Why does AS3001:2008 Transportable Structures explicitly deny a separate earthing electrode?

Question

As the title says, if a generator is providing a TNS earthing system connected to a transportable structure through a detachable connection, why does AS3001 state:

Isolation of the protective earth conductor. No fixed connection shall be made between the protective earthing conductor and the general mass of the earth, e.g. by means of a driven electrode. No connection shall be made between any live (active or neutral) conductor and the protective earthing conductor of the electrical installation.

Is there a safety reason? When the transportable structure is metal and it sits on the the ground, isn't that just another form of earthing? Why would it be different if there was an extra earth electrode?

Answer 1

When the transportable structure is metal and it sits on the the ground, isn't that just another form of earthing?

It is. And that sort of answers your question. Then you add a couple earth rods here, and there, and there, and ...

Why would it be different if there was an extra earth electrode?

Grounding systems with multiple earthing points are complex beasts. They require design, field measurements, and so on.

In a temporary system, the earthing conductors can carry enough current that disconnecting them may expose the users to dangerous potential differences. As far as I understand, it basically can't be done right in practice in the situations transportable structures are used in. So it's straight forbidden.