this makes me wonder why they mark them as 6KA at all?

Also the same MCBs will be sold across CENELEC land - only the UK has this conditional 16kA rating. That's down to our peculiar supply system - if you want a 22kW supply in the rest of Europe you'd usually get 3-phase 32A on much thinner supply conductors, and consequently lower fault currents.

   - Andy.

Newfutile said:

this makes me wonder why they mark them as 6KA at all?

The breaker isn't capable of breaking 16kA.  The fully assembled consumer unit is capable of surviving 16kA for long enough for the service fuse to blow.  But it's the service fuse that breaks the current in the end, not the 6kA-rated breaker.

Indeed, basically the are saying that the  breaker will not be damaged by the maximum energy that a 100 BS1361 or BS88-1 fuse might let through while breaking a fault current of up to 16kA PSSC, and also for faults of 6kA or less, it will do the disconnection itself safely, at least once, without relying on any upstream device to actually interrupt the fault current.

Despite statements to the contrary, this has very little to do with makers boxes, use of bus bar versus cable or anything else about the breaker really, and  has far more to do with the let-through energy limiting properties of the typical UK service fuse.

Mike.

mapj1 said:

has far more to do with the let-through energy limiting properties of the typical UK service fuse.

... and the bit about "has to be in the correct manufacturer's box it's been tested in" of course relating to the way the product has been tested (or, in the case it's in another box, not tested) for the UK-specific market conditions.

Arguably we should also be looking at the upstream fuse and the compliance or not of its installation with the test assumptions with the same or more perhaps scrutiny - however this is usually the DNOs and therefore considered to be off limits.

Mike.