Electricity supply cable: why are E and N combined?

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

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

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.

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

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.

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.

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! :-)

Reply to
wanderer

(quite a bit)

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.

Reply to
BigWallop

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.

Reply to
Martin Angove

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.

Reply to
Christian McArdle

On a related note here's one explanation of why we have three prongs in a plug (with a US bias)

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Reply to
MBQ

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