dave wrote:

I have some questions regarding domestic electrical earthing - and would
appreciate some enlightnment.

Yes, you seem mighty confused...

http://www.diyfaq.org.uk/electrical/....html#earthing may shed
some light.

The house is a 50's semi. Earthing is provided by a conductor bonded to the cold
water supply where it enters the house. There is an RCD fitted in the CU. Both
rings and lighting circuits have mcb's.

Are you sure you're not confusing the "means of earthing" with the
(required) main bonding connection to the water pipe? Is there any
other connection to the main earth terminal? Is your supply from an
underground cable, or overhead wires? A photo of the supply intake,
meter and consumer unit would help if you have a digital camera and some
Web space.

Q1: Is this an illegal installation because of the earthing?

If the water pipe is the only means of earthing it would certainly be
illegal to install now. Right now it requires urgent investigation and
possibly remedial work.

Q2: Is this a TTS system?

TTS? There's TN-S (cable sheath earth), TN-C-S (PME) and TT (own earth
rod). These are explained in the above-cited FAQ article.

Several years ago we had a quote from the electricity company to rewire the
house (which we should have got around to but didn't). In that quote they say
(under comments), "PME Earthing". I'm not sure if that meant they intended to
provide PME earthing as part of the quote - or that (for some reason) they
thought we already had it. I think the former. Anyway...

Perhaps. On a 50s property PME will only be possible if the service
cable has been replaced, or an overhead network upgraded.

Q3: Given that (afai can see) the only conductors into the house a single phase
and neutral cable, how/where would they have obtained that earth from?

From the neutral - that's what PME is - or from a local earth electrode
(TT system). TT is a non-preferred option now and you should use it
only if the supplier cannot give you PME, or TN-S. What does the cable
look like, by the way?

I'd like to install an earthing rod. If I do,

Before you do that, let's be sure it's necessary.

Q4: Is this still TTS?

It would still be TT, yes, provided there was no supplier's earth
connection. (The bond to the water pipe doesn't affect this, and is
still necessary.)

Q5: If I do install the earthing rod, can/should the bond to the cold water
supply remain *as well*. Would it do any harm to leave it in place
(electrically) anyway?

Yes, it's a requirement - main equipotential bonding. There should be
one to any gas pipe too, and any other incoming metallic services.

Q6: Is it possible, and is there any virtue in, asking the electricity company
to provided PME earthing to the property? (How they would get that to the house
I have no idea - maybe is expensive).

Yes, use PME if available. If necessary, you will need to bring the
earth bonding, both main and supplementary, up to modern standards
before the supplier will connect a PME earth.

Q7: If PME is installed, would the installation then be TN-S?
Thanks

No, it would be TN-C-S.

If you're going to DIY this you need to learn an awful lot more. Could
I suggest using a qualified electrician? Start by getting a PIR
(periodic inspection report) done, as there may well be other nasties
lurking in your old wiring.

--
Andy