Upon reading Rationale for sizing of lightning down conductor a correspondent asks whether his premises 4mm^2 main earthing conductor ok as a lightning down conductor.
Intended purpose of electrical installation ground electrode
The usual source of current on the premises main earth conductor would be a fault energised by the incoming supply. To understand the implications, lets review the supply system.
Most residential premises in Australia use the MEN supply system, and they have an earth electrode (typically a 1.2m driven copper clad steel rod) which is bonded to the supply neutral at the main switchboard. The supply is usually protected by Service Fuses which are typically HRC fuses commonly rated up to 100A in residential premises. Fuses are rated for a pre-arc i^2t product, and these are typically rated for up to about 25,000 A^2s.
So, the energy delivered to a severe fault (one that blows the Service Fuse almost instantly) is limited by the fuse’s rating of say 25,000A^2.
A 4mm^2 V75 PVC insulated main earth conductor will withstand that fault scenario with very little temperature rise, and in many fault cases, much of the fault current will return via the neutral rather than through the premises ground electrode (which is rarely low resistance).
As an aside, the minimum main earth conductor has been 6mm^2 for decades now, but as you can see there is no reason to loose sleep over an installation compliant with older rules.
Lightning
Lightning introduces another source of drive to the electrical installation, and if it comes from a local antenna system, it is not effectively limited by the electrical supply Service Fuses.
This can and will drive very high currents into the premises protective earthing system which may cause damage to persons and property. The extent of damage would of course depend on the severity of the lightning event, and they can vary over a very wide range.
The premises protective earthing system is designed to be a part of the premises electrical installation, and not designed to provide lightning protection.