:

Isb = I\(\rm\sqrt{\frac{T}{ t}}\)

It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.

For a 50 Hz supply,

 \(T={1\over f}={1\over 50}\)

T = 20 ms

But for surge, half of 20 ms = 10 ms will be the value of the surge period.

Isb = \(\sqrt{10\times 10^{-3}\over t}\)

Isb = I\(\rm\sqrt{\frac{1}{100 t}}\)

where, I_sb = Subtransient surge current

I = Surge current in complete one cycle