6mm incoming earth

[DNO1981]

Hi all

This is a follow up from a previous thread I posted about 16mm tails.

Just had two electricians round to quote for a board change, they were both happy to do it with 16mm tails but the incoming earth is only 6mm they have advised me to get onto the electricity supplier and upgrade the earthing and wont do the board change until done. Internal bonding is also 6mm

Sounds right? (not that I don't have faith in the sparks just seeking thoughts on this)

 

[telectrix]

photo of earthing arangement please.

 

[timhoward]

@telectrix just beat me to it, I'll post anyway -

This is hard to answer concisely! I'll have a go....
Generally speaking it sounds like good advice. If you follow it, your installation will look 'normal' and match the tables in the wiring regs and the on site guide.

The precise answer depends on what sort of earthing arrangement you have. If you stick a photo up we can help.
If you have the very common arrangement called TNCS then the advice is spot on, end of story really. 16mm earth. 10mm bonding.

If you have a (generally speaking) older arrangement called TNS then there is a choice between using a table to keep things simple or a formula to work out the required earthing conductor size.
If using the table, then the advice is still spot on. 16mm earth, 10mm bonding.
If using the formula it is certainly theoretically possible for 6mm earthing to be deemed adequate, and if it was then the 6mm bonding would also be adequate. But I stress this is only ok if it's the right earthing arrangement and various measurements support this.

 

[DNO1981]

photo of earthing arangement please.

 

[i=p/u]

Looks like 16mm earth on tncs.

 

[GBDamo]

Looks like 16mm earth on tncs.

At least 10mm², although photos can be deceiving.

 

[timhoward]

Well certainly PME sticker so tncs, it's always hard to tell size from photographs.
It does look bigger that 6mm csa, my money is on 10mm.

(Also note the seal fairy has really let her hair down!)

 

[i=p/u]

Zoom in and it looks 16?

 

[timhoward]

This is one of the more interesting ones in terms of regs.
I think the bottom line is:
1) If either main earthing or main bonding are less than 10mm^2 csa then it certainly needs sorting as it's PME/TNCS. (reg. 542.3.1 , 544.1.1 etc.)

2) If tails are 16mm^2 csa and earthing is less than 16mm^2 csa then the only possible way it can comply is using the 543.1.3 adiabatic equation. This stands a good chance of deeming 10mm to be adequate, but would need another electrician to visit to prove this, and they would need to be happy to entertain this slightly unusual approach too. Many simply never use this method and may not even remember it!

Even though it's a standard size that gets installed regularly I'm not aware of anything else in BS7671 requiring it to be 16mm main earthing on a PME as long as 543.1.3 is met. Happy to be corrected.

3) DNO rules may independently require it to be 16mm (for new connections they certainly want it).

4) To avoid this discussion at every twist and turn in the future I'd personally just get it changed to what is universally agreed to be a compliant setup.

 

[Dustydazzler]

Tails leaving the meter look 25mm and the earth looks like 10 or 16mm (its no 6mm)

if I were changing the board I would pull back the suppliers earth , put a block next to the head and run my new 16mm and connect both at the block

 

[i=p/u]

Has to be a minimum of 10mm due to mpb..

 

[DNO1981]

Tails leaving the meter look 25mm and the earth looks like 10 or 16mm (its no 6mm)

if I were changing the board I would pull back the suppliers earth , put a block next to the head and run my new 16mm and connect both at the block

Thats a good thought

 

[Pretty Mouth]

There is a note in Best Practice Guide 1 (CU changes) that says a 6mm bond on PME can be left in place if it has been there a long time and shows no thermal damage:


3. A 6 mm2 main protective bonding conductor size could be deemed adequate where the minimum size required by Regulation 544.1.1 of BS 7671 is 10 mm2, if the bonding conductors have been in place for a significant time and show no signs of thermal damage

 

[i=p/u]

Yeah he started of saying he had 6mm earthing conductor.... then we jump all over the place.

But a Welsh man wont the darts on diff channel 180v

 

[Dustydazzler]

The ferret

 

[cliffed]

Definitely looks bigger than 6mm…its not usually connected like that,always from the side terminal…

 

[DNO1981]

Definitely looks bigger than 6mm…its not usually connected like that,always from the side terminal…

Definitely a 6mm that's been confirmed funny thing is underneath next to this looks like a chopped earth cable which is much bigger

 

[GBDamo]

Definitely a 6mm that's been confirmed funny thing is underneath next to this looks like a chopped earth cable which is much bigger

Yes, I'm rolling back on this.

Here is a 6mm² in relation to my 25mm² tails and 16mm² main earth.

 

[DNO1981]

Yes, I'm rolling back on this.

Here is a 6mm² in relation to my 25mm² tails and 16mm² main earth.
View attachment 86266

So I guess it wouldn't stop any CU change, they could cut the 6mm earth put in a block then run a 16mm from the CU to the block

 

[Dartlec]

It's the sort of situation that might result in a Catch 22 unless some common sense is applied.

When you go ahead with the fuse upgrade it is entirely possible that the DNO will demand an up to date electrical certificate for the property to show that everything is in place and suitable for 100A, before they do any work. So then you'd end up in a Chicken vs Egg situation.

In most cases (having carefully assessed the situation) I'd personally feel comfortable with doing as suggested, with 16mm earth from the new board to a block near to the main head (and likewise with 25mm tails to henley blocks connecting to the existing 16mm ).

That would be on the understanding that the earth was to be upgraded (or that the DNO would decide they were happy with existing) - as well testing the existing Ze (test of the level of the current earthing) and a quick check with the adiabatic. I'd upgrade main bonding to 10mm too where it was reasonably practicable to do so.