Main Earth Problem

[Piratepete]

Hi Guys
I'm working in a house which has 2 consumer units. One supplying the original house is by the DNO's cutout and meter through Henley blocks. Installation is TN-C-S.
The other is some way away and supplies an extension. It is fed with a 10 sqmm T &E from a 45 amp DP switch fuse by and connected to the same Henley blocks.
IMO the 4 sqmm earth in the T&E is inadequate and it should be 10 sqmm.

Assuming I'm correct, it will be a challenge to route a 10 sqmm earth all the way through the house or even around the house from CU to the MET.

A simple solution would be to make this part of the house TT (the outside wall is only 2 metres away).

Would this be permitted?

Note: Gas and water bonds currently are only connected to the original CU.

 

[Leesparkykent]

adiabatic equation?

 

[Andy78]

How have you determined that the CPC is undersized ? It works out in most situations, that's why they put that size CPC in the cable.

 

[telectrix]

work out your fault current then apply the adiabatic equation. from what you say in the OP, the bonding is alrteady in situ and i assume it;s 10mm. or could you install 10mm cpc to connect to the gas and/or water if it'ds close to CU2, thus combining cpc and bonding for CU2?

 

[Wilko]

Oops - telectrix covered my thoughts - I shoulda read it first

 

[Piratepete]

Hi Guys
Thanks for your replies.
1. This cable carries the main earth for the extension installation. As such, Table 54.7 could apply. For line conductors smaller than 16 sqmm, the protective conductor has to have at least the same cross-sectional area.

2. Is my adiabatic calculation correct? k=115. PFC at the DB is 1.3 kA. Fig 3A2(b) suggests t=0.1 for a BS3036 45 amp fuse. This gives 3.57 sqmm for the earth conductor. So 4 sqmm is ok? Though not exactly best practice?

3. The regs suggests that you can use the size determined by the adiabatic equation or the minimum value in table 54.7. The OSG only has the table, which is why I thought the earth was too small.

4. Where in the regs does it say that the earth and the main protective bonding can be combined? I could do this, but it would seem to me to be very bad practice. The stopcock is quite close!

That's all.

Pete

 

[telectrix]

4. Where in the regs does it say that the earth and the main protective bonding can be combined?

where in the regs. does it say they can't? if you think of a sub DB (e.g.to an outbuilding)fed by a 3 core SWA, the earthing and bonding conductors are combined in the 3rd core. at the end f the day. it's all just an equipotential zone.

 

[Wilko]

Hi Pirate - just my thoughts (and apologies if I'm wrong) but if this 2nd installation 'some way away' and if it has metal gas and water pipes entering the area, they should be bonded to that installation's board and not rely on the bonding done elsewhere? What do you think? Cheers, David

 

[Wilko]

I get 3.57mm as well

 

[bigspark17]

Nothing wrong with selecting from T54.7 but you cant fail a existing install without proving conductor doesnt meet adiabatic equasion. Adiabatic =3.57mm. So 4mm is adequate cpc. Bonding would need to be ran back to MET or EMT in main db01 providing earthing conductor to that db is correctly sized.

 

[Wilko]

On the changing to TT question, I would not change earthing systems within the same building. But that's just me. Back to bonding conductors - having thought 4mm OK, I read chapter 5, section 544 Protective Bonding Conductors, first para 544.1.1 saying "Except where PME conditions apply, a main protective bonding conductor shall (!!!) have a cross sectional area ... and not less than 6mm2" . Am I reading that right? Cheers

 

[bigspark17]

The 10mm t&e is a circuit so conductor is a cpc not a main earthing conductor or a bonding conductor.

 

[Midwest]

The 10mm t&e is a circuit so conductor is a cpc not a main earthing conductor or a bonding conductor.

Is it not actually a distribution circuit?

 

[acvc]

What is the PFC at the sub DB?

I agree about not combining TT and TN in the same dwelling.

 

[westward10]

If the cpc in that 10.0 T&E is to support main protective bonds then it is undersized. You will either have to put in place a TT earthing system or install a separate 10.0 cpc. GN8 figure 5.16 shows an example of this.

 

[westward10]

If bonds are not an issue there is nowt wrong with a 4.0 cpc.

 

[Piratepete]

Thanks Guys for all your replies and discussion. I'm going with the adiabatic. It's not being used to support bonding. That has been done correctly at DB1.
Combined earth/bonding ok provided cable size is adequate. I did have a chat with the NICEIC support guy as this raised some issues I hadn't come across before. No probs.

TT and TNCS in same building - possible but bad practice.

 

[Piratepete]

The 10mm t&e is a circuit so conductor is a cpc not a main earthing conductor or a bonding conductor.

f the supply to DB2 had been 2 x 10mm single cores and a seperate earth, would you say the same?

 

[Piratepete]

On the changing to TT question, I would not change earthing systems within the same building. But that's just me. Back to bonding conductors - having thought 4mm OK, I read chapter 5, section 544 Protective Bonding Conductors, first para 544.1.1 saying "Except where PME conditions apply, a main protective bonding conductor shall (!!!) have a cross sectional area ... and not less than 6mm2" . Am I reading that right? Cheers

Correct, but 10 mm bonding already in place to DB1