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

Andy Wade wrote:

TTS? There's TN-S (cable sheath earth), TN-C-S (PME) and TT (own earth
rod).

Another decent article with helpful explanations and diagrams
http://www.iee.org/Publish/WireRegs/...s_answered.pdf

On Tue, 04 Apr 2006 13:59:43 GMT someone who may be dave
wrote this:-

Hang on. You say I am "mighty confused" based on the assumtion that *you think*
I *may* be confusing the means of earthing with an equipotential bond! I think
maybe remedial English comprehension and elementary logic class would benifit
you.

I think that given that reply few people will be keen to give you
any more advice. The advice which you were given is good advice, all
of it including the bit about you seeming mighty confused.

I just didn't have the detailed
info of the coding and schemes in use today.

The TT and so on terminology goes back at least 25 years, probably
more.

One last thought. Proper earthing is important and can be made worse
by those that don't know what they are doing. I suggest reading the
articles you have been pointed to, followed by one of the various
guides that are available.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

In message , Andy Wade
writes
dave wrote:

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.

I don't think it's that uncommon Andy, it older properties not recently
rewired. Our last house - 1938 semi, some wiring done in the 1960's ish
probably, but still old rubber stuff on the lighting circuit.

Our only earth was via the lead water main.
--
Chris French

chris French wrote:

still old rubber stuff on the lighting circuit.

and you're practically *boasting* about that?

chris French wrote:

I don't think it's that uncommon Andy, it older properties not recently
rewired. Our last house - 1938 semi, some wiring done in the 1960's ish
probably, but still old rubber stuff on the lighting circuit.

Our only earth was via the lead water main.

Yes, I've never doubted that that used to be quite common practice.
OTOH confusion has often arisen between the earthing conductor and the
main bonding connection to the water service in threads such as this in
the past, hence I felt that the possibility should be considered and
eliminated (or otherwise) at an early stage in the discussion.

--
Andy

dave wrote:

Hang on. You say I am "mighty confused" based on the assumtion that *you think*
I *may* be confusing the means of earthing with an equipotential bond!

No, I only said you _seemed_ (to me, at that point) confused about many
aspects of the subject. The other source of possible confusion, between
earthing conductors and bonding conductors, has been common in threads
like this, so it helps if it's resolved early in the discussion, that's
all.

[...]

Well of course it would be to install NOW. That's why I clearly stated the age
of the property. Do you think I ripped out the PME and installed a equipotential
bond to the water supply in place of it? My actual question was, is it illegal
NOW. It may well be unsafe - that's clearly a separate point. (remember the RCD
though).

I know of no reason which makes it illegal now. The building
regulations, Part P of which which now cover the electrical
installations of dwellings, are not retrospective.

Right now it requires urgent investigation and
possibly remedial work.

That's right. The past 30 years it's been teetering on the edge of black-hole
collapse. Yes I know that just because it's been a long time doesn't mean it
safe etc etc. Anyway, that's what I'm "looking at" now.

So long as the water service pipe remains metal and provided that the
RCD is regularly tested to ensure it works, then it's probably not
electrically unsafe (although any competent electrician doing a PIR
would flag it as 'code 1' - "requires urgent attention" - if only to
cover their own arse).

It also sounds as if you have a single whole-house RCD (30 mA?) and
that, nowadays, is not regarded as a safe arrangement since a single
earth fault will result in loss of supply to all circuits, including
lighting, which may be dangerous (trip/fall hazards) and will almost
certainly be inconvenient.

[See reg. 314-01-01 of BS 7671 which says:
"Every installation shall be divided into circuits as necessary to:
(i) avoid danger and minimise inconvenience in the event of a fault,
and
(ii) facilitate safe operation, inspection, testing and maintenance."]

For upgrading you have two earthing options, either install a proper
earth electrode (usually one or more rods), keeping it as a TT system,
or get the supply and installation upgraded for PME. At the same time
you could consider replacing or upgrading the consumer unit to provide a
split-load arrangement complying with 314-01-01. For TT this should
usually have a 100 mA RCD for all circuits except those socket circuits
"reasonably expected to supply portable equipment for use outdoors,"
which must go via a 30 mA RCD [reg. 471-16-01]. For a TN system only
the latter, 30 mA, RCD is required.

A closing thought is that I may not be allowed to do it anyway - given the
latest legislation.

Part P doesn't forbid DIY wiring work of any kind. If the work
concerned is notifiable then a building notice should be submitted to
the local authority, who are supposed to arrange for inspection, testing
and certification of the completed work. For non-notifiable work you
are responsible for doing (or arranging) your own inspection, testing
and certification. Installing or upgrading equipotential bonding is not
notifiable work, but altering the earthing arrangements and consumer
unit replacement are notifiable.

--
Andy

dave wrote:
On Tue, 04 Apr 2006 14:09:52 +0100, Andy Wade
wrote:

dave wrote:

I have some questions regarding domestic electrical earthing - and would
appreciate some enlightnment.
Mmm while I thank you for the purely informative parts of your reply I object to
your presumptuous and patronizing statements.

Fie!

You are receiving excellent expert advice.

My first impression from your post was that you have an elderly
TNCS/PME supply, so may be AW is a little alarmist, however it is also
clear you have some homework to do to familiarise yourself with the
theory and practicalities of domestic power wiring.

snip....

.. I am more than happy to debate
electrical theory with you (if I have the time anyway).

snip...

.. I have been involved in electronics for 30
years

There is considerable difference between electronics and power supply
practice.

Ohms law stays the same, but after that everything is at opposite
poles: eg

generally in electronics: currents are low; conductor size is seldom
critical; frequencies may be high; impedances contain R,L & C
components; compenents get hot but conductors don't; heat is dissipated
with fans; circuit fault protection is ignored.

in power supply work (at least for domestic and small offices) :
currents are high; corrrect conductor sizing is vital; frequency is low
and fixed (50Hz); impedance is pure resistance; components (fittings)
stay cool but conductors get hot; heat is dissipated to ambient
surroundings (and thus much attention is paid to routing conductors);
circuit protection, especially from unforseen external causes, is
important.

There is also a significant difference in bonding practice: in
electronics common practice is to bring all grounds to a common point
and avoid mesh circuits because of eddy currents: in power supply work
everything in sight is bonded at every opportunity creating a huge mesh
network.

it might help to peruse:

http://www.kevinboone.com/cableselection_web.html + other items on
Kevin Boone's website. [highly recommended if you are know your elec
theory but are new to power cable selection and protection].

IEE On-Site Guide

The Electricians Guide John Whitfield

HTH
..