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?

 

[Richard Burns]

When I had my assessment I was asked what the minimum size was for an earthing conductor (and no clarification on type of earthing was forthcoming) and the assessor was non plussed and confused when I said 2.5mm².
However this was the correct answer; as far as I am aware the absolute minimum permitted for an earthing conductor is 2.5mm². This would necessitate the conditions referred to above: A TT system where the earth rod current will not overload the conductor and the conductor is copper, mechanically protected and sheathed.
The discussion then went on to say that they wanted to encourage installers to use 16mm² in all cases as it was safer and accounted for changes that may be made to the installation, which, frankly, was utter rubbish.

 

[Ian1981]

When I had my assessment I was asked what the minimum size was for an earthing conductor (and no clarification on type of earthing was forthcoming) and the assessor was non plussed and confused when I said 2.5mm².
However this was the correct answer; as far as I am aware the absolute minimum permitted for an earthing conductor is 2.5mm². This would necessitate the conditions referred to above: A TT system where the earth rod current will not overload the conductor and the conductor is copper, mechanically protected and sheathed.
The discussion then went on to say that they wanted to encourage installers to use 16mm² in all cases as it was safer and accounted for changes that may be made to the installation, which, frankly, was utter rubbish.

How can they even push this rubbish out to contractors is beyond me.
Do they work to a different standard?

 

[Wilko]

I'm listening to some nice music here (Alberto Guirioli) and reading post #16 just made me laugh with joy. Bless you Richard Burns

 

[davesparks]

The discussion then went on to say that they wanted to encourage installers to use 16mm² in all cases as it was safer and accounted for changes that may be made to the installation, which, frankly, was utter rubbish.

This kind of one size fits all, paint by numbers, no thinking approach just can’t work for electrical installations.
It’s going to lead to customers being ripped off for unnecessary ‘upgrades’ and occasionally could lead to undersized protective conductors.

 

[Pete999]

Was the assessor a Electrical Trainee type assessor, who could only look at diagrams and take

How can they even push this rubbish out to contractors is beyond me.
Do they work to a different standard?

As long as you don't question what they say, they will, get them to prove what they say, it may take some time but you may be surprised with the results, I'm waiting for all the Dumb's, Disagrees or whatever frankly say what you like I'm not bothered, far to long in the tooth to worry about peoples thoughts, until someone stands up to the BS they tell they will carry on taking your hard earned.

 

[Octopus]

Most of have 10mm and 16 mm to hand .... 10 mm nearly every time for me

 

[telectrix]

as a dedicated scouser, the min size should be 50mm, bare copper preferred.anything less just aint worth taking to the weigh-in.

 

[Gavin John Hyde]

I see you are with Stroma @happyhippydad I havent had this conversation with the local NICEIC assessor, but when ever I question what the assessors or other such folk say, I say can you show me where that is in the books so I can read up on it later. They quickly move on! Similar things happens with building control poking there noses in when doing visits.

 

[Deleted member 26818]

Yes, minimu CSA is 2.5mm2 if provided with mechanical protection. In other words, a sheathed single.

 

[Deleted account]

I said I'd be back with reg no

Yet again a contradiction.

On site guide table 4.4(iii) states the 4mm

Under neath in notes the contradiction :
The main protective bonding conductors shall have a cross sectional area not less than half that required for the earthing conductor and not less than 6mm.

So the main protection bonding conductors have to be min6mm and its ok for earthing conductor to be 4mm or even 2.5mm.
Shirley the earthing conductor should have the path of least resistance.

Have I interpreted that correctly?
I hope not.

 

[D Skelton]

Yes Pete, you have.

The earthing conductor and bonding conductor serve completely different purposes.

The former is there to carry current and is sized accordingly, the latter is there to equalise potential and is sized accordingly.

It's perfectly reasonable to expect that on many TT installations the bonding conductor/s will be larger than the main earthing conductor.

 

[Deleted member 26818]

Yes that is correct.
The CSA of the earthing conductor with both TN-S and TT can be less than that of the bonding conductors.

 

[Octopus]

Yes that is correct.
The CSA of the earthing conductor with both TN-S and TT can be less than that of the bonding conductors.

Yup - shame the muppets who are assessing people don't seem to understand this

I blame the pretty pictures in the front of the OSG

 

[Deleted member 26818]

Yup - shame the muppets who are assessing people don't seem to understand this

I blame the pretty pictures in the front of the OSG

There was one edition of the OSG which said the minimum CSA for the earth in TN-C-S was 6mm2.
I think line and neutral were listed as 25mm2, so it could have been a mis-print for 16?

 

[Deleted account]

I have got to admit I didn't know this. Though with the explanations given I totally understand why.

I don't know why I didn't know it.

Anyone else out there didn't know it also? Go on admit it.

Or is it just me and the assessors?

Thanks