Earthing conductor csa/size (oh no, not that again I hear you cry)

[Rob Smith 643]

Firstly, appologies for raking this one up again. Although not new to electrical work the earthing arrangements in some properties, including my own, still mystify me.

My home was built in 1997 by Bellway. It has a 60A service head with a 1361 fuse, is TNCS and has a PME sticker in place. The tails are 25mm and the earthing conductor is 10mm. A friend in a younger house in a neighbouring town ((different county and LABC) has the same set up but with a 100A head. His home was built by Barratt.

I have read various threads on the forum that discuss this issue including some fairly heated ones and we appear to have split camp. Some stick to the tables, some prefer the adiabatic equation, some have regard to both but err on the side of caution. And in some cases I doubt that pistols at dawn would have solved it for some parties!

But surely Bellway and Barratt are reputable builders and had their designs/plans passed by different LABC's which might indicate that the earthing arrangement is sound otherwise there are literally hundreds, nay thousands of newish homes throughout the area and possibly the country that were incorrectly wired?

I no longer have my 'red' regs but don't recall any changes with regard to earthing conductors (main) since the arrival of 'green' regs or the subsequent amendment.

Any further thoughts? :dizzy2:

 

[RISElectrical]

With regards PME adiabatic is irrelivent

 

[Rob Smith 643]

With regards PME adiabatic is irrelivent

Thats my take on it but knowing that coupled with the existing earthing arrangement is what has created the mystification. :30:

 

[oldtimer]

I would not worry when the 15th came out they wanted to earth bond everything until they got real remember there are 30-40 year old properties with 6mm main earths and nobody is beating down their door demanding it gets changed to 16mm or else

 

[Snapester]

With regards PME adiabatic is irrelivent

Why is that?

 

[IQ Electrical]

Why is that?

On a PME supply, the main protective bonding conductor CSA must be selected from table 54.8 based on the suppliers neutral conductor CSA (not the 'tails').
If the installation has extraneous-conductive parts then the earthing conductor will also be subjected to the same table regardless of adiabatic results.

 

[RISElectrical]

Yep what he said

 

[Mainman]

On a PME supply, the main protective bonding conductor CSA must be selected from table 54.8 based on the suppliers neutral conductor CSA (not the 'tails').
If the installation has extraneous-conductive parts then the earthing conductor will also be subjected to the same table regardless of adiabatic results.

I was quite happy with earthing until I opened this thread, I'm going for a lie down. Isn't the adiabatic only for the earthing conductor and the cpc? 543.1.1 states that the csa of every protective conductor other than a protective bond can either be calculated or selected from table 54.7. 54.8 is only for bonds on a PME or am I completely off the track?:behead:

 

[telectrix]

IQ's post is correct table 54.8 relates to main bonding conductors. i still can't see any reason not to calculate the size of the main earthing conductor in accordance with 543.1.3 OR 543.1.4

 

[Mainman]

Never too old to lurn sumfink!!

 

[IQ Electrical]

On a PME system, the earthing conductor is also a main protective bonding conductor and that is why we are stuck with 54.8 as a minimum

 

[Deleted member 26818]

As far as the adiabatic equation is concerned, 10mm² will be sufficient in the majority of cases for supplies up to 100A.
If the main conductors are bunched together or incorporated in a single cable, then a larger CSA may be required for 100A supplies.
In such cases it will depend on the disconnection times.
In general, domestic PME heads are rated up to 100A.
That does not necessarily mean that the main fuse is 100A

 

[Rob Smith 643]

I think I need to sit down over a few drinks with someone who has a better grasp than I, beers on me. In these threads I never know who is answering which part of the thread :rolleyes4:! Which creates further confusion for me, sadly. Can anyone offer a whole/complete answer on whether 10mm is okay under the system I outlined at the start. Love this forum :love:

 

[telectrix]

10mm is usually more than adequate for the job. it may not comply, but , then again you can still drive a 1950's car to kabul and back. it won't pass emissions tests, but it probably won't break down half way there requiring spare parts that take land rover 6 months to supply, then it's certain to be the wrong parts. think another beer is called for before i ramble on too much.

 

[Deleted member 26818]

I think I need to sit down over a few drinks with someone who has a better grasp than I, beers on me. In these threads I never know who is answering which part of the thread :rolleyes4:! Which creates further confusion for me, sadly. Can anyone offer a whole/complete answer on whether 10mm is okay under the system I outlined at the start. Love this forum :love:

Where PME conditions apply, where the supply neutral conductor is 35mm² or less, the minimum CSA for the earth conductor is 10mm².
As the supply neutrals in both cases are 25mm², then 10mm² is fine.

When applying the adiabatic equation.
Assuming that in both cases the tails are not bunched together, or incorporated in a single cable.
For a 60A BS1361 fuse, a maximum CSA of 5.16mm² is required.
For a 100A BS1361 fuse, a maximum CSA of 9.85mm² is required.
As such in both cases a CSA of 10mm² for the main earth conductor is again fine.