80A PME Fuse

[JohnW2]

My I remind us that when some rule or reg or law states some particular thing - in this case being the csa of a particular conductor - it does not invarably mean you may not use something better by some reason ...

Sure - the sort of regs we're talking about always dictate minimum requirements, and there is rarely, if ever, anything wrong with 'exceeding' those minimum requirements.

However, if, BS7671 says that a certain conductor must have a minimum CSA of, say, 16mm², I don't see in what sense a DNO could argue that 10mm was 'better' (or even 'adequate') Am I missing something?

 

[ebee]

No John I do not think you are missing something but I do think some "rules" could be better made

 

[mikehammer69]

Regarding upgrading 80-100a fuses, in SSE DNO areas at least, 100a cutout fuses are no longer permitted, if found they have to be changed to 80a on domestic installations. This is to protect the metering and customers equipment from overload as 100a fuses can withstand much higher load than 100a for a long time which could lead to damage/fire. This came about because of additional loads with all the EV and heat pumps being put in these days.
An 80a fuse will blow before things get dangerously overloaded.

UK should fit separate consumer main overload protection at the DNO fuse location

DNO fuse for disconnection and short circuit protection only

 

[EFLImpudence]

I am still confused.

Right, so I had exactly the same thoughts over a decade ago

Did you?

For example:
Adiabatic calculation, 13mm²
Table 54.7, 16mm²
Table 54.8, 10mm²

When you write:

For non-PME, the minimum CSA of Earthing Conductor is given by (adiabatic calculation OR Table 54.7)
For PME, the minimum CSA of Earthing Conductor is given by the higher of (adiabatic calculation OR Table 54.7) AND (the CSA required for a PME main bonding conductor as given in 54.8 ).

Did/does that not mean that in both cases the CSA of EC shall be 16mm²?

 

[JohnW2]

I am still confused. ....
For example: Adiabatic calculation, 13mm² ... Table 54.7, 16mm² .... Table 54.8, 10mm²
When you write:
For non-PME, the minimum CSA of Earthing Conductor is given by (adiabatic calculation OR Table 54.7)
For PME, the minimum CSA of Earthing Conductor is given by the higher of (adiabatic calculation OR Table 54.7) AND (the CSA required for a PME main bonding conductor as given in 54.8 ).
Did/does that not mean that in both cases the CSA of EC shall be 16mm²?

Yes, with those figures it would mean that.
However, Table 54.7 is only relevant if one does not want to undertake the adiabatic calculation, so one can ignore it if, as in your example, the calculation has been done. However (2), since your adiabatic calculation has indicated the need for at least 13mm², in practice that obviously means 16mm² cable.

However (3), if your adiabatic calculation had resulted in a figure of, say, 9mm², then 10mm² cable presumably would (per Table 54.8) be adequate for an Earthing conductor, regardless of (the irrelevant) Table 54.7

 

[JohnW2]

UK should fit separate consumer main overload protection at the DNO fuse location .... DNO fuse for disconnection and short circuit protection only

"Overload protection" of what?

 

[EFLImpudence]

Yes, with those figures it would mean that.
However, Table 54.7 is only relevant if one does not want to undertake the adiabatic calculation, so one can ignore it if, as in your example, the calculation has been done.

Agreed.

However (2), since your adiabatic calculation has indicated the need for at least 13mm², in practice that obviously means 16mm² cable.

Agreed.

However (3), if your adiabatic calculation had resulted in a figure of, say, 9mm², then 10mm² cable presumably would (per Table 54.8) be adequate for an Earthing conductor, regardless of (the irrelevant) Table 54.7

Agreed.

BUT -

this all started because you said the Earthing conductor could be 10mm² with 25mm² tails.

And a 16mm earth, they were most concerned about that when ours was upgraded!

That presumably is a rule of their own?

As far as BS7671 is concerned, even with a 'PME' (TN-C-S) supply, Earthing and Main Bonding conductors only have to be greater than 10mm² if the incoming PEN is bigger than 35mm² - which I imagine is very rare in a domestic installation.

So, which is it?

 

[JohnW2]

Agreed. ... Agreed. ... Agreed.

Sounds promising

BUT - this all started because you said the Earthing conductor could be 10mm² with 25mm² tails. ... So, which is it?

No it didn't. I said nothing about tails, 25mm² or otherwise. My comment (which you correctly quote) was specifically (and only) in response to phatboy's comment that his DNO's greatest concern was that he should have a 16mm 'earth'.

In any event,m what have 'tails' got to do with this part of the discussion?

 

[EFLImpudence]

No it didn't. I said nothing about tails, 25mm² or otherwise. My comment (which you correctly quote) was specifically (and only) in response to phatboy's comment that his DNO's greatest concern was that he should have a 16mm 'earth'.

Yes, after someone (Chivers?) was wanting upgrading to 100A.

In any event,m what have 'tails' got to do with this part of the discussion?

Because that is why the DNO insisted on a 16mm² earthing conductor.

 

[mikehammer69]

"Overload protection" of what?

Installation overload

There's no good reason to back up main overload protection to the DNO fuse

 

[JohnW2]

Yes, after someone (Chivers?) was wanting upgrading to 100A.

Indeed - but what phatboy was talking about, and I was responding to, was that when he had a 'supply upgrade' his DNO was "most concerned" that there should be a 16mm 'earth'.

Because that is why the DNO insisted on a 16mm² earthing conductor.

Surely (in their mind - but see **) because they were upgrading the supply (fuse),m rather than anything directly to do with 'tail size' ?

What I did say was that if an adiabatic calculation indicates that a CSA ≤10mm² is adequate, then the Earthing conductor could be 10mm² - since that is what the regs appear to 'actually say'.

[** what I don't understand is why upgrading the supply (cutout fuse rating) should result in any change in requirement for the size of the Earthing conductor. As above, the 'actually required' CSA of EC effectively depends upon the adiabatic calculation - which, being based only of the PFC, does not change when the DNO fuse is changed ]

 

[JohnW2]

Installation overload

What do you mean by that?

There's no good reason to back up main overload protection to the DNO fuse

I don't really understand that, either.