Hi. My 1963 house is supplied with a PME earth system according to the label on the main incoming supply. I suspect it was upgraded at some point in the past as the floor shows signs of repair etc.
I have been researching the best way to replace the power supply from the consumer unit to an outside metal shed but have found new problems / questions.
Regarding the shed, it appears I could run an RCD protected live, neutral and earth within a PVC conduit secured to my car port and fence into the shed. I shall fit a small secondary CU in the shed. It seems I should earth the metal of the shed using the earth wire which should be no less than 10mm. This will bond it to the PME system.
My "internet" understanding of the earth bonding now makes me question the existing set up in the house - a heavy earth runs from the incoming supply to the main CU and this connects to the ring main and lighting circuits etc. The electric shower is supplied by an RCD protected 10mm T & E supply from the CU with the earth terminated within the shower enclosure and linked to the water supply pipe internally. However, I have no separate earth bonds to my 2 cold water supplies (1 in the kitchen, 1 in the utility room). I have no gas.
Is the 10mm earth to the shower acceptable as the earth bond for the cold water supply (kitchen)? Or does it have to be at the point of entry in the back of the kitchen cupboard?
The water supply into the utility room presumably needs a 10mm earth connection directly to it near the point of entry.
Any advice / thoughts appreciated.

Your water services should be bonded within 600mm of point of entry, the 10mm twin and earth has only a 4mm earth so thats not upto the job regardless of where its connected.

You should get the bonding sorted and then the supply to the remote building should be put on a earth rod and disconnected from the main earth.

I have had problems in the past with an earth (4mm) run to a shed which was earthed independent in that case an earth mat consisting of four earth rods and 1" by 1/8" bare copper one foot deep between them. (Installed for the Ham radio used in the shed).
The shed had been wired by an electrician who for reasons unknown had run the earth independent to the supply cable following same route neatly cleated below the main feed.
I was called when the owner had found the plastic melting from the earth cable and in spite of turning off main supply at the main incoming isolator the wire was still heating up.
By time I arrived it was copper globules on the path.
Phoned supplier and told them they had lost the neutral/earth on the PME supply to be told it was impossible. Within minutes whole power was lost but not before I was able to measure with my meter a voltage varying 0 - 400 volt.
When the cable in the road hit by contractors was repaired his was only house not to have blown up equipment.
As a result I looked into the whole idea of "extraneous-conductive-part". With a PME system an earth rod is not called an earth rod it is a "extraneous-conductive-part" as is the water pipe, gas pipe, and any cast iron soil pipes. Should the PME earth be lost these items will try and carry the neutral current and if all houses in block all have bonding to these services then the load will be shared. But where in his case all the houses have plastic services then the whole block will try and earth through what ever it can find.
Because his cable was not protected from mechanical damage it should have been 16 mm². At that size it would not have melted. The alternative which was used after is to turn the supply into a TT supply for the shed. However in the case as described there will be a gradient between the house and shed under fault conditions so much depends on the distance from the house or at least "extraneous-conductive-parts" connected to house.
It's not just read the rule book job but using some common sense and working out the likely scenario should the neutral/earth be lost. I know it's very rare but it can happen.
So with a house with metal water, gas, and other services I would not worry about extending the PME supply. Also where a large supply cable is used 10 mm² SWA or above I would not worry.
However as far as the shed goes it's down to the guy doing the work and issuing the certificates with DIY then local authority building control (LABC) to decide what is required. They are responsible for site safety.

Internal again depends on services what is metal and what is plastic. What we want to avoid if for a fault in one room to be transmitted to another room through metal services. What one has to realise if a radiator for example is supplied in plastic pipe to earth the radiator means if you touch something live and the radiator you will get a shock. If you don't earth it then no path so no shock. However if metal pipe work a lamp falling onto the radiator in one room could give you a shock touching a radiator in another room unless earthed. So the rules take this into account and the whole idea of earthing everything even with one edition of wiring regulations metal windows has gone and what is earthed is very dependent on which edition of the wiring regulations your house is wired to. In my case 17th Edition which means all circuits are protected by RCD's.

Where the problem lies is where old houses used water, gas, and soil pipes as the main earth and as time has gone by electrolysis has eroded the soil pipes and the supplies have moved to plastic leaving the house without an earth. It is therefore recommended that an electrical condition report or periodic inspection report (old name) is carried out every 10 years or change of occupier which every is the shorter. So your house should have been inspected at least 5 times since it was built?

I think it was in the 1960's the earth to lights rule was changed. It seems unbelievable that today when owners have know for at least 30 years that their lighting needed rewiring (assuming 10 year test) that we still have houses without an earth to the lights. Yet we still find this during inspections. One wonders at what point we will stop just placing stickers on the consumer unit warning of no earth on lights and when we will finally say "You have had enough time sorry turning off the lights"? But of course we still have cars built in 1930's on the road without duel line breaking or seat belts so I guess the same applies to a house.

However as my father-in-law said to me you will never forgive yourself if some one is injured due to you not fitting safety features. In my house RCD on all circuits back in 1990's. No one can force you but clearly by the fact you have posted you are worried.
When I check a house as well as the visible checks I also use a loop impedance meter which is an expensive bit of kit and also if used incorrectly can give some one a shock. So really the inspection and testing is not a DIY job.

Although Part P allows anyone to do electrical work and use the LABC to issue the completion certificate in real terms it is to allow some one like me who is an industrial electrician to do the odd domestic job. It's not really designed for Joe Public. What you are suppose to do is to fill in the minor works or installation certificate and the LABC guy will decide if he trusts you or not and can confirm any of the readings taken. Even to hire the meters required will cost around £75. Plus the LABC charges so allow at least £200 to DIY. Once you knock off that £200 from the scheme member electricians bill it just is not worth doing the work as DIY. Even for me.

Thanks for your replies.
Interesting to note that 10mm twin and earth only has a 4mm earth. I've been reading up on armoured SWA cable which I guess is different in that each core is generally the same size.
It looks like a whole can of worms could be opened up here but as is rightly pointed out - it needs doing as safely as possible. I've just looked at the lighting circuit - and just as described - there are no earths!
I may take to wearing rubber boots or indeed an entire rubber outfit around the house - for safety of course.
Regards.

Out of interest it seems wearing rubber boots was giving people wearing them shocks due to static build up so now graphite is added to the mix so rubber boots don't insulate completely although the resistance is still high. So you can still get shocks wearing rubber boots.

The lighting can only really be corrected by a re-wire. The regulations say the earth wire must follow same route as feed and where not part of feed cable must be at least 4mm sq csa so replacing whole cable is nearly as easy as adding an earth wire.

In real terms until the re-wire it means the lights used must be double insulated. Florescent lamps are normally not double insulated although there are some 2D (round or square types) that are so it means no kitchen strip lights. Also no fancy metal lamps in other rooms. All lights will have to be mainly made of insulating material. There is one shop which specialises in Class II lamps which can be used where the earth is missing.

What I found was there seemed to be no difference between the 2D lamp I got for may dads house marked as double insulated to the ones sold in screwfix without the double insulated marking so I paid double the price just to have one with the sticker on it.