I can't see where any new regulation states that max Zs figures do not have to be reached on a TN system?
I have thought on occasion 'why' is an RCD not deemed acceptable on TN system for fault protection when it is fine for a TT, but I came to conclusion that I was thinking about it in the wrong way.....
I'm pretty rubbish at analogies but lets say a TT system is like a classic car without a seat belt (pre 1966) and a TN system is a modern car.
Sticking to the speed limit is the RCD and wearing a seatbelt is ADS (i.e meeting max Zs figures).
Both cars can stick to the speed limit (have RCD) but that's no reason not to wear a seatbelt (ADS). The only reason you don't wear a seatbelt legally is if you don't have one fitted and it's pre 1966, but that's no excuse to not wear one in a modern car.
Yep, that all makes perfect sense