Unfused spur.

In this 1/2 cycle, the energy let through is the RMS current squared x 10 ms. A 6kA breaker then has 6kA squared x 10 ms which is 360,000 A²seconds. This number is not that given by the manufacturer, so what is wrong? They say typically 45,000A² seconds. The only variable to be incorrect is the trip time,


Quite so - and the contacts start to open much faster than 10ms, start in quite a bit less than 1ms in fact- the 10ms will be the total extinction of the arc on the zero crossing, and compared to the PSSC, quite a modest current flow

As soon as the contacts have opened even a little bit (and the same applies to blowing a fuse,more or less) an arc is drawn. This volume of gas and ionic plasma (atoms with electrons torn off and free electrons) is very far from a zero resistance - which is both good and bad - good in that the fault current is now quite a bit more limited if rather uncertain in value, bad in that all that energy is heating a very small and angry volume, so angry in fact that more atoms are ionised by impact, and the cloud of plasma grows rapidly - the plasma volume grows more or less at the speed of sound (not so surprising as that too is determined by the speed of atoms bumping into each other.)

This can be seen as a series resistance whose value would fall with time if nothing moved as more plasma is made (the number of atoms ionised is proportional to the energy input, but that is like filling a leaky bath, as there is also  the recombination time, which is when the atoms and the free electrons getting back together again, giving off some light, one photon of light or heat per electron that clicks back home into its orbit .. ? well, very roughly  )

But the contacts have not stopped moving (or the fuse wire burning back) and the aim is to stretch that thread of plasma, driving the voltage drop up. and ideally 'quench' it by letting it hit some large metallic mass that absorbs some of the energy as heat (and may get a bit cooked in the process - the impact damage from an ion beam leaves the surface looking like the moon at microscopic scale) If the plasma can be stretched faster than it can grow (and the rate of growth is nothing more than the available energy as V*I) the resistance will not fall, and the current stays high-ish but acceptable until the next zero-crossing, when it goes out (not an option for DC, so bigger gaps, maybe supersonic contact speeds,  and more arc catching metal work...)

Really big contactors, the ones with a winding handle or a motor for a firing spring that makes your toes curl to think of the stored energy, often have 2 sets of contacts in parallel, a clean set that open and shut while the other contacts are still connected, that never get arc damage, and a smaller working set of contacts that suffer all the flash and pop, but mean that the clean set only ever open and close with very small contact voltages.

Other tricks include deflecting the ion beam of the plasma with magnetic  fields  so that it burns some less important part of the mechanism.


There is a lot to it hidden in an innocent rectangular box.

M.

In this 1/2 cycle, the energy let through is the RMS current squared x 10 ms. A 6kA breaker then has 6kA squared x 10 ms which is 360,000 A²seconds. This number is not that given by the manufacturer, so what is wrong? They say typically 45,000A² seconds. The only variable to be incorrect is the trip time,


Quite so - and the contacts start to open much faster than 10ms, start in quite a bit less than 1ms in fact- the 10ms will be the total extinction of the arc on the zero crossing, and compared to the PSSC, quite a modest current flow

As soon as the contacts have opened even a little bit (and the same applies to blowing a fuse,more or less) an arc is drawn. This volume of gas and ionic plasma (atoms with electrons torn off and free electrons) is very far from a zero resistance - which is both good and bad - good in that the fault current is now quite a bit more limited if rather uncertain in value, bad in that all that energy is heating a very small and angry volume, so angry in fact that more atoms are ionised by impact, and the cloud of plasma grows rapidly - the plasma volume grows more or less at the speed of sound (not so surprising as that too is determined by the speed of atoms bumping into each other.)

This can be seen as a series resistance whose value would fall with time if nothing moved as more plasma is made (the number of atoms ionised is proportional to the energy input, but that is like filling a leaky bath, as there is also  the recombination time, which is when the atoms and the free electrons getting back together again, giving off some light, one photon of light or heat per electron that clicks back home into its orbit .. ? well, very roughly  )

But the contacts have not stopped moving (or the fuse wire burning back) and the aim is to stretch that thread of plasma, driving the voltage drop up. and ideally 'quench' it by letting it hit some large metallic mass that absorbs some of the energy as heat (and may get a bit cooked in the process - the impact damage from an ion beam leaves the surface looking like the moon at microscopic scale) If the plasma can be stretched faster than it can grow (and the rate of growth is nothing more than the available energy as V*I) the resistance will not fall, and the current stays high-ish but acceptable until the next zero-crossing, when it goes out (not an option for DC, so bigger gaps, maybe supersonic contact speeds,  and more arc catching metal work...)

Really big contactors, the ones with a winding handle or a motor for a firing spring that makes your toes curl to think of the stored energy, often have 2 sets of contacts in parallel, a clean set that open and shut while the other contacts are still connected, that never get arc damage, and a smaller working set of contacts that suffer all the flash and pop, but mean that the clean set only ever open and close with very small contact voltages.

Other tricks include deflecting the ion beam of the plasma with magnetic  fields  so that it burns some less important part of the mechanism.


There is a lot to it hidden in an innocent rectangular box.

M.