Good morning all.
A bit of along shot this one.
Would anyone out there have data on - Maximum permitted Zs values for MCCBs & MCACBs that were fitted in ECC (Electric Construction Company) cubicle boards in the early 1960s?
Many thanks in advance,
CPC
Indeed, and a result very close to the pass/fail boundary should trigger some further consideration - with fuses and the thermal part of breakers, the world was simpler, as when checking for a 'must blow in 5 seconds' limit, a marginal answer just meant it would blow anywhere between perhaps 4 and 6 seconds instead - which is usually within the experimental uncertainly of what matters anyway, as the 5 second figure is itself plucked from the air of convenient round numbers. And the fast blow with fault currents at ten times the rating would vary from 0.2 secs to 0.5, again not perhaps so big a deal. And in an era of re-wireable fuses where so much depends on the tension of the wire, if it is cooled by rubbing the ceramic guides and so on, there was so much slop in the figures that no one would ever know. It just had to be sized well enough to work every time and fuses to blow before cables melt.
In the same way, exceeding the PSSC was less of an issue - with no contacts to weld, the fusewire always goes open circuit, what varies is if you are just changing the wire, or also sweeping up bits of shattered ceramic out of the holders with a dustpan and brush...
With a breaker where one is waiting for the near instant magnetic part to operate, except for a few cunning but very expensive designs with hydraulic damping, the magnetic part either fires or it doesn't, so being over the line in the wrong direction could mean the near-instant trip becomes waiting for the thermal part to wake up.
It is fair to say that it took until the 1990s for regulations and testing practice to catch up.
Mike
PS
Modern breakers are not actually tested in a way that plots a curve either - its actually a very awkward test to do during manufacture and repeated operation tends to 'life' the mechanism, so only a small sample of go-no-go cases on either side of the lines are tested to give reasonable confidence that each one is good. Samples from the production line may be taken out and tested to death, but not very often.
The curves only represent the upper and lower limits of accepted performance but their shape is theoretical and based on the likely physics of operation of a near adiabatic operation from the thermal part when faster than a few seconds, and a vertical take off for the magnetic part.
If you were to test a real breaker and plot the breaking time results on top of the curves you should expect it to meander between the limits, rather than to follow the curve shape exactly.
The only thing that is sure is that any test to the left of the left limit should be a 'no-go' and any test to the right of the right limit should be a 'go'. Between the lines, well, it may or may not operate, and somewhere in that grey zone it will make the transition from one to the other.
Mike.
Indeed, and a result very close to the pass/fail boundary should trigger some further consideration - with fuses and the thermal part of breakers, the world was simpler, as when checking for a 'must blow in 5 seconds' limit, a marginal answer just meant it would blow anywhere between perhaps 4 and 6 seconds instead - which is usually within the experimental uncertainly of what matters anyway, as the 5 second figure is itself plucked from the air of convenient round numbers. And the fast blow with fault currents at ten times the rating would vary from 0.2 secs to 0.5, again not perhaps so big a deal. And in an era of re-wireable fuses where so much depends on the tension of the wire, if it is cooled by rubbing the ceramic guides and so on, there was so much slop in the figures that no one would ever know. It just had to be sized well enough to work every time and fuses to blow before cables melt.
In the same way, exceeding the PSSC was less of an issue - with no contacts to weld, the fusewire always goes open circuit, what varies is if you are just changing the wire, or also sweeping up bits of shattered ceramic out of the holders with a dustpan and brush...
With a breaker where one is waiting for the near instant magnetic part to operate, except for a few cunning but very expensive designs with hydraulic damping, the magnetic part either fires or it doesn't, so being over the line in the wrong direction could mean the near-instant trip becomes waiting for the thermal part to wake up.
It is fair to say that it took until the 1990s for regulations and testing practice to catch up.
Mike
PS
Modern breakers are not actually tested in a way that plots a curve either - its actually a very awkward test to do during manufacture and repeated operation tends to 'life' the mechanism, so only a small sample of go-no-go cases on either side of the lines are tested to give reasonable confidence that each one is good. Samples from the production line may be taken out and tested to death, but not very often.
The curves only represent the upper and lower limits of accepted performance but their shape is theoretical and based on the likely physics of operation of a near adiabatic operation from the thermal part when faster than a few seconds, and a vertical take off for the magnetic part.
If you were to test a real breaker and plot the breaking time results on top of the curves you should expect it to meander between the limits, rather than to follow the curve shape exactly.
The only thing that is sure is that any test to the left of the left limit should be a 'no-go' and any test to the right of the right limit should be a 'go'. Between the lines, well, it may or may not operate, and somewhere in that grey zone it will make the transition from one to the other.
Mike.
