Having just had a new electricity supply laid on, I was kind of
surprised to see that the incoming cable has only one core; ie the
earth and neutral are combined. Having just googled a bit I can see
that this is perfectly normal practice, but what I don't understand is
how this configuration translates to my domestic supply. Evidently
neutral and earth are at the same potential, right? So how does
earthing work within my house? Sounds daft to be asking, but what is
the advantage of earthing the circuits in the house? ie, why do we
have a separate earth within the house but not from beyond the
consumer unit?

Would be grateful for some clarification!

Thanks
David

On 7 May 2004 12:27:25 -0700, Lobster wrote:

Having just had a new electricity supply laid on, I was kind of
surprised to see that the incoming cable has only one core; ie the
earth and neutral are combined.

It's called PME, or Protective Multiple Earthing (of the neutral
conductor).

Having just googled a bit I can see
that this is perfectly normal practice, but what I don't understand is
how this configuration translates to my domestic supply.

Protection of your installation against the effects of earth leakage
currents is *your* responsibility. In the vast majority of cases the
leccy company will provide an earth terminal for you to connect to.

Your electrical installation *must* have separate earth conductors for
each circuit, and all extraneous metal work such as water pipes, taps,
metal sinks, etc, has to be connected to the earth wires in your
installation.

Evidently
neutral and earth are at the same potential, right?

More or less - there may be a few volts difference between neutral and
the general mass of earth, but normally nothing to bother you.

So how does
earthing work within my house?

The idea is that if you have an earth fault on an appliance or in the
installation, the fuse protecting that circuit will blow and isolate the
fault. If however, the fuse doesn't blow coz there ain't enough current
flowing, then the bonding of all the metal work means that everything -
including you - is at the same voltage above earth so you won't get a
shock.

Sometimes, however, the leccy company won't be able to provide an earth
terminal, particularly in older rural properties. Then you have to use a
residual current device. Put very simply, this makes sure that what goes
in on the live comes back on the neutral. If it don't then the device
recognises the imbalance, thinks 'there must be a fault to earth' and
opens. You still have earth conductors, but these are connected to a
separate earth rod, to give a return path for leakage currents,
otherwise the rcd won't work.

As it happens, modern installations use these devices as well as well
as earth terminals, as they are are very sensitive and have time factors
associated with them as well.

Sounds daft to be asking, but what is
the advantage of earthing the circuits in the house? ie, why do we
have a separate earth within the house but not from beyond the
consumer unit?

You are not allowed to use the neutral conductors in the installation
for an earth conductor unless with the specific approval of the
Secretary of State, and that's as rare as hen's teeth.

The leccy companies make the neutral very secure (usually with
compression joints) and also connect it to earth at several places as
well at the distribution transformer. They *are* permitted to use the
neutral to prvide you, the customer with an earth terminal at your meter
position.

Would be grateful for some clarification!

HTH - I've simplified things quite a bit but the basics are there - the
pedants will be along in a minute to argue about the terminology! :-)

"wanderer" wrote in message
...
On 7 May 2004 12:27:25 -0700, Lobster wrote:

snipped (quite a bit)

Would be grateful for some clarification!

HTH - I've simplified things quite a bit but the basics are there - the
pedants will be along in a minute to argue about the terminology! :-)


No. You're right. All the basics are in there and you explain it to the best
understanding you can without going into all the techie thingies.

In message ,
"BigWallop" wrote:


"wanderer" wrote in message
...
On 7 May 2004 12:27:25 -0700, Lobster wrote:

snipped (quite a bit)

Would be grateful for some clarification!

HTH - I've simplified things quite a bit but the basics are there - the
pedants will be along in a minute to argue about the terminology! :-)


No. You're right. All the basics are in there and you explain it to the best
understanding you can without going into all the techie thingies.



The scary thing about PME is that there is a possible failure mode where
the power is off, but everything is still live, and unearthed. This
would be the case where somehow someone cuts through the supply
company's neutral just before it enters your house, but not the live.

As has been said, the supply company goes to great lengths to earth the
neutral (hence "multiple") as securely as possible to prevent this, and
the use of concentric cable makes it extremely unlikely, but having PME
makes it even more important that main and supplementary bonding is
correctly used.

Actually, come to think of it, there is a possible failure mode in a
TN-S system where the earth disappears but L&N are still live. That
could be quite dangerous too, if a fault develops in your
installation...

Maybe we should all go back to candles :-)

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... I've used Basic so long, my brain has gonesub permanently

Actually, come to think of it, there is a possible failure mode in a
TN-S system where the earth disappears but L&N are still live. That
could be quite dangerous too, if a fault develops in your
installation...

Indeed. It is more dangerous than the PME case, as with PME you would have a
power cut that causes the fault to be fixed. If you lost only the earth but
not the neutral, you could continue in that state for some time without
anyone realising that a fault has occured.

Christian.

(Lobster) wrote in message . com...
Having just had a new electricity supply laid on, I was kind of
surprised to see that the incoming cable has only one core; ie the
earth and neutral are combined. Having just googled a bit I can see
that this is perfectly normal practice, but what I don't understand is
how this configuration translates to my domestic supply. Evidently
neutral and earth are at the same potential, right? So how does
earthing work within my house? Sounds daft to be asking, but what is
the advantage of earthing the circuits in the house? ie, why do we
have a separate earth within the house but not from beyond the
consumer unit?

Would be grateful for some clarification!

Thanks
David

On a related note here's one explanation of why we have three prongs
in a plug (with a US bias) http://amasci.com/amateur/whygnd.html

MBQ