Generally we do not care if a device has to be replaced so long as users are not put at risk.

In practice the two ratings may the same, for example a breaker may survive and be re-used a few times at 6kA but not disconnect reliably at the next common rating of 10kA, so both would be set to 6kA.


Mike

If you go above I_cn, the breaker may not stay in one piece.


If you remain below I_cs, the breaker should remain serviceable - i.e. you fix the short circuit and reset.


If the short circuit current is between I_cs and I_cn, the breaker should remain in one piece, but the innards cannot be guaranteed to work again, so it must be replaced.


For BS 60898 breakers rated at <= 6 kA, I_cs = I_cn; and for > 6 kA and <= 10 kA, I_cs = 0.75 x I_cn.


Fairly obviously, the I_cs should be less than the PSCC, which is almost always the case for domestic supplies.

Thanks to both for detailed response.


I said this is a trivial comment.

 

it’s a comment on logic - a simpler example.


A)

if   Condition a then the bomb explodes.


B) 

if condition b then the bomb does not go bang.


Under a specific measurement - you cannot say A = B as the bomb either goes off , doesn’t go off, or you have another answer - it might go off.

Quite so.

As you mention bombs...


When fusing ordnance one defines the currents for electric fuses as


An "All fire" current  above which all devices should detonate, any that do not are dud.


A "No fire" current below which no  devices detonate,  any fuses that do should not have passed factory inspection before the thing was assembled with the main charge.


Something safely less than the No fire current is used to put the "primed" lights on  and give confidence the circuit is intact.


Something quite a bit more than the All fire current is used when the buttons are pressed.


There is indeed a range of currents, in many cases more than an order of magnitude in fact,  which you should try and avoid dwelling on, that are totally indeterminate, as not all devices have exactly the same thickness of fuse element, or the same mass of primer to heat up. If for any reason you do you supply a current in the indeterminate range you may have an unexpected event after an unknown delay.?

The penalty of getting it wrong may not be as bad with our sort of fuses and breakers , but the physics and variation in manufacture means the same ideas apply - certainly some devices will survive a current higher than the one printed on it, but how many - well who can say.

In some case we do not care. Inside a lot of domestic electronic kit, fuses are used that are thin glass walled efforts,  the mains short circuit current is likely to not just melt the wire but also destroy the glass tube. But if the outer enclosure provides shrapnel and blast containment we might not care, so long as the current is interrupted, so we can use a fuse with an apparently inadequate maximum rupture rating. (if you try and repair the thing, you need to remove the shards of glass or ceramic and so on carefully before restarting, and take great care if powering up open top )

Mike.

As you may have guessed , I worked in a large explosive factory for several years and there were a number of incidents. 

There is no faulty logic in the OSG or indeed BS 60898.


When I_cs = I_cn, it means that there is no range where an MCB would safely break a current but not remain serviceable.


It's all about design.


If the PSSC is 5 kA, you may select a 6 kA MCB.


If it is 7 kA, you may select a 10 kA MCB because I_cs = 7.5 kA.


If it is 8 kA, you need at least a 16 kA MCB because where I_cn is > 10 kA, I_cs = 0.5 x I_cn. I suspect that when you get into this territory, there will be something before the MCB.

when you get into this territory, there will be something before the MCB.

Indeed, perhaps an energy limiting "death or glory"  fuse, cunningly selected so the I2t (or if you prefer to think of that as joules per ohm) of the fuse means that the (however many milliohms) contacts of the breaker do not get hot enough  (joules) to weld even in the unlikely event of a silver stake zero ohms fault right next to the origin.

In a domestic situation the company cut-out fuse will perform this damage limiting function to a degree, as well as protecting the tails and the meter.

M.

I was thinking of the cable. Domestically, unless you live right next door to the transformer, PSSC is unlikely to exceed 6 kA. In the industrial context, I was thinking that by the time that you have distributed through a main panel and so on down to a BS 61439 board, you may dropped below 6 kA or so.

Looking at all the great replies, I think we are saying that if we we fall below 6KA then the device is safe to re-use. 


thanks to all - it was very informative for me.