Best solution for primary distribution in residential property

I also wonder how they would handle sitting at full load for 12 plus hours with a couple of cars charging.

KMF type switchfuses have been used for donkey's years to supply complete installations - including some with night storage heaters, which are probably as bad if not worse than EVs -  without any problems that I have heard of.

In some ways using mcb's has it's appeal because presumably they won't generate as much heat when running close to limits. If a C curve was used what's the chance of nuisance tripping.

MCBs have in-built thermal elements (in effect a small heater wrapped around a bimetallic strip) - so certainly not free of the self-heating problems. I'd have to check some manufacturer's data for actual figures, but I doubt it's massively better than a good HBC fuse.

The curve type (B,C,D) doesn't make much difference at all to the overload behaviour of an MCB - the thermal response is normally the same - the difference is just the point where the 'instantaneous' magnetic trip takes over. E.g.

   - Andy.

...just to add, I've done a little research -

Lawson say their 63A BS 88-3 (BS 1361 as was) fuses dissipate between 4 and 5W (https://www.lawsonfuses.com/app/uploads/2021/09/ME-MF-Datasheets.pdf)

while Schneider say their iC60N 63A MCB dissipates 5.6W per pole (https://ckm-content.se.com/ckmContent/sfc/servlet.shepherd/document/download/0691H00000FKkQrQAL)

That won't directly tell you how the terminal temperatures will compare as that'll depend on how well the devices can loose heat as well, but they do seem to be in the same ball park.

Other manufacturers and ranges may differ of course - these were just the first I found through Google.

   - Andy.

Andy

Thanks for all the useful information. I am learning quite a bit from this.

My thinking

The curve type (B,C,D) doesn't make much difference at all to the overload behaviour of an MCB - the thermal response is normally the same - the difference is just the point where the 'instantaneous' magnetic trip takes over. E.g.

The circuit will be supplying 2 ev chargers, one with dynamic load limiting plus garage sockets and lighting. Most of the time it would be 2 chargers, set total load limit to 60amps on circuit and 80amps on house. It's actually very unlikely that there will be a long term overload. As you say if there is a short circuit then it could take out the final circuit and the sub main circuit breaker. Agreed a fuse on the submain would prevent this, but loosing the garage feed isn't a big deal and a circuit breaker can be easily reset.

I just took a look at heat dissipation for Hager file:///C:/Users/alanb/Downloads/TECHINFO_MCBS.PDF. It's actually higher at 6.4 watts for 63 amp.

I had assumed that as bi metallic strips bend in proportion to temperature they would run at lower temperatures. But I guess not and they would have far less insulation and more dissipation than a fuse.

I now have a full understanding of why fuses are a good idea in the application. But also an appreciation that plastic enclosures for multiple fuses or circuit breakers with high ratings is probably a bad idea. Probably explains why I can't find a plastic enclosure with bus bars capable of taking 16mm cable.

Given metal enclosures are a bad idea due to open pen issues I am going to need to go down the path of individual fused switches using something like this. With everything contained in a meter box providing plenty of space for heat dissipation.

Still going to be interesting what the load shedding plan is going to be as I think it will turn everything on it's head again.

But this has still been a very useful learning exercise for me.