Jim Lux wrote in
:

The purpose of the National Electrical Code (National, here, referring
chauvinistically to the U.S.) required AWG 6 (diam 0.15 inches, 3.8
mm) bonding wire for grounds is NOT to carry the lightning current
(which it wouldn't, in most cases) but to carry fault currents from
things like shorts from line to grounding conductor, which are usually
in the hundreds of amps range. Say an energized power line falls down
and hits the antenna. You want the antenna's grounding conductor to
carry the likely fault current and not go open, and carry enough
current to trip any overcurrent protective devices.

I meant to comment:

I think that it is common in electricity distribution level networks,
that they are designed to hold fault current to about 20 times the
maximum working current.

In this part of the world, a single phase 240 home probably has a 80A
rated service, and fault current would usually be not worse than than
about 1600A, so the specified 6mm^2 earthing conductor and 4mm^2 bonding
conductor should withstand that current for 100ms until the protective
device operates. (4mm^2 withstands 3200A for 0.1s with a safety factor of
3.)

You probably know the numbers for your own distribution network practice,
they may be of interest to readers.

So you raise a good point, that if your tower falls onto power lines, it
would be good if your earth system could withstand the likely fault
current to take out the protection on the power lines and leave the tower
un-energised. In this part of the world with LV distribution, the
protection is probably a 500A HRC fuse with a fault current level of 10
+kA. It may be much lower for you if power lines are HV where the fault
current level should be a good deal lower.

Owen