Sub Main Issue on Condition Report!

[hoppy]

Hi,

i have been to complete a condition report in a flat today and would appreciate a bit of advice.

The db was fed via a 16mm twin and earth sub main approx. 12m run, protected by the 6o amp main fuse, tncs system. Gas bonded in 10mm at main cutout, water bonded in 10mm at db.
Main earth at db is the 6mm in the twin and earth. Dual rcd db.

I have c2 the set up, but an electrician i work with is saying c3. Do you agree it is a c2.

I believe a c3 may apply if it was a tns system and the adiabatic equation was satisfied.

Also, to rectify this would running a separate 10mm earth from the cut out to the db be ok, giving a combined csa of 16mm, or would we have to run a separate 16mm earth to comply.

Just wondered what everyone's thoughts were on this?

All advice is appreciated.

Thanks

Craig

 

[Octopus]

C3 is what I would report, backed up with the calculation

 

[Wilko]

Hi - personally I'd put in a new 16mm and be done.
Perhaps also check the run, is it through walls < 50mm etc so may also need RCD from 522.6.203 ?

 

[westward10]

C2, the 6.0 cpc is inadequate for PME bonding purposes. There is no calculation in this case as the Adiabatic equation is not used for calculating main protective bonding which the cpc of the cable has become.

 

[Wilko]

Hi - personally I'd put in a new 16mm and be done.
Perhaps also check the run, is it through walls < 50mm etc so may also need RCD from 522.6.203 ?

Oops, meant to say .202 ... no tea yet

 

[westward10]

I am counting down the seconds until some hairpin mentions Best Practice Guide.

 

[Ian1981]

Hi,

i have been to complete a condition report in a flat today and would appreciate a bit of advice.

The db was fed via a 16mm twin and earth sub main approx. 12m run, protected by the 6o amp main fuse, tncs system. Gas bonded in 10mm at main cutout, water bonded in 10mm at db.
Main earth at db is the 6mm in the twin and earth. Dual rcd db.

I have c2 the set up, but an electrician i work with is saying c3. Do you agree it is a c2.

I believe a c3 may apply if it was a tns system and the adiabatic equation was satisfied.

Also, to rectify this would running a separate 10mm earth from the cut out to the db be ok, giving a combined csa of 16mm, or would we have to run a separate 16mm earth to comply.

Just wondered what everyone's thoughts were on this?

All advice is appreciated.

Thanks

Craig

My understanding is Cables ran in parallel must be the same CSA same construction and follow the same route as would be the case if you wanted to try and add the 2 cpcs together.
Both cables would have to be able to withstand the fault current.
For the sake of running a 10mm cable in you might as well run it in 16mm if that is what you are going to end up doing.
10mm combined cpc and protective conductor would be acceptable tho as it's a cpc your installing not the main earthing conductor.

 

[AndyL]

So does the cable come straight from the meter or is there at main switch by the meter?

 

[Octopus]

I am counting down the seconds until some hairpin mentions Best Practice Guide.

why? what does it say?

PS I do need to go to spec savers.. thought the OP said TNS

BUT there are countless installs, in flats with this type of "issue" and some you simply can't upgrade.......

 

[Ian1981]

why? what does it say?

PS I do need to go to spec savers.. thought the OP said TNS

BUT there are countless installs, in flats with this type of "issue" and some you simply can't upgrade.......

Best practice guide is put together with contributions by industry personnel such as the niceic, napit, the IET, beama, city and guilds, EAL.
It is as it says a guide only but to totally dismiss using, I don't understand.
As always bs 7671 is the standard we follow.

 

[westward10]

why? what does it say?

PS I do need to go to spec savers.. thought the OP said TNS

BUT there are countless installs, in flats with this type of "issue" and some you simply can't upgrade.......

No idea what it says I was just anticipating it.
It is TN-C-S and I agree blocks of flats are a pain in this situation, just do the Report and hope no one asks for a remedial quote.

 

[hoppy]

So does the cable come straight from the meter or is there at main switch by the meter?

It comes from an isolator

 

[hoppy]

Thanks for all the replies, c2 it is with recommendation a separate 16mm earth is installed