TN-S or TN-C-S which would you have?

[dokidoki]

Hi guys. I'm new, be gentle!

I currently doing a major referb on my house. Whole thing will need a re-wire (LABC aware & doing inspects for part P). Not at that stage yet but need to move my TN-S supply to an outside meter box. DNO have said I can either have have TN-S or TN-C-S (PME) supply – my call. Question is what to ask for and why? My initial thoughts is to go with PME but I have a large shed in the back garden and wondered if this would be a problem exporting the CPC, although I have no problem using 3-core 10.0mm² SWA to extend the equipotential zone or am I talking rot?

 

[ericmark]

Seems odd to offer both as normally one is only guaranteed a TN-S supply with petrol stations.

But TN-S is safer than TN-C-S which is why it has to be used for petrol stations, caravan site and marinas. However the earth loop impedance can raise from 0.35 for TN-C-S to 0.8 for TN-S but you will have RCD protection anyway so no real problem.

So I would select safe option and go for TN-S.

 

[JohnD]

why it it safer?

I have seen old installations where the armour wrap has rusted or been damaged

 

[MalcofArabia]

Quite supprised they are giving you a choice as TN-C-S is now the modern way to do it as it saves cost.

Both systems have pro,s and con's as like everything in life.

You get a more reliable Ze with TN-C-S, as JohnD pointed out on a TN-S system you rely on the cable sheath for the earth and that can fail.

There is a theory that if the neutral failed on a TN-C-S system it could cause a problem with raising potential during a fault, but I've not known this to happen in my experience, others may have.

In pre 17th days a TN-C-S system would give you a lower impedance path outside the installation and therefore disconnection times are met easier on the protection devices. Now with 90% of domestic circuits being RCD protected this is not so critical.

I would go with the TN-C-S system to be honest.

Yes though there is not a regulation against exporting the earth from a PME system quite a few DNO's don't allow you to. So you could ask them this before you decide it may make you go for a TN-S system if they allow the earth to be exported.

If you are allowed to export the earth then your extending the zone. What you would need to do is, if the shed will have either gas or water in it you will need to run a 10mm^ Bond back to your Main Earth Terminal (MET) at your header.

If you decided TT then you can bond local at the MET in the shed. So like everything in life loads of if's and but's which you can only answer.

 

[ericmark]

If at any point the same conductor is used for earth and neutral then the supply is a TN-C-S and since the point of separation does not have to be in the customers premises it is impossible to tell if the supply is TN-S only a deceleration from the DNO can tell you which system. There are because of safety issues a list of locations where TN-C-S is not permitted for example a construction site, petrol station, caravan site, and marinas. It seems rather odd for the ESQCR would have been unlikely to have prohibit the use of a TN-C-S system for the supply to a caravan or similar construction if there was not a safety issue.

Yes I have come across failures and it did cause problems but it has only caused a real problem due to earthing on one property and that was due to the DNO not following the rules on PME.

But it seems a strange option and I wonder if it was really TN-C-S or TN-S or was the option TT or TN-C-S?

The rule says:-
(2) In addition to the neutral with earth connection required under regulation 8(3)(b) a distributor shall ensure that the supply neutral conductor is connected with earth at -

(a) a point no closer to the distributor's source of voltage (as measured along the distributing main) than the junction between that distributing main and the service line which is most remote from the source; and -----

If that had been followed then the problem I came across would not have caused the damage it did. However if everyone followed the rules than to require caravans to be fitted with RCD protection would be OTT as they could never plug into any supply which was not already RCD protected to 30ma at 40ms.

The same goes for the TN-S v TN-C-S in both cases if the DNO does their job correct and the house holder does his job correct with PIR's every 10 years neither system can cause any safety problems. However any metal in the ground that is connected to other bits of metal in the ground can and do have electrolysis taking place if dissimilar and the more the voltage is raised over the grounds potential the greater the problem be it metal soil pipes, gas pipes, water pipes or any other extraneous-conductive-part and to use diodes to prevent it will only work if the voltage is within the threshold voltage and with TN-C-S that threshold voltage is more likely to be exceeded so to my mind TN-S every time.

 

[dokidoki]

Thanks for the replays guys. All very interesting. Good point about the earth loop Ze but as you all rightly point out I don't think this is going to be a problem.

Seems odd to offer both as normally one is only guaranteed a TN-S supply with petrol stations.

I agree. The chap from the DNO said I had TN-S which I didn't question as to me it looks to be - which is why he offered to keep it or change it to TN-C-S.

If you are allowed to export the earth then your extending the zone. What you would need to do is, if the shed will have either gas or water in it you will need to run a 10mm^ Bond back to your Main Earth Terminal (MET) at your header.

Shed doesn't have gas or water, just a few sockets and lights. I'm assuming I still need to bond back to the MET in case any appear in the future? Sounds like TT at shed would be easier.

If at any point the same conductor is used for earth and neutral then the supply is a TN-C-S and since the point of separation does not have to be in the customers premises it is impossible to tell if the supply is TN-S only a deceleration from the DNO can tell you which system.

So even though it looks like a TN-S (CPC is cable sheath) the DNO may have changed it to TN-C-S anyway? If so I'm thinking it once was a TN-S back in 1934, when the house was built, but is now actually a TN-C-S in disguise. In which case the chap from the DNO can't offer me a TN-S and has it wrong?

But it seems a strange option and I wonder if it was really TN-C-S or TN-S or was the option TT or TN-C-S?

He definitely didn't give an option for TT. I wonder if he just looked and thought TN-S but didn't check. Any ideas how the DNO checks from “true” TN-S?

 

[ricicle]

He definitely didn't give an option for TT.

He wouldn't - he would decline to give an earth and then you would have no choice but to TT. Even if a TN earthing system is offered you wouldn't necessarily have to use it. you could still TT !

 

[Lectrician]

god.

 

[mfarrow]

Shed doesn't have gas or water, just a few sockets and lights. I'm assuming I still need to bond back to the MET in case any appear in the future? Sounds like TT at shed would be easier.

Only if you run services in metallic pipe, and plastic's (e.g. MDPE) so much easier to work with.

 

[ricicle]

god.

Where ?