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.

 

[JohnW2]

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

I'm starting to wonder whether you have understood my questions and the thinking behind them ...

... the 'customer-side' can do nothing about 'protecting' the meter or anything upstream of it. All of the final circuits in the installation should be adequately protected (against both overloads and 'faults') by the devices (MCBs or RCBOs) in the CU. That only leaves the connection between meter and the devices in the CU (i.e. 'tails' and Main Switch') and, as I mentioned (which started this bit of the discussion), DNOs will usually only upgrade their fuse to 100A if those tails are at least 25mm², since (like BS7671) they don't believe that cables any smaller than that would be adequately protected by a 100A fuse.

Hence, if the DNO fuse is 100A, the meter tails are 25mm², the Main Switch rated for100A and the rest of the installation correctly designed and installed, it would seem that absolutely everything will be adequately 'protected' (against both overload and 'faults').

That's the reason why I've been asking what you feel needs further "overload protection", given that, as above, it would seem that everything would already be appropriately 'protected'. I'm therefore wondering whether you perhaps don't actually mean 'protection' in the usual sense but that you are, rather, referring to "current limitation", which is not something we do explicitly in this country - at least, not in domestic installations?

Kind Regards, John

 

[mikehammer69]

I'm starting to wonder whether you have understood my questions and the thinking behind them ...

... the 'customer-side' can do nothing about 'protecting' the meter or anything upstream of it. All of the final circuits in the installation should be adequately protected (against both overloads and 'faults') by the devices (MCBs or RCBOs) in the CU. That only leaves the connection between meter and the devices in the CU (i.e. 'tails' and Main Switch') and, as I mentioned (which started this bit of the discussion), DNOs will usually only upgrade their fuse to 100A if those tails are at least 25mm², since (like BS7671) they don't believe that cables any smaller than that would be adequately protected by a 100A fuse.

Hence, if the DNO fuse is 100A, the meter tails are 25mm², the Main Switch rated for100A and the rest of the installation correctly designed and installed, it would seem that absolutely everything will be adequately 'protected' (against both overload and 'faults').

That's the reason why I've been asking what you feel needs further "overload protection", given that, as above, it would seem that everything would already be appropriately 'protected'. I'm therefore wondering whether you perhaps don't actually mean 'protection' in the usual sense but that you are, rather, referring to "current limitation", which is not something we do explicitly in this country - at least, not in domestic installations?

Kind Regards, John

I understand your point

If the tails and main distribution board are adequately rated then main overload protection should be safely provided by the dno fuse

My point was in the context of EVs being added to an installation causing a main overload condition at the dno fuse

A separate consumer main overload protection is better than backing up overload to the DNO fuse

 

[JohnW2]

I understand your point. If the tails and main distribution board are adequately rated then main overload protection should be safely provided by the dno fuse

Indeed. If the main switch of the DB/CU is 'rated' for 100A and the tails are 25mm², then everything would adequately protected by a 100A DNO fuse. That is theoretically not the case if the tails are only 16mm² - which is why, as I said, DNOs will usually insist on 25mm² tails before they will upgrade their fuse to 100A

My point was in the context of EVs being added to an installation causing a main overload condition at the dno fuse .... A separate consumer main overload protection is better than backing up overload to the DNO fuse

Yes, I understand what you are/were saying but, again, you are using the word 'protection' in an unusual way (assuming adequate tails and Main Switch), the only thing being arguably 'protected' being the DNO fuse (everything else being adequately protected, in the conventional sense) ... so, as I wrote, I would say that you are talking more about 'current limitation' than 'protection'

In any event, as has been said, most/all EV charging facilities monitor the total demand on the installation and adjust charging rate accordingly in order to avoid total demand exceeding the 'available supply' (i.e. the DNO fuse rating).

Kind Regards, John