Understanding minimum CPC requirements?

[HappyHippyDad]

My assessor said that the minimum CSA for a Main earthing conductor for a TT was 16mm. I questioned this as I thought it was 4mm.

Regulation 542.3.1 states every earthing conductor shall comply with section 543.

Section 543 refers mainly to the adiabatic (reg 543.1.3) or reg 543.1.4 (table 54.7), so if the CPC meets the adiabatic then its ok.

The on site guide even has a Table ( 4.4(ii) ) on page 41 (green copy) showing that 4mm is acceptable for a TT. I'm assuming this is from 543.1.1.

The assessors justification for the 16mm on a TT was that there is a diagram in OSG showing the CPC on a TT with 16mm written next to it (see page 16, fig 2.1(iii) - green OSG). Also the Electricians guide to the building regs - reprinted 2012 version (Table 3.3.2b - page 40) clearly shows that a 16mm CPC is required for a TT (assuming 25mm tails)??

How do you guys read the regs with regards minimum size CPC for a TT?

Also, just as an aside, why do we always have to have 16mm CPC for PME or TNS? If the regs say we can use EITHER reg 543.1.3 OR 543.1.4 why cant we just use the adiabatic (reg 543.1.3) and perhaps use a 6mm if it meets the reg?

 

[davesparks]

The pictures in the OSG etc are supposed to represent a typical domestic or small commercial arrangement, they don’t set out the minimum cable sizes.
My first thought when someone says it must be x size because that’s whats in the on-site guide pictures is ‘what if it needs to be bigger than the picture shows?’

PME is a special case because of the bonding requirements and increased risks. The bonding and earthing conductor need to be large enough to cope with the theoretical situation where a bonded extraneous part has such a low resistance to true earth that a large proportion of the neutral current flows through it rather than through the combined neutral and earth of the service cable.

 

[Deleted account]

Yes assessor is partly correct, but it's never as simple as that.
If you can prove cross sectional area is acceptable with adiabatic equation then it's fine.

Also guidance states if the existing is 6 mm or greater it can stay as is, if less up grade to the full 16 mm.

I know your going to ask where is this guidance I am talking about. To be honest I cannot remember. When get home I will look it up and report back.

As for why it is 16 mm. No idea.

 

[Octopus]

The assessor is wrong for the reasons stated above

 

[mhar]

16mm on pme is a dno requirement. Part of your supply agreement (along with 10mm bonding conductors). Nothing to do with 7671

 

[Octopus]

16mm on pme is a dno requirement. Part of your supply agreement (along with 10mm bonding conductors)

The op states TT system .... do keep up

 

[mhar]

The op states TT system .... do keep up

Oops!

 

[Strima]

Assessor is wrong. He can quote guides etc but he needs to back it up with a reg number.

 

[Dillb]

Seems more and more assesors are getting things wrong lately? Does this mean the scams are now employing the Electrical Trainee they seem to like.

 

[Wilko]

I've had the same thing from an Assessor. I'd used 10mm for rod and bonding and he didn't like it. I asked him how much current was going to go down a rod with 50 Ohms Ra and he moved swiftly on . Perhaps it's lack of familiarity/ experience with TT ?

 

[telectrix]

from the green osg.....

" for TT systems protected by an RCD with an earth electrode resistance > 1 ohm, EC size need not exceed 2.5mm if protected against corrosion by a sheath and also protected against mechanical damage "

seems clear to me.

 

[D Skelton]

16mm minimum if buried and sheathed but not mechanically protected.

542.3.1 states it must comply with section 543.

543 tells you what you can use as a protective conductor, how to preserve its continuity and as per sub section 543.1 how to size it, either by selection as per table 54.7 or by calculation using an adiabatic equation.

Assuming we're talking about a bog standard domestic TT here, 2.5mm is normally more than adequate so long as it's buried in some galv tube. Note the two key words there however; 'assuming' and 'normally'.

 

[westward10]

TT the earthing conductor often only serves the purpose of earthing the electrode, fault currents often find another path out.

 

[Deleted account]

Assuming we're talking about a bog standard domestic TT here, 2.5mm is normally more than adequate so long as it's buried in some galv tube. Note the two key words there however; 'assuming' and 'normally'.

I would nearly always upgrade to 16mm as it is good practice, but the few times I have done the adiabatic out of curiosity I agree with you Skelton, I often get results Between 1.3mm lowest and 2.9mm.

 

[davesparks]

I would nearly always upgrade to 16mm as it is good practice, but the few times I have done the adiabatic out of curiosity I agree with you Skelton, I often get results Between 1.3mm lowest and 2.9mm.

Why is it good practice? And why would you say it is an upgrade?

Removing a cable which is a suitable size and replacing it with an oversized one doesn’t sound like an upgrade to me, more like unnecessary work.