80A PME Fuse

[plugwash]

The total of the 'ratings' of MCBs/RCBOs in a CU will nearly always be considerably greater than the 'rating' of the main switch - but 'so what?' ! Much the same is true of the situation when the MCBs protected by an RCD 'add up' to al lot more than the 'rating' of the RCD - but, again, "so what?", given that the final circuits are never going to all be loaded to anything like the total ratings of their OPDs.

We should not design installations we anticipate will be overloaded, but we have to consider the possibility that people will use an installation in ways the designers did not anticipate. One of the functions of overcurrent protective devices is to protect the installation against overloads caused by unanticipated usage,

While it's not normally listed as such on the circuit schedule, there is effectively a "circuit" consisting of the tails, isolators, busbars RCDs etc between the main fuse and the OCPDs. I see no reason why the components in this circuit would be any less deserving of protection against overload than any other components in the installation.

Earlier editions of BS7671 required overcurrent protection for cables, but had a blind spot around switches and RCDs, the 18th edition has apparently tried to fix that, though was somewhat ham-fisted reffering to manufacturer guidance. The manufacturer guidance seems to be that the RCD rating should be based on the lower of either the upstream protective device, or total downstream proective devices.

https://www.beama.org.uk/static/uploaded/6861a2e1-d8d4-4d81-b5fa710ba60ca4a7.pdf

 

[JohnW2]

Seem to remember this was one of the things clarified by BS7671? Which means not a change, just clarified, so if you read it wrong, what one has done is non-compliant even with older editions.

What "this" are you referring to?

 

[JohnW2]

.... We do not need to add up the ratings of our fuses/breakers etc but can apply diverity on circuits dependant an maximum possible loads of what is actually connected or maximum expected loads per circuit and then sometimes apply some diversity to the whole system.

Indeed, and it's not only 'diversity'. Many circuits in domestic installations have 'design currents' (hence OPD ratings) well in excess of likely loads. As you go on to imply, now that we are in 'LED era', this is perhaps most dramatic in relation to lighting circuits - since even a 1A circuit would be more than adequate for the average domestic property. Indeed, given that there are usually at least two lighting circuits, a lot less than 1A would probably be appropriate! Less dramatic, but similar, can probably be said of sockets circuits, since it would be incredibly rare that such a circuit would be asked to supply anything like 32A in a domestic property!

.... Or we might think it more realistic to apply a sensible Watts per square Metre of floorspace approach. .... This applies in similar fashion to our maximum cut out rating and maximum consumer unit Mainswitch rating and RCD switch rating. .... Working it all out realistically then adding a reasonable "safety factor" can be a bit of a Dark Art but given that the bog standard Cooker Circuit Diversity calculation usually works very well then all methods may have their merits.

Quite so - and although I don't want to 'put words into your mouth', I think what you are really talking about is 'common sense' - which I definitely think is needed here!

Kind Regards, John

 

[phatboy]

There's obviously no electrical downside to upgrading to a 100A fuse/supply, so it really comes down to whether there are any significant financial issues. I imagine it varies between DNOs, but it seems with mine ("National Grid") that, provided their supply has adequate capacity and one's installation has 25mm² tails, they will usually upgrade to 100A for little or no charge. One can but ask!

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

 

[JohnW2]

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.

 

[ebee]

Yes they do insist on 16.0 Earthing conductor with 25mm tails even though bonding at 10.00mm is sufficient and even when their connection from the incomming earth to the earth block is not much bigger than a piece of wet string. I often think of it as an untended earth fusewire. To befair though, if you ask the bloke onsite and mention it they either do it themselves or offer to get it booked in for reneal and if it gets done it`s 16.0mm

 

[EFLImpudence]

543.1.4 and Table 54.7

 

[ericmark]

I note there are two figures given with the likes of RCDs, one in the working amps, the other the fault amps, often something like 6kA which may damage the RCD but not in a dangerous way within the time allowed.

This was seen as being the case where one had MCBs exceeding the value of the RCD and the supply fuse, as long as it was not designed to exceed the RCD rating, however we saw with the likes of heat pumps, EV charging, and immersion heaters situations where the 63 amp RCD could be overloaded for an extended time, so the rules were clarified, which is the bit I hate, if rules changed, then the installation is still compliant as it's the design date that counts. But when clarified, it means it was never permitted, so the design was in error, so not compliant and never was.

As to if designers are expected to issue a recall notice and someone should visit to correct it I don't know, when I was doing domestic we collected the consumer unit from the supplier, and we did not stipulate 63A, 80A, or 100A, so not sure how after 6 months we would know if any installations did not comply?

Until my meter was changed, I had no idea if DNO fuse was 60, 80, or 100 amps. And even then only found out because I asked. And who would have got it wrong if not fitted with a 60 amp fuse, the old Wylex fuse box had an isolator rated 60 amps, so anyone who had an old Wylex fuse box should have been on a 60 amp or less supply,

 

[JohnW2]

543.1.4 and Table 54.7

That's for protective conductors in general. However, in the case of Earthing and Main Bonding conductors with PME that seems to gets trumped by 542.31 and 544.1.1 / Table 54.8.

As I said, those requirements per BS7671 seem to differ from those which DNOs are imposing.

 

[JohnD]

I note there are two figures given with the likes of RCDs, one in the working amps, the other the fault amps, often something like 6kA which may damage the RCD but not in a dangerous was within the time allowed.

I remember bring told that devices could break an enormous current, once, but that did not mean they would be usable afterwards.

 

[ebee]

That's for protective conductors in general. However, in the case of Earthing and Main Bonding conductors with PME that seems to gets trumped by 542.31 and 544.1.1 / Table 54.8.

As I said, those requirements per BS7671 seem to differ from those which DNOs are imposing.

"

As I said, those requirements per BS7671 seem to differ from those which DNOs are imposing.. Yes indeed it seems they (or Some) Do

 

[JohnW2]

I remember bring told that devices could break an enormous current, once, but that did not mean they would be usable afterwards.

The ability to break 'enormous currents' presumably depends upon the extent of measures taken to avoid/suppress arcs?

Very many moons ago, when MCBs first started appearing in domestic installations, I was 'taught' by an 'old-boy electrician' that if a MCB operated 'in anger' (i.e. in response to an over-current) it should be replaced, because it could not be 'trusted' to work again properly in the future!

 

[EFLImpudence]

That's for protective conductors in general. However, in the case of Earthing and Main Bonding conductors with PME that seems to gets trumped by 542.31 and 544.1.1 / Table 54.8.

As I said, those requirements per BS7671 seem to differ from those which DNOs are imposing.

Table 54.8 is for bonding conductors; not earthing conductors

 

[ericmark]

I remember bring told that devices could break an enormous current, once, but that did not mean they would be usable afterwards.

which may damage the RCD

I did say that, only once have I needed to look at the let through current, where the loop impedance was too low, decided to fit a 20 amp fuse up front from memory, and found the let through current no longer mattered as the resistance of the fuse had raised the loop impedance enough to be within the 6 or 10 kA limit.

It was a battery charger for a handy used with a tower crane at T5 building, so unlikely to have the same problem with domestic.