Two things are at times referred to as duel earth. Where RF signals may travel on earth cables we have clean and dirty earths and a little beyond me to explain exactly what the procedure is.
Second is where there are two supplies and as a result both the supply authority earth and a second earth is required.
Wind generators and solar panels are becoming a little of a problem as when you lose supply and the earth and neutral are bonded together then a fault could send power back down the supply line and electrocute the power companies workers etc. So earth rods are required for when PME is lost.
When power has not been lost then the earth rod is just the same as any other Extraneous-conductive-part.
This is rather a technical question and it may be better to post on one of the technical forums like the one on IET web site.
Although I think you would need to word the question better.
I suspect you are refering to dual earths with regard to circuits with high leakage currents such as those found in offices with several PC's per circuit;
If so then the ref is to using accessories such as socket outlets which have 2 seperate earth terms. on each and two seperate CPC's (earth conductors) each going to a different connection in the consumer unit.
There is some confusion I believe even within the IET as to what is intended.
The regulations verbage says for instants that the normal ring conductor should go to say all the left hand terms as a conventional ring and a second conductor which could be another ring of earth wire or use the metal conduit connected to the other terms(RHS) thus giving 2 sets of earth circuits for higher integrity.
BUT the drawing in OSG show one circuit going in one side and out the other, thus doubling the chance of loosing the continuity, imho not the best way to safeguard a circuit! in fact the only drawing I think is correct is the spur showing 2 independant earths,
Bet you wish you hadn't asked!!!
If I misunderstand your ? please ask again,
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