Transformer Half Crest Time

Dears,
during reading power transformer (0.71/33) kv data sheet, I found term "half crest time" which equals to 0.8 s,
the question is: what is that time and why is important?
thanks and kind regards.

I can't completely answer this question so if someone else can provide further information this would be great.

This is a test for the dielectric properties of power transformers as they can be expose to external element need to withstand lightning surges. I am not studying electrical power so this is as far as I can go but to say it has something to do with lightning surges experienced in power transformers will surge to full crest voltage quickly then die away to half that crest voltage. I think it provide a transient wave shape of the surge.

Now that is the closest explanation I can think of of what half crest voltage is but what affect lightning surges have in relation to power transformer is a bit out of my depth.

thank you Malcom, this was helpful.