On Tue, 26 Dec 2017 10:05:56 -0800, George Miles wrote:

I dont know why Iggy is talking about Lightening:
My On Site Guide says the earthing conductor need only be 2.5 mm-squared
for TT if protected, I think that'd melt with lightening.

Most probably.

But 2.5 mm-squared does seem very thin compared to the new meter tails
to the consumer units and earth wires i was sold with them. I dont
understand why OSG Table 4.4.ii says only 2.5 mm-squared

It's all to do with the fault current the earthing conductor will be
carrying under fault conditions, i.e. a Line to earth fault. As the Earth
Fault Loop Impedance will be pretty high (20 ohms+?), it will only be
required to carry a low fault current, and can withstand that current
long enough to allow the RCD to trip.

I thought the earthing conductor is for use in case of an electrical
fault, a loose live wire, earthing most of the current through copper
not through me until a fuse blows or RCB trips.

Yes, that's right.

There's an 80 or 100 amp main fuse isnt there,
which would be of no use for lightening.
Or would it?

No, it'd be no good, as it would blow fast enough to stop any lightening
surge current damaging any equipment in your house. However, the odds of
that happening are so remote, that we mostly dont even consider it a risk
in a typical house. A large house on top of a hill, then maybe.

The 80 or 100 amp fuse will also be pretty useless for you if you touch
live and earth at the same time, as it'd never blow. In a typical TT
installation, a 32Amp circuit breaker is also unlikely to trip when you
touch L + E simultaneously.
That's because your Earth Fault Loop Impedance will be so high that there
will not be enough fault current to trip the circuit breaker. A 32A CB
will trip instantaneously (~0.1 second) with ~160A flowing through it.
Your total fault current with a 20 ohm EFLI (your earth rod resistance)
will be about 80 Amps, so you'll be holding that L+E for quite a while
before the circuit breaker trips. You'll be holding it forever for the
main fuse to blow.
This is why RCD protection is needed for TT installations. The RCD will
trip in less then 300mS, probably faster for a 30mA RCD.
Your current rod EFLI will be less than 200ohms, possibly as low as 10
ohms, but, IME, usually in the range of 30-50 ohms. Adding more rods, and
deeper rods will bring it down, but there is a limit, especially in rocky
areas, where it just is not possible to get a lower EFLI.