TN-S and TN-C-S

1 Dec 2010
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United Kingdom
Hi, can someone tell me how to tell the difference in this in real life?
if the earth goes to the armoured sheathing then its TN-S but how do you identify a TN-C-S system?would there usually be a separate earth going down the main incomer with the live and neutrals?

I've heard that if the armoured cable gets corroded then you can convert to TN-C-S. I would expect this is done by the electric board but, how do you go about it in practise? Replace the whole cable or add in a very large earth to the substation?

Also, If the Earth and Neutrals are both grounded at the substation, arent they all really TN-C? - or this depend on where the Earth actually joins the ground in the substation?

And finally, doesnt a TN-S affect RCD's? or is this not a problem because of the neutral joining the earth before the RCD?

You answers are eagerly awaited!
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TN-C-S has a COMBINED neutral and earth upto the service head and a SEPERATE earth and neutral after the service head.

A TN-S has a SEPERATE neutral and earth throughout the whole system.

All substations have the neutral connected to an earth rod/matt/tape. If the substation is supplying TN-S it will have a seperate earth leaving it, if it is supplying TN-C-S it will only have a neutral leaving it, the earths in the properties being taken from the neutral.

RCD's are always after the combined part of the network where you have seperate earths and neutrals in your installation.
As already said and TN-S requires an earth loop impedance of 0.8 ohms or better and TN-C-S needs 0.35 or better.

TN-C-S is banned for caravans, boats, and petrol stations.

You have no control and although inspection can sometimes identify a TN-C-S system since the combining and separating need not be at your premises only the DNO can confirm you have a TN-S system.

With a TN-C-S also called PME the supplier uses many earths with the TN-S the neutral is only bonded to earth at the sub-station.

As to rust and the steel wired armour failing in most cases it's not SWA but a coaxial cable with centre line and outer half copper painted black for neutral and half bare for earth. Not steel but copper.
Generally it's easy enough to spot the difference, on a TN-S system the earth wire is clamped to the outer sheath of the main cable just below the cut out head with a special clamp. On a TN-C-S system the earth connects to the neutral terminal of the cut out, modern cut outs are made to accept this connection and have an extra terminal for it (for example cut outs made by Lucy have a four leaf clover shaped cover on the neutral side that covers the connection.

As others have said TN-C-S is also known as PME, which is what the DNO's still call it. It's up to them if you can have it or not and as you suggest it's them that do the conversion . It's a popular conversion for TT services, in the past the DNO used to require an earth on every pole (overhead) to do a PME conversion but they have now relaxed this and seem happy with an earth up to 5 spans away, the slight problem with this is that if a storm brings down the line and breaks the neutral and not the live you have a live system and no earth!

Interstingly the DNO has no responsibility to provide an earth, thats up to the consumer, so sometimes if a TN-S fails they simply tell you theres nothing doing and you have to convert to TT.

Whatever you do don't DIY this, the DNO will do a conversion for you if needed and possible and normally its free (speaking from my experience in SEC area)
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As already said and TN-S requires an earth loop impedance of 0.8 ohms or better and TN-C-S needs 0.35 or better.

Just to clarify TN-S does not require an earth loop of 0.8 and TNC-S does not need to be 0.35 or better. These are just typical values that supply authorities try to achieve and can be exceeded there are no regulations covering the maximum values allowed for either supply type.

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