4 submains with one common CPC

[atm84]

I have a query. 4 distribution boards, each fed by 25mm2 double insulated tails (approx 30m away) in a roof void. Only 1 16mm2 earthing conductor for all 4 submains. The earthing conductor is then connected at the first board and looped to the other three boards. Is this acceptable? I thought BS 7671 states that each circuit should have its own CPC.

 

[shanky887614]

You have probably read a regulation that each individual circuit must have a cpc and interpreted as each individual circuit must have its own cpc.
There is electrically nothing wrong with supplying the distribution boards using the same piece of cable.

true there is nothing electrically wrong with it if the cable is large enough but personally i would rather have a seperate earth with each supply.

 

[davesparks]

It is effectively the main earthing conductor to each DB.

An installation can only have one earthing conductor, it is defined as being the conductor which connects the MET to the means of earthing.
The term 'main earthing conductor' does not exist in bs7671

 

[atm84]

true there is nothing electrically wrong with it if the cable is large enough but personally i would rather have a seperate earth with each supply.

I agree but then one doesn't want to create unnecessary work if it's not required.

 

[Deleted member 26818]

To be precise, BS7671 does allow for an installation to have more than one Earthing Conductor.

 

[atm84]

An installation can only have one earthing conductor, it is defined as being the conductor which connects the MET to the means of earthing.
The term 'main earthing conductor' does not exist in bs7671

Dave please refrain from trying to be smart and always trying to catch people out. I apologise for mister meaner. I'm sure you understood what I meant.

 

[Richard Burns]

true there is nothing electrically wrong with it if the cable is large enough but personally i would rather have a separate earth with each supply.

Not a problem for you to do that if you are installing it, I would think it would increase the quality of the installation in many cases and exceeding the minimum standards is generally a good thing, but it is important to be aware that it is permitted to have a single (suitably rated) cable.

 

[shanky887614]

I agree but then one doesn't want to create unnecessary work if it's not required.

? it would take more work to join the cable and link it than to pull it in with the tails.

same route afterall and bs7671 is a minimum standard afterall

 

[atm84]

? it would take more work to join the cable and link it than to pull it in with the tails.

same route afterall and bs7671 is a minimum standard afterall

Its an existing installation. How I would have done it in the first place is kind of irrelevant. I'm just query its suitability as a basis to make additions to the DBs.

 

[Risteard]

The term 'main earthing conductor' does not exist in bs7671

Indeed. I often wonder where the supplementary Earthing Conductors are if one is the main one!

 

[atm84]

Just looking in the regs to find where it says each circuit must have its own CPC and I've come across 543.1.2 in the green book (installation complete in 2011) and it clearly suggests that it is acceptable to have a common protective conductor.

 

[Spoon]

Just for reference, its the same in the BYB (page 164)

543A.2 Where a protective conductor is common to two or more circuits, its cross-sectional area shall be:
(i) calculated in accordance with Regulation 543.1.3 for the most onerous of the values of fault current and
operating time encountered in each of the various circuits, or

(ii) selected in accordance with Regulation 543.1.4 so as to correspond to the cross-sectional area of the
largest line conductor of the circuits