TNS or TNCS - which is preferable?

[Siddell]

Hi there,

I'm the proud owner of a Victorian, 2-bed semi and recently discovered that my only means of earthing look to be my water supply and (redundant, non-conducting) gas pipe.

I managed to get UKPN to come out and do a PME availability check, having been told all sorts of things by them on the phone beforehand. Anyway, the fellas turned up and promptly fitted a PME terminal and I gladly let it continue, as it was free of charge! It was only afterwards that I realised (on phoning the engineer) that the very fitting of the PME terminal now excludes TNS as an option and that I am now committed to PME, the disadvantage being that I will now have to upgrade all the bonding which is 6mm.

My questions therefore are:

1) Given that UKPN installed PME free of charge, would they have installed TNS free of charge using the lead sheath of the cable, if the impedance was O.K? If not, then I don't care about the time/money of upgrading the bonding.

2) Would TNS necessarily require upgrading to the bonding or is this dependent on testing?

3) What are the main advantages and disadvantages of TNS/TNCS?

Thanks for your input! This is all new to me, but I have really taken an interest in it. Not least because it concerns my safety - on which note I am aware there is still no main earthing to my house as the CU is not connected to the PME terminal. Need to get an electrician round to finish, but would like to choose the best balance of practicality, safety and cost.

Jake.

 

[Guitarist]

Did UKPN connect up to an earthing block in 16mm, then reconnect your supply with no earth to your consumer unit?

 

[WPD Lad]

Did they dig up your drive? How did they PME it?

Got any pics?

 

[Richard Burns]

I do not think UKPN would install a TNS earthing system now they are all hot on the PME / TNCS earthing as being a better method.
However it is not necessarily a better method, just more reliable for them to do.
TNS is good because loss of the neutral does not mean loss or earthing. It is bad because sheath decay / disconnection could mean there is no viable earth.
PME is good because you only need to send a single core sheathed cable to a single phase installation. There will also very likely be a reasonable earth since it is earthed all along the incomer back to the substation. Disconnecting the neutral does mean that you have lost all earthing for the installation which is why bonding becomes so important.

Although you now theoretically have bonding that does not comply with the current regs this does not necessarily mean you must upgrade it if it is still adequate for the installation.

You do need to get a main earth to your CU as soon as possible , presumably UKPN just provided a main earth terminal from the head for this purpose. But (as it is not their area of work) did not provide a CU earth!

 

[mechelec]

If your bonding and earth conductors do not comply with bs7671 then they should not have given you a pme earth terminal.

 

[telectrix]

with PME, the minimum size of main bonding conductors is specified as 10mm . however, on an existing installation, if the 6mm bonding shows no sign of deterioration, and is verified as connected correctly, then it may well be that it can be left. your electrician can confirm this.

 

[Deleted member 26818]

You may find, that the ESQCR 2002 doesn't allow the provision of TN-S, If the distribution line has been made PME.

 

[Edd]

with PME, the minimum size of main bonding conductors is specified as 10mm . however, on an existing installation, if the 6mm bonding shows no sign of deterioration, and is verified as connected correctly, then it may well be that it can be left. your electrician can confirm this.

I thought this would have meant if an existing PME installation showed no signs of deterioration of the 6mm bonding then no need to upgrade. But this is now a new PME instalation so I think it would need to be upgraded. but as you say his sparks can confirm it.

 

[Siddell]

Cheers guys,

I had posted a picture and more info but it said the moderators need to check my message first. I'm new on this forum so don't know when that message will appear if at all!

I'll double check with my spark, although he mentioned 10mm so must lean that way. He's gonna stick an RCD in too. Said if buy one they'll connect it! What would people recommend? He had just said "63A RCD" but UKPN installed a 100A fuse now.

Also, any issues with finding space for the 16mm earth on my old, 4-way Wylex rewireable fuseboard (to connect to earth block)?

Apologies for repetition if my other message gets posted too.

Related question: does supplementary bonding have to be bonded itself to the earth from a nearby circuit?

Many thanks.

 

[Richard Burns]

Cheers guys,
I'll double check with my spark, although he mentioned 10mm so must lean that way. He's gonna stick an RCD in too. Said if buy one they'll connect it! What would people recommend? He had just said "63A RCD" but UKPN installed a 100A fuse now.
You will have to check with him on that one because I do not know the method he is using to do this.

Also, any issues with finding space for the 16mm earth on my old, 4-way Wylex rewireable fuseboard (to connect to earth block)?
Should be OK, even if the cable needs to be fed to two terminals.Though sometimes the holes are a bit small and there can be a lot of CPCs in the earth bar.

Related question: does supplementary bonding have to be bonded itself to the earth from a nearby circuit?
Yes

Hope it all goes well.