No earth tncs

[anth125]

Hi all,

I am just looking for some advice or whether I am possibly missing something here. After isolating the supply and taking the cover off an Wylex consumer unit, there is no direct earth from the DNO cut-out to the board, and is only one 6mm bonding cable connected on the MET which is a 6mm2 earth connecting to the gas pipe.

There is a 6mm2 earth from the DNO cutout that bypasses the board and I believe it runs to the Water pipe. If this is disconnected there is no continuity between the gas bond at the board to the cutout itself. This is a TNCS earthing arrangement.

Has anyone come across wiring such as this before and can give me any clues.

 

[radiohead]

How are you establishing that a TNCS arrangement is in place?

 

[snowhead]

There is a 6mm2 earth from the DNO cutout that bypasses the board and I believe it runs to the Water pipe. If this is disconnected there is no continuity between the gas bond at the board to the cutout itself

So the 6mm must go Cutout - Water - Gas - Wylex?

 

[anth125]

How are you establishing that a TNCS arrangement is in place?

Line/Neutral brought into the service head with a seperate earth terminal used
[automerge]1583682505[/automerge]

So the 6mm must go Cutout - Water - Gas - Wylex?

There is only one bonding cable on the Water pipe I located, unless the water is bonded to the gas somewhere else.

 

[pc1966]

It sounds as if the earth connectivity is dependent on the gas and water pipe being connected (even if just by plumbing joints all the way to the boiler) and a risk if any future repair adds plastic pipe!
Many years ago a friend complained of getting shocks from his dad's garage door. Some investigation shows the whole house "earth" was live as a new socket had a screw puncturing the live insulation, and there was no earth bonding back to the DNO. That extended to the electrics in the garage (lights and 13A sockets) and hence to the metal door. I never found out why (never fitted, some muppet removed it) but that case was one wet shoe away from a fatality!

 

[anth125]

It sounds as if the earth connectivity is dependent on the gas and water pipe being connected (even if just by plumbing joints all the way to the boiler) and a risk if any future repair adds plastic pipe!
Many years ago a friend complained of getting shocks from his dad's garage door. Some investigation shows the whole house "earth" was live as a new socket had a screw puncturing the live insulation, and there was no earth bonding back to the DNO. That extended to the electrics in the garage (lights and 13A sockets) and hence to the metal door. I never found out why (never fitted, some muppet removed it) but that case was one wet shoe away from a fatality!

From what I have discovered with this installation, do you think I should recommend connecting the water to MET with gas and run a cable from cutout to the board?

Also the cable sizes are 6mm2; for a tncs isn't the minimum 10mm2 or is it adequate to leave it as it is knowing there are no signs of thermal damage

 

[pc1966]

There are better qualified folks on this forum than me to give chapter & verse, but AFIK it should be at least 16mm^2 from the earth connection from the fuse board / consumer unit (i.e. Main Earth Terminal) back to the DNO point. For the bonding from the MET to the external metalwork (to both gas & water pipes) it should be at least 10mm^2.

In my own case recently I just used 16mm^2 as well since I had a 25m reel from Toolstation (part no. 29323) and it would do all jobs. If space is tight in the Wylex board to put in the additional earths then you could use a separate earth block just for the job (such as Toolstation 62785 4-way or 63372 8-way, similar from Screwfix, etc)

But wrangling that wire down through a wall / false ceiling without an assistant was a major pain and while I cursed I would have preferred paying more to get a reel of the more flexible 10mm^2 at that time!

 

[anth125]

There are better qualified folks on this forum than me to give chapter & verse, but AFIK it should be at least 16mm^2 from the earth connection from the fuse board / consumer unit (i.e. Main Earth Terminal) back to the DNO point. For the bonding from the MET to the external metalwork (to both gas & water pipes) it should be at least 10mm^2.

That is what I understood it to be as per Table 54.8 BS 7671. For alternative arrangements, the main bonding conductor should be not less that half that of the earthing conductor of the installation and not less than 6mm2. reg 544.1.1

 

[pc1966]

It is not quite current, but there is an IET article has various examples and wire sizes, just search for "2005_16_autumn_wiring_matters__complete_no_adverts.pdf" and you should find it (last in the PDF document) .

 

[davesparks]

From what I have discovered with this installation, do you think I should recommend connecting the water to MET with gas and run a cable from cutout to the board?

Also the cable sizes are 6mm2; for a tncs isn't the minimum 10mm2 or is it adequate to leave it as it is knowing there are no signs of thermal damage

The first thing to do is get a direct connection between the means of earthing (cutout earth terminal) and the CU in place.
You can worry about sorting out what size the bonding needs to be and organise this with the customer later, but the first priority should be achieving a basic level of safety.

 

[anth125]

The first thing to do is get a direct connection between the means of earthing (cutout earth terminal) and the CU in place.
You can worry about sorting out what size the bonding needs to be and organise this with the customer later, but the first priority should be achieving a basic level of safety.

Cheers Dave. I will look at following your advice and see what they would like to do.