What Earthing System is This?

[andrewpreece]

I've looked in the FAQ's etc but am puzzled whether my electricity supply has TN-S or TN-C-S earthing. I thought that TN-S had the earth supplied by the incoming cable sheath, and that TN-C-S had the earth supplied by tapping an earth off the neutral core as it enters the service head thingy.

Now then, my system has an incoming cable with a central red insulated core surrounded by 16 black insulated 2.5mm2 wires. The red core is used as live,

12 of the 16 black cores are used as neutral, and four of the black cores have had their insulation stripped and they have been directed to an earth terminal.

Although at face value this means that my neutral line is being used to provide an earth, the 4 earth cores are not connected to neutral in my house, possibly they are only connected back at the substation - which would happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Andy.

[Dave Jones]

I would say TNC-S

[Dave Jones]

If your not sure phone the supplier up, they should tell you

[Stefek Zaba]

I've no experience of supply cables to speak of; but it sounds more PME (TN-C-S) than TN-S, simply because - going on your description - there'd be no way for the installer to distinguish between the 12 cores used to provide N and the 4 used for E. For all we know the cores you describe are bound together where the final supply to your place tees off the feeder in the road...

But Dave has it most right when he says,

The "should" here refers to the fact that they have a legal obligation to tell you what earthing arrangement you have, what the maximum earth loop impedance of the supply is, and what the maximum prospective short-circuit fault current is. (Don't expect those last two figures to be consistent with a 240 (or 230)V supply and Mr Ohm's most excellent Law, btw: they're both worst-case figures from opposite ends of the same spectrum, and suppliers allegedly most often say "0.35ohm" (for TN-C-S) or "0.8ohm" (for TN-S) (which would imply L-to-E fault currents - admittedly not exactly the same as a PSC, but damn close for PME at least - of about 690A and 300A respectively), while simultaneously quoting PSCs of 10kA or 16kA (the Conventional Maximum figures).

Of course, getting through to anyone in "engineering" at an electricity supply company in these days of micro-privatised sub-sub-outsource-contracted Competitive Electricity Supply is a patience-stretching experience...

Stefek

[Stephen Dawson]

The cable sounds as if it is a split concentric cable. This is generally used to provide a TN-S supply. This is not PME. The only way to be certain is to contact your local network operator and ask them.

[The Natural Philosopher]

Whilst you ar all talking, tell me what I have.

I have a substation in the garden. A coaxial cable comes from that with the neutral pretty much attached to the Earth at the transformer, and my earth busbar goes to a stake I drove into the ground about 5ft deep.

What do I have, and what the **** difference does it make to anything?

I am not sure, but I think that the earth and nuetral are also joined in the meter cabinet

[John]

Sounds like standard concentric cable. The stripped cores are seperate from the black insulated neutral cores so the "earthing system" really depends on just how the connection of your bit is made into the underground supply system. Thus it could be either TN-S or TN-C-S and you ought to ask your regional supply company who made the initial connection to the distribution system for a definitive answer. We can only guess from afar

[Stefek Zaba]

Having got the previous one probably wrong (the estimable Mr Stumbles tells us the std concentric cable *does* make it easy to distinguish the N cores from the E ones, so it really really is an ask-the-supplier one), let's see if I can c*ck this one up too.

Given "e and n also joined in the meter cabinet", it's pretty definite that your supplier is trying to give you TN-C-S, or PME. Your extra earth stake either does naff-all, if it's supplementary to the supplier's earth (other than creating an extra path for faults to earth in your local area, which may not be what you want!) or have created a TT supply, without an assurance that the earth resistance is low enough or (I assume) the usual "well we'll put a 100mA RCD across the whole lot" response.

The FAQ describes the earthing types and their effect. PME means you need to be a tad more cautious in exporting the house earth to shed, remote garij, and the like. TT means your supplier leaves you to sort out a local earth, and these days you need to supplement with the aforementioned whole-system RCD.

[Alistair Riddell]

If the earth and neutral are combined in a single conductor right up to the supply head then it is TN-C-S.

If there is no metallic connection between your main earth terminal and the neutral of the supply transformer (i.e. you have a metal stake in the ground as your sole earth connection) then it is TT.

It makes a difference because in the case of a TT supply, there is a good chance that the earth loop impedance will be sufficiently high that a live-earth fault in the customer's installation will not trip a protective device (circuit breaker) quickly enough or indeed at all. It is possible that "earthed" metalwork could be humming away at voltages significantly above true earth potential. To mitigate against this a Residual Current Device must be installed covering the entire installation.

[andrewpreece]

distribution

Thanks for the replies folks! I see now that what I thought was a simple question depends on things that might be buried in the ground i.e. how far back along my concentric cable are the four earthy cores connected to the 12 neutral ones - if back at the substation ( which is only one house away ) then it's TN-S, but if it is nearer than that, or multiply connected, then it's TN-C-S (PME).

Andy.

[Andy Wade]

Agreed. It's not usual though to be able to see the termination of a service cable; it's normally sealed inside the cut-out. Would the OP care to post a photograph somewhere?

[The Natural Philosopher]

Ah. Thanks for that. All makes total sense now.