Flex for vac.

Andrew, would you mind if I play the devils advocate on this one? Because every tradesman and DIYer would never be able to work if extension leads were banned:-) I am making the assumption that anyone with a brain cell uses a RCD at the head end of their extension lead and so only live neutral shorts are now relevant.

I say "You can make extension leads safe if you have good fixed wiring" and fixed wiring is something I am good at doing and designing.

If you did a recalculation using the resistance of the 2.5mm of the live-neutral cores and not the live-earth cores on the fixed wiring you do get different fault currents between a live-neutral and a live earth short when using 2.5mm T&E..

A 16A radial should have a maximum length of 40m 2.5mmT&E to allow for a 5% voltage drop. So that's a 0.15 ohms resistance in the live neutral cores plus a maximum of 0.85 ohms for the incoming supply. That's maximum of 1 ohm where the extension lead is plugged in.

106m is the "maximum" allowed length of a final ring circuit for a 5% voltage drop. So at 7.41milli ohm per meter. 7.41 x 106 / 1000 = .785. Divide that by 4 for the mid point of the circuit and you have 0.20 ohms, add on the 0.85 ohms maximum allowed for a TNS system and you have 1.05 ohms.

I have taken the worst case scenario and assumed that the PSC and PFC are the same and used 0.85 ohms on an TNS supply.

In reality the PSC is always lower than the PFC on TNS supplies (and it's a long time since I saw any PSC reading above 0.5 ohms on any supply).

For a PME supply (PSC and PFC being the same) and having a maximum of 0.35 ohms Zs there is no problem with using a 60m extension lead with either a

16A radial or a 32A ring on a correctly designed circuit. Worst case scenario is a midpoint connection to a ring circuit 0.35 + 0.20 ohms = 0.55 ohms LN resistance at the midpoint socket.