Upstream protection for short circuit current

jbrameld said:

This of course is not a simple matter as the product is CE/UKCA marked and such a modification would likely require a re-certification by the manufacturer with significant cost and time impacts.

Why do you need a CE marking? My understanding is that a CE marking is required to be applied by the manufacturer when selling a product within the EU/UK. I'm not aware of any requirement for the user to install only CE marked products - although I'm happy to be corrected.

mapj1 said:

Yes, bur given its a steel box, one can very quickly set a very high bar, to the total liberated energy permissible, as all that is required is to contain the arc so the flash does not set fire to the curtains, and to catch all the flying bits  - nothing has to survive to work again, as by then it is a repair job anyway.

It may be informative to consider that more in the manner of a blast calculation.

That is up to the manufacturer to consider that, it's outside the scope of BS 7671.

This is the rub for the installer.

mapj1 said:

Unlike much European data, they give curves that go far enough into overload to really show the time limiting of the mechanism. For the 'C' type, a factor of 20 in current gets you from about 10msec to about 5msec.

The I²t curves will usually be provided separately for discrete ranges of prospective fault current.

mapj1 said:

"max back-up fuse 125A gG" so there is already a reliance on upstream limiting at the 6ka level.

But that's the breaker, not the dimmer ... therefore, if it were that simple, the dimmer manufacturer would quote an I_cc value ... unless they have not considered it? The breaker in the dimmer might be able to take it, but can the rest of the dimmer?

In this particular case, unless the assembly (dimmer) manufacturer is willing to declare an I_cc value, along with an appropriate protective device upstream, I'm not sure this goes anywhere sensible, as far as BS 7671 goes?

The pertinent issue being, the device that's driving the limit of prospective fault current is an assembly outside the scope of BS 7671, so without the manufacturer's agreement, or a consultant (or designer) taking responsibility for it, there doesn't appear to be a simple mechanism for conformity?

The pertinent issue being, the device that's driving the limit of prospective fault current is an assembly outside the scope of BS 7671, so without the manufacturer's agreement, or a consultant (or designer) taking responsibility for it, there doesn't appear to be a simple mechanism for conformity?

Indeed not - but actually this is a very silly situation, as the person who knows most about the particular situation and condition of use actually is the installer, as they are the only one actually there. 

My personal feel is that there is almost certainly not any really dangerous problem , but there is certainly an admin  one !

Also it is a bit naughty of Chint / Noark to declare their devices as 6kA and then hide the equivalent  of " but only when backed up by 125Gg or smaller fuse" about 2/3 of the way down the datasheet !!!

To me that  is an admission that even if the supply pssc had measured as 5.9 kA, an upstream fuse is required somewhere in any case, to operate and give non destructive coverage of the full range of fault currents. I presume without it the breaker de-latches but is then damaged beyond further use. I'd like to hope that flames don't shoot out the front but you never know.

And that small but key detail about the essential fuse really ought to be copied into in the dimmer makers instructions too.

Mike

mapj1 said:

And that small but detail about the essential fuse really ought to be copied into in the dimmer makers instructions too.

Yes, I think that is a pertinent point.

Yes - this comes down to who will sign it off from a design perspective, and I agree that to me this is more of a theoretical and admin problem than a real danger, but finding a full documented/calculable trail to compliance is the challenge here.

Is one interpretation of the max back-up fuse statement on the data sheet that there must be an upstream protective device that has no greater energy let-through than a 125A Gg fuse, as of course in many instances you may not see a fuse until you get as far as the supplier's intake but there could be a series of MCCBs, MCB,s or other devices in a typical installation.

That Gg fuse let-through looks to be around 1.7 x 10⁵ A²s.  The let-through of the currently specified upstream MCB on a 9kA fault current is 1.2 x 10⁵ A²s so would that be compatible (purely from the perspective of the MCBs - we'd still need something from the manufacturer about the rest of the dimmer though - but it is fundamentally a metal box?

Surely the Icn of a EN 60898-1 circuit-breaker is not reliant on any upstream current limiting device?

Or does that mean that the 125A gG fuse will protect the breaker when the fuse is supplied from an infinite bus?

Tim Boler said:

Surely the Icn of a EN 60898-1 circuit-breaker is not reliant on any upstream current limiting device?

Or does that mean that the 125A gG fuse will protect the breaker when the fuse is supplied from an infinite bus?

In this case, however, the assembly manufacturer has provided an Icn ... if they intended a specific upstream protective device, they ought to have used Icc (unless I've missed something)?

If I'm reading this correctly then the I2t value for gG fuses flattens out at high prospective fault currents with a maximum of 140 A2s for a 125A fuse.

So I might interpret the manual of the Noark MCBs to mean:

Icn = 6kA

Icc = Infinity kA (in practice limited by the rated breaking capacity of the fuse)

I think it's 140,000 A²s but your observation on the flattening out of the energy let-through is correct.

The I_cc statement made could be correct, but we would really need that set out by the manufacturer to be able to rely on it.

It is an unusual line for an MCB data sheet  certainly, and while I've seen it in other Chint group derived kit, in particular their half width (9mm) MCBs its not seen in fancier brands like Hagar Schneider etc.

The "CN" subscript is the fault current at which under the stated test conditions, (so only with that fuse I presume) the breaker will break, but it may never work properly again. Beyond that it is not guaranteed to break safely  or indeed to break at all so it may all just blow up.. Usually they don't however ;-)

Given those breakers, I'd be reserving a space on the wall for some local 80A 'death or glory' fuses in the line myself.

Mike

It is an unusual line for an MCB data sheet  certainly, and while I've seen it in other Chint group derived kit, in particular their half width (9mm) MCBs its not seen in fancier brands like Hagar Schneider etc.

The "CN" subscript is the fault current at which under the stated test conditions, (so only with that fuse I presume) the breaker will break, but it may never work properly again. Beyond that it is not guaranteed to break safely  or indeed to break at all so it may all just blow up.. Usually they don't however ;-)

Given those breakers, I'd be reserving a space on the wall for some local 80A 'death or glory' fuses in the line myself.

Mike