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.

 

[rich.250]

Service intake to MET.
And service entry into flat to each flats met.
As for the existing 6mm in the t and e.
I'd say it needs to be 10mm without checking the bgb.

 

[rich.250]

Service intake to MET.
And service entry into flat to each flats met.
As for the existing 6mm in the t and e.
I'd say it needs to be 10mm without checking the bgb.

Obviously only if bonding is required!!
Have you tested it??

 

[SYKRAPS]

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??

No as I mentioned I would bond all incoming services in dwelling where I carry out work back to the MET at origin using 10mm g&y but I did a job only yesterday where there was only 1 gas meter and 1 water meter serving 3 flats both in the landlords/communal area.

I was just asking the settup at the OP's job

 

[spark 68]

but I did a job only yesterday where there was only 1 gas meter and 1 water meter serving 3 flats both in the landlords/communal area.

I was just asking the settup at the OP's job

Makes no difference, each flat requires it's own local bonding back to it's own CU/DB, see GN8 section 5 for further details (reg 411.3.1.2), this can be done either from the entry point of the services to each flat back to the MET at the origin, or more commonly/sensibly from each flats entry point to that flats CU EMT.

As I said, each flat is classified as an individual installation as far as the regs are concerned.

 

[SYKRAPS]

Makes no difference, each flat requires it's own local bonding back to it's own CU/DB, see GN8 section 5 for further details (reg 411.3.1.2), this can be done either from the entry point of the services to each flat back to the MET at the origin, or more commonly/sensibly from each flats entry point to that flats CU EMT.

As I said, each flat is classified as an individual installation as far as the regs are concerned.

As I said I was asking for more info regarding the job, out of interest why would you have to bond the gas in every flat if there is only 1 meter at the property? We don't even know if there is gas in every flat. Also as I have also said I would bond every incoming service to every flat. More info that is all I was asking for fgs................

 

[SYKRAPS]

To make it clearer about my job (heaven knows why I should) in my earlier post at my job there was communal heating, cooking and washing and therefore no need for every flat to be bonded by its own rights as there was no gas and water in each flat...........................

 

[spark 68]

It is in GN8 mate, this tells us each flat is to be considered as a seperate installation, this also goes for HMO's (houses of multiple occupancy), bedsits and the like, and it refers you to reg 411.1.3.2 for the electrical definitions, I have posted the relevant info. on the forum before (pages from GN8).

There have been some rather longwinded posts on here before also regarding this subject.

It is so each flat has it's own equipotential zone, and as BS7671 requires each installation to have main bonding conductors, GN8 elaborates on this, and specifically says this applies to multi occupancy buildings and classes each flat as a seperate installation and also elaborates on the definitions given in the reg number above.

Edit:, I was not having a dig at anyone, just posting the information which some people may find informative and useful, as not everyone has GN8, even GN8 itself states that the regs regarding this are not clear, and are often misunderstood/misinterpreted.

 

[flyingsparc]

Many thanks to all for the recent replies.

Yes each flat has their own local CU.

When you ask - how many water stops and gas valves does the property have, do you mean the block of flats as a whole or the flat in question? Either way the flat has one water stop, I don’t know about the other 3 flats (they are all privately owned) but I would expect to find one in each flat, and one or four where the water enters the block. With the gas valve/meters there is one per flat, so four in total.

I will have another look for the water stops tomorrow, would the amount of water stops in the block affect the answer to my question about the 6mm earthing conductor?

Another point worth mentioning is that from the DNO head to each meter and onto each switch fuse are supplied via 10mm CSA line and neutral conductors.

One other point I would like to clarify is that it has been suggested I could run in a 10mm conductor alongside the 16mm t&e. I thought there was a regulation (can find it though as yet) about if running parallel conductors that they should be the same CSA? Don’t know why that’s in my head though!?

Thanks again.

 

[MDJ]

I would certainely run a 10mm2 earth wire with the 16mm2 Twin and Earth, good practice IMO, if you can aford to do so within your quote you may as well.

 

[johnboy6083]

The minimum size of Bonding conductor is 6mm for TNS, and 10mm for PME. Its common practice and the diargram in the front of the OSG that has everybody beleiving that a main earthing conducotr has to be 16mm min, and the main bonding 10mm min. Thias isnt the case. A main earthing conductor for a tns can be sized by selection from table 54.7, or calculated vie the adiabatic. The main bonding has to be half of this ( well it says half of the required size. Even if somebody has used a 35mm as a main earth, if your calcs say it could be a 16mm, then a 10mm bond will do. Useful to know when working in older installations)
For PME you have to go with the requirements of your local DNO, and a far as im aware, theyre usually happy to go with the table in BS7671.

 

[Amp David]

GN8 is a great read for this very subject.