Earthing conductor and main bonding conductor clarification wanted please.

[flyingsparc]

Hello, please can anyone advise me to help decide what would be the best way to go with this earthing/bonding issue? I apologies in advance as I know there are many threads covering this, but they often deviate and do not always decisively conclude.

I have been asked to upgrade an existing Wylex BS3036 fuse board to a dual rcd unit. The fuse board is in one of 4 flats which are all supplied via a 3 phase TNS intake.

This flat is supplied via a 60A switched fused (BS3036) isolator located near to the intake. From the switched fuse is 16mm t&e to the flats fuse board.

There is currently no main bonding to gas and water.

Using the adiabatic equation, (using both tabulated and my measured values) the minimum CSA for the earthing conductor (6mm within the 16mm t&e) is adequate.
I will need to install some main bonding conductors to the gas and water as they enter the flat, and in this case I feel they could be run in 6mm CSA.

My confusion is the difference between compliance with BS7671, and adequacy defined by calculation.

Its not a problem to install 10mm main bonding as gas and water enter flat near to fuse board location, but as there is only a 6mm earthing conductor in my opinion it would not improve the safety etc of the installation.

So….. with the existing tails should I install 6mm or 10mm CSA main bonds? and might it be necessary to run in a separate 10mm or 16mm earthing conductor?

And finally if upgrading the earthing conductor is thought necessary, the obvious route (to avoid taking up carpet and floors on two levels of communal stairs) would be out, up & along the outside of the building before coming back into the flat, so the customer will need to gain permission from the freeholder.

Sorry if my post is a bit long and waffeley, but wanted to include as much info as possible to help anyone answer my question/s.

Many thanks for any replies.

 

[RyanCheetham]

I would do it in 10mm. The cost difference is minimal and it future proofs the bonding should the service supply increase.

 

[MDJ]

I would run a 10mm2 earth wire with the 16mm2 twin and earth for a start, It is my understanding the earth should be the same size as the neutral conductor up to 25mm2 and then half after that, and I would run 10mm2 earths from the db to the gas and water.

 

[smu_diddly]

I think 6mm bonding with a tns supply is ok as long as ze is low enough for devices to operate quick enough (ADS). But I could be wrong. My regs books in the van and I'm too ill to be bothered to check

 

[flyingsparc]

Thanks for the replies so far.

Ok then 10mm main bonds, future proof etc no problem, done!

Ze & Zs's are all good!

So would like to get a few more views re the 6mm earthing conductor if poss.

thanks again.

 

[Engineer54]

I would run a 10mm2 earth wire with the 16mm2 twin and earth for a start, It is my understanding the earth should be the same size as the neutral conductor up to 25mm2 and then half after that, and I would run 10mm2 earths from the db to the gas and water.

Think you need to read your BS 7671 more often!!

 

[Engineer54]

Thanks for the replies so far.

Ok then 10mm main bonds, future proof etc no problem, done!

Ze & Zs's are all good!

So would like to get a few more views re the 6mm earthing conductor if poss.

thanks again.

Any bonding present at the main intake MET, to the incoming water and gas pipes??

 

[MDJ]

Think you need to read your BS 7671 more often!!

you could expand on your reply.

 

[Engineer54]

you could expand on your reply.

Yes... what you stated (in bold) is Wrong!! Check your BGB/Guide...

 

[MDJ]

your certainely a very helpful polite chap thats for sure.

 

[Engineer54]

Well, ...I've politely tried to help you. :wink5:

 

[spark 68]

Each Flat is also considered a seperate intallation electrically, and requires bonding from the water and gas services at the point of entry (within 600mm etc...) in each flat, to that flats CU.

Does each flat have a local CU?

 

[SYKRAPS]

I personally would bond the gas and water from the MET at the metering position using 10mm g&y, how many water stop taps and gas meters does the property have out of interest? Also even though it is over the top I usually run a seperate 10mm g&y from origin to sub-main along with a 16mm t+e as I prefer to air on the side of caution. In the experience of my previous installations more often than not on a TN-S system a 6mm bond is sufficient for bonding gas and water for PIR's and now EICR's but if I was installing from new it's a 10mm every time.

Hope this helps...........

 

[Engineer54]

Each Flat is also considered a seperate intallation electrically, and requires bonding from the water and gas services at the point of entry (within 600mm etc...) in each flat, to that flats CU.

Does each flat have a local CU?

More importantly does each flat derive it's supply to a local CU via a DNO meter ??...

 

[Engineer54]

I personally would bond the gas and water from the MET at the metering position using 10mm g&y, how many water stop taps and gas meters does the property have out of interest? Also even though it is over the top I usually run a seperate 10mm g&y from origin to sub-main along with a 16mm t+e as I prefer to air on the side of caution. In the experience of my previous installations more often than not on a TN-S system a 6mm bond is sufficient for bonding gas and water for PIR's and now EICR's but if I was installing from new it's a 10mm every time.

Hope this helps...........

As many flats/apartments as there is in the building, plus a main gas and water stop valve...

Not sure what your saying here, ...are you saying there is no need to bond these services to the EMT'S at each flat??