Saturday evening discussion - Part 1 (of 1 or 2!)

[JohnW2]

It is the maximum breaking capacity when backed up by the 100A BS1361 fuse, it will take into account the maximum energy let through of the BS1361 fuse (which will probably also operate as well as the MCB if a 6 to 16kA fault occurs). The MCB will probably also be trash with this sort of fault level. It is unlikely, given cable impedances in households that these sort of fault levels will occur that often.

It still makes no sense to me. 'Let through' (I²t is not 'energy') has surely got nothing to do with the maximum breaking capacity of the MCB?

If the fuse doesn't operate first, then it clearly has no effect on what's going on at the MCB, and therefore cannot possibly affect the MCB's maximum breaking capacity. If the fuse does operate first, then there is no current for the MCB to break, so its maximum breaking capacity is moot.

What am I missing?

Kind Regards, John

 

[RF Lighting]

I would imagine it's saying it can cope with a 16kA fault without exploding. It'll probably render the breaker u/s as it's exceded the breaking capacity quite significantly. If the fault is greater than 16kA then the device may explode.

 

[Adam_151]

And Im sure we have all seen breakers which have been rendered usless by tripping on a fault level far below what they are rated to!

Bearing in mind that the 16kA conditional test puts the fault a little bit of a way along a final circuit cable and quite likely at 16ka, render the bit of T+E between the con/unit and fault usless as well (but of course that would likely end up replaced as well.

One might consider that > 6ka in domestic is most uncommon, >10ka even more uncommon. A designer may consider that 6ka MCBs, regarless of the conditional rating may not be the best way forward

 

[JohnW2]

I would imagine it's saying it can cope with a 16kA fault without exploding. It'll probably render the breaker u/s as it's exceded the breaking capacity quite significantly. If the fault is greater than 16kA then the device may explode.

That may well be true, but it doesn't make their statement "the breaking capacity of the MCB is increased to 16,000A" any more sensible, does it? 'Maximum breaking capacity' surely means what it says? Do you agree that the statement, as written, is basically nonsense?

Kind Regards, John

 

[JohnW2]

One might consider that > 6ka in domestic is most uncommon, >10ka even more uncommon. A designer may consider that 6ka MCBs, regarless of the conditional rating may not be the best way forward

Indeed. In fact, I reckon that 6kA in a domestic installation in next to impossible. Even if the fault were at the CU, that would only allow for the equivalent of about 22m of 25mm² cable from CU back to the transformer (and the feed in to most domestic installations often/usually isn't even 25mm²). As you say, there might be other reasons for wanting high-rated MCBs.

However, this discussion is not about how high fault currents may exist in domestic installations (it's not even specifically about domestic installations) - it's about the statement in MK's Product Information.

Kind Regards, John

 

[Adam_151]

Its important to note that the breaking capacity of the MK itself ins't increased to 16ka, but that the consumer unit as a whole type tested assmebly is. So if you are one of these who mixes and matches MCBs then you might be in trouble!

Its in Annex Za of BSEN 60439. Not 60898

 

[Spark123]

It still makes no sense to me. 'Let through' (I²t is not 'energy') has surely got nothing to do with the maximum breaking capacity of the MCB?

I²t is proportional to the energy let through, you are correct in that I²t isn't a direct measurement of energy.
As I said before allowing a PFC up to 16kA is to do with the energy let through of the BS1361 fuse and what the MCB can handle, not necessarily remaining serviceable afterwards. The manufacturer will have done the maths so they will know how much energy their devices can safely handle hence we don't need to.

 

[JohnW2]

Its important to note that the breaking capacity of the MK itself ins't increased to 16ka, but that the consumer unit as a whole type tested assmebly is.

Exactly, BUT ....

I am quoting from MK's Product Information for their MCBs, not CUs, and (with apologies for quoting it yet again!), what MK wrote is "the breaking capacity of the MCB is increased to 16,000A". I assume that we're just seeing badly/inorrectly written Product Information - quite possibly written by a non-technical person!

Kind Regards, John

 

[JohnW2]

As I said before allowing a PFC up to 16kA is to do with the energy let through of the BS1361 fuse and what the MCB can handle, not necessarily remaining serviceable afterwards

Totally agreed. As you say, MK may well have worked out that the let-through of the fuse enables the MCB to 'handle' 16kA (without 'exploding') - but I still do not believe that they are correct in saying (which they do, explicitly) that the breaking capacity of their MCB magically becomes 16kA in the presence of an upstream fuse - unless, of course, they have an understanding of 'breaking capacity' which is different from my (common sense) one. As I'm saying to everyone, my only quibble is with MK's words, not the practicalities of what their MCBs can 'handle'.

Kind Regards, John

 

[Spark123]

I see your point, it's just badly put into words.
Maybe it should have read "the assembly can be considered to have a maximum breaking capacity of 16kA where the upstream fuse is a 100A BS1363" or similar.

 

[JohnW2]

I see your point, it's just badly put into words. Maybe it should have read "the assembly can be considered to have a maximum breaking capacity of 16kA where the upstream fuse is a 100A BS1363" or similar.

[I take it you mean BS1361]. Yes, they could have written that - but, as I said, I don't know how appropriate that would be in the Product Information for their MCBs - which could be used in all sorts of ways - in a type-tested CU, a mix-and-match CU, some other sort of DB, a standalone enclosure or whatever - so I don't know exactly what "assembly" in your suggested wording above would refer to.

Kind Regards, John.

 

[EFLImpudence]

the breaking capacity of the MCB is increased to 16,000A". I assume that we're just seeing badly/inorrectly written Product Information - quite possibly written by a non-technical person!

Do they mean that when the stated circumstance apply/arise then the MCB breaking capacity IS increased to 16kA?

I.e. not miraculously changed but must be replaced by an MCB with that capacity.

 

[EFLImpudence]

Or to word it better -

the REQUIRED breaking capacity of the MCB is increased to 16,000A".

 

[Spark123]

Do they mean that when the stated circumstance apply/arise then the MCB breaking capacity IS increased to 16kA?

I.e. not miraculously changed but must be replaced by an MCB with that capacity.

The first part sort of - the MCB (as part of a whole type tested consumer unit) can be considered to be rated at 16KA when backed up by a BS1361 100A fuse even though the MCBs have 6000 on them.
(And yes - I meant BS1361 before John , don't think plugtop fuses come in 100A flavour )

 

[JohnW2]

(And yes - I meant BS1361 before John , don't think plugtop fuses come in 100A flavour )

... and, even if they did, they'd be BS 1362 (not BS 1363), wouldn't they?

Kind Regards, John