Generator backup
discount,
I'm puzzled by your assertion that diesel generators are used for backup. We certainly don't use them like that.
Our SOP is to use a ring main as its own backup. Thus, a break in the ring DURING any period of LVPs is covered by the supply coming around the other way, so to speak.
If the ring is interrupted before the onset of LVP, then it can't be used as its own backup. What we do then is to use the Diesel generators as the PRIMARY source and the mains as BACKUP. That way, we get the 1 sec changeover required for CATIII. This doesn't cost any more, just a bit of lateral thinking to give the result.
I'll also repectfully disagree with you about there being a difference between LVPs due low cloud and low horizontal surface visibility. Yes, we often get situations where we are 200' cloud ceiling but several km under it. We still need LVPs in this circumstance, so that when crews cloudbreak at 200', there is a reasonable chance that the a/c is aligned with the runway. I reckon you've got less than 15 secs between a 200' cloudbreak and the flare in a big a/c. That's supposed to be possible to hand-fly for a CATI approach. Thus we protect for this condition.
Cheers,
TheOddOne