Some interesting reading I also found
This PDF which helped to explain.
It would seem a house holder does not have to use the earth supplied but where they do I can't see how any metal connected to the earth system will be anything other than exposed-conductive-part or an extraneous-conductive-part it would not matter if it's a water pipe supplying garden shed, a fuel oil pipe going to tank or a lump of metal hammered into the ground until such point as the DNO earth is disconnected it's not the earth electrode.
I do see the point where earth electrodes should be fitted where the voltage gradient under fault conditions will not produce a danger which is why we use pits to stop any one getting too close but there is nothing to stop anyone fitting a stand pipe in the garden using metal pipe. OK freezing could be a problem but fuel oil tanks could very easy use metal piping.
The problem would be electrolysis where the pipe could be eroded away causing a leak and as a result I would want some insulating device to stop this but I have seen many houses with copper plumbed outside taps which one would assume would some where be connected to the electrical earth.
It is pointed out the 13th Edition allowed use of a water pipe as an earth (1955) but by 1966 the 14th Edition prohibited this for new installations but did not require old installations to be upgraded. So there may still be properties using the water pipe and originally it did not require RCD protection but with water authorities changing to plastic the water pipe now would not be good enough and alternative earthing arrangements are now required.
I have only once come across a TN-C-S supply where the neutral was lost and it resulted in the 4mm² earth wire feeding my friends shed to melt due to the electrician who had wired it bonding exposed-conductive-parts and extraneous-conductive-parts which included an earth mat connected to earth bar used for a radio transmitter. Clearly the shed supply should have been TT but it did point out one of the problems when earthing extraneous-conductive-parts.
I have often wondered what would happen if any fuel pipe is earthed which could not take the current under fault conditions with a street of houses with a gas supply it would be shared between all the houses it is only when one odd house has a gas or oil supply which has been bonded and even then a 10mm diameter pipe would be well over 4mm² CSA so unlikely it would melt.
What is the big question is with a loss of the neutral/earth how long would it take before the supply was cut?
The second is what danger would it cause?
On the second with no extraneous-conductive-parts bonded there would be very little danger only with extraneous-conductive-parts bonded is there a significant risk.
So it introduce unnecessary extraneous-conductive-parts would clearly reduce safety not increase safety so I would not fit any earth rods as under fault conditions they could be dangerous. I would also want plastic pipe to any outside tap or fuel supply.
Never been involved with LPG it would be interesting to hear how that is bonded and piped to the house. That is about the only case where I can see extraneous-conductive-parts would need bonding and I just don't know how this is done.
So with the exception of a house with LPG I would not consider fitting earth rods would be better than using the DNO earth.
