Thankyou peetee this is what i was getting at. Surely a higher current carrying circuit is potentially more dangerous under fault conditions, so why give it 5 seconds to bad day explode you? And surely cables with larger surface area of copper will return betterohms
As stepse implies.. and IMHO the 'WHY' bit of 0.4 sec -vs- 5 sec Disconnection times comes back to some of the historic reasons in earlier regs..
e.g. If you look back at the yellow cover On-site guide 16th edition page 19 it states that a 0.4 disconnection time is required for all final circuits supplying:-
i) Portable equipment intended to me moved by hand while in use.
ii) hand held metal cased equipment requiring an earth supplied though a socket.
iii) Fixed equipment outside the equipotent zone.
iv) fixed equipment in bathrooms.
As we then move toward the brown cover 16th edition O-S-G..
page 19 is the same except it excludes the item iv) bathroom equipment.
(which is probably covered by RCD anyway!)
Now as we get to our current red book 17th edition (page 21) it is just ALL final circuits up to 32A..
So IMHO a large proportion of this pivots around the realistic likely risk of electric shock and minimising it down to a short duration, preventing the build up of dangerous touch voltages, to avoid a shock becoming a fatal shock!
So If you did a general risk assessment of the various circuits and those circuits more likely to give an electric shock Final circuits over 32A and Distribution circuits are quite lower down the risk list..
i.e.
Mr