The thing that is odd, is that in the UK an MCB stops working when you take it out of the makers enclosure.

By all means test an assembly with many breakers in the box mounted on the maker's bus bar.

But the whole question could be resolved, I suggest, with one other cheese cloth test, where the MCB  is not in box,  but with wired leads that are long and thin, so they do not significantly cool the device. If the cloth catches fire when the the MCB breaks the maximum fault, then it cannot safely be  used in an unknown containment, as the case alone has not retained the highest temperature arc products.  All MCBs fire hot gas out (a hole in the back somewhere, never the front or sides) during such an event, but the internal arc traps should catch the hot metal that might ignite something.

Any device meeting this test is OK for any kind of containment from the perspective of ignition. The only other aspect is self heating or neighbour heating, and this is only a question of knowing the voltage drop on load or dissipation, and max permissible temperature. Both could be on the data sheet.

When used on a rigid bus bar, is it beyond the wit of man to mandate the offset from the DIN rail reference plane to the tooth, and say that if that condition is not met then only a flexible link may be used.

Mike.

The thing that is odd, is that in the UK an MCB stops working when you take it out of the makers enclosure.

By all means test an assembly with many breakers in the box mounted on the maker's bus bar.

But the whole question could be resolved, I suggest, with one other cheese cloth test, where the MCB  is not in box,  but with wired leads that are long and thin, so they do not significantly cool the device. If the cloth catches fire when the the MCB breaks the maximum fault, then it cannot safely be  used in an unknown containment, as the case alone has not retained the highest temperature arc products.  All MCBs fire hot gas out (a hole in the back somewhere, never the front or sides) during such an event, but the internal arc traps should catch the hot metal that might ignite something.

Any device meeting this test is OK for any kind of containment from the perspective of ignition. The only other aspect is self heating or neighbour heating, and this is only a question of knowing the voltage drop on load or dissipation, and max permissible temperature. Both could be on the data sheet.

When used on a rigid bus bar, is it beyond the wit of man to mandate the offset from the DIN rail reference plane to the tooth, and say that if that condition is not met then only a flexible link may be used.

Mike.

mapj1 said:

The thing that is odd, is that in the UK an MCB stops working when you take it out of the makers enclosure.

Well, there is the UK National Annex for 16 kA requirement, which comes out of ENA ER P23 ... but the value is not new, and not based around the latest version ... it's been in place for a very very long time, and we have been cautioned about changing breakers, and the conditional fault rating of the assembly, since we moved over to BS EN 60439.

I think really you'll find that's one of the key issues behind it. Many many years later, we're still sucking through our teeth at it.

True, but  that is only a formal acknowledgement of the energy let through (limiting action) of 100A company fuses.

There is the odd question of if the company fuses are supposed to respond to faults more than 3m down the wire, but we will ignore that. *
This ensures that the damage done to the MCB and any downstream wiring for very low impedance faults is limited by the fuse going first. In many ways this too is in the wrong set of regulations, as again there is also nothing magical involving different physics in the UK - a death or glory fuse up ahead of anything where otherwise the PSSC would be too high is a general technique used the world over to safely reduce the weight of cables and switchgear that would otherwise have to be oversized just on PSSC grounds.

The equivalence of I²t and j/Ω of damage energy per ohm of fault path is not well articulated, and the same concept appears in Germanic countries texts as something that might translate as restriction of 'the action integral', a turn of phrase that in English I have only seen in Lightning related documentation by DEHN in relation to melting conductors and cutting holes in steel roofs etc. perhaps revealing their original authorship. (and if lightning is not an example of a short duration high current pulse that deserves to be adiabatically considered I'm not sure what is)

Mike.

* I think there can be an element of trying to  codify laws of physics and that is not always sensible (as an example and an amusing aside the Indiana Pi Bill comes to mind as a case of historical overreach ) Just because a committee can publish advice without giving an obvious reason, it is not always wise.

mapj1 said:

True, but  that is only a formal acknowledgement of the energy let through (limiting action) of 100A company fuses.

And the fact that we use mcb's that are only in themselves rated for 6 kA prospective fault current ... absolutely fine for most installations, but the very few "near the transformer" as it were need to be covered by the test in the National Annex to BS EN 61439-3 (and BS EN 60439-3 prior to that) ...

Indeed, but the same combination  - a high breaking capacity energy limiting fuse and 6kA rated devices on supply with pre-fuse 16kA PSSC would be just as safe anywhere on the planet, not just here, and indeed is common, but more often with bottle fuses rather than house cut-outs, as those seem to be a unique UK thing.
We could just say that the 6kA only limits the fault that the breaker itself can be relied upon to disconnect, above that  you accept it might decide to  fuse solid and never work again, and relegate the ADS functions to the company fuse, the rating becomes that which ruptures the case or melts the interconnecting wires. That is what I mean when I said the annexe is probably in the wrong place - it could probably be deleted if something like the above went in BS7671 instead. As written it sounds like 'oh we  do not care about the 6ka rating and take a terrible liberty' in fact it is safe and justified, but the reasoning is not clear,  and it is not always well understood by the right group of folk, and the knock on consequences as in this thread, are many-fold.
Mike.
Edit

PS I know you probably could not say that in a standard, but you could say that where an upstream energy limiting fuse is present, the MCB PSSC rating may be ignored, so long as the fuse let-through is below the damage level of the breaker, which is presumably only a small multiple of its own maximum let-though without failure at 6kA. (else it would be rated for 10ka ;-) )

mapj1 said:

PS I know you probably could not say that in a standard, but you could say that where an upstream energy limiting fuse is present, the MCB PSSC rating may be ignored, so long as the fuse let-through is below the damage level of the breaker, which is presumably only a small multiple of its own maximum let-though without failure at 6kA. (else it would be rated for 10ka ;-) )

Well, isn't that, in a round-about way, what BS 7671 says in Regulation 536.4.201?