Sure, but when one is talking in general (without knowing the actual Zs) one really has to do it 'that way around'.
We are all agreed that we don't have enough detailed information about the operating characteristics of MCBs (although I am doubtful whether reliable data of the type we'd like to have necessarily exists).
I fully understand your point and I, too, would like to understand how/why (on the basis of what you're telling me) they seemingly base their calculations on an assumed disconnection time of 0.01 sec.
Further to my recent response, I think I may have discovered why the 'general practice', which most of us follow, does not assume a disconnection time of 0.01 secs (and probably why we usually use 0.1 secs).
Yes, I almost mentioned that last night. One thing is fairly clear. Although I realise it has not been suggested, to assume any disconnection time less than 0.01 seconds for these simple calculations would almost certainly not be safe. If one gets down to less that half a cycle, behaviour is bound to be at least somewhat unpredictable, since it becomes dependent on the point in the cycle at which the fault arises - if current flows for less than a half cycle, there is no guarantee that the potential peak current (of the whole wave) will ever flow.
Sure. Above about 5 seconds, the whole process becomes non-adiabatic (i.e. there is time for some thermal movement to occur), so the whole adiabatic approach becomes inappropriate, and much more complex calculations would be required. If current flows for very short periods (e.g. <0.01 seconds), the process is very much adiabatic. However, if one gets down to less than half a cycle, the simplistic adiabatic equation as given in 543.1.3 can no longer be used, since the square of the RMS current multiplied by time no longer represents the ‘let through energy’. The same theoretically also applies whenever ‘t’ is not an exact number of half cycles, but the consequences of that are trivial if ‘t’ is at least a few cycles in length.
I would say that answering that question requires two additional bits of information - firstly, as discussed, a proper understanding of how the t/I relationship of a MCB behaves at disconnection times below 0.1 sec and, secondly, knowledge of the In of the MCB protecting the circuit. If calculated conventionally using 230V, your Zs of 0.26Ω corresponds to a PFC of 884A. IF the curve for a Type B MCB is similar to the Type D curves I posted last night, then that PFC would be enough to achieve a disconnection time of 0.01 seconds (or less) for any Type B MCB with an In up to around 70A - in other words, any domestic Type B would achieve a disconnection time of 0.01 seconds (or less). One would therefore be justified in using t=0.01 for the adiabatic calculation, which would result (assuming 253V) in a ‘minimum Zs’ of about 0.15 Ω. So, if the above assumption about the t/I curve is correct then the circuit you describe would be ‘safe’ for any MCB with an In less than about 70 A. However, I can only say that because I have looked at curves not readily available to most electricians (and assumed Type B ones are similar!) - it is not true that a calculation undertaking assuming a 0.01 second disconnection time will necessarily give a correct answer asti whether or not the conductor is 'safe' for any circuit.
It’s probably not so much arbitrary as ‘simplified and conservative’, particularly given that most electricians do not have easy access to t/I curves for MCBs which go below 0.1 seconds. If the Zs were such that PFC were much closer to the ‘magnetic trip current’ (e.g. 5*In for a Type B) than in your example then, if those Type D curves I posted last night are typical of all MCBs, the disconnection time could be appreciably above 0.01 secs (although, I agree, not as high as 0,1 seconds). Given the wide spectrum of knowledge, abilities and resources of electricians, it probably does make sense to suggest, as a ‘guideline’, calculations on the basis of an assumed disconnection time appreciably greater than 0.01 seconds, although perhaps not as high as 0.1 seconds. Those with adequate knowledge, skills and access to the relevant data can always undertake ‘proper’ calculations, as per those above.
That is not necessarily true - it depends crucially upon the actual shape of the t/I curve in the region when currents exceed the minimum required to achieve a magnetic trip. The required CPC depends upon I²t. If, for example, you double the current, the required csa will actually increase unless the effect of that increase in current is to reduce the disconnection time by a factor of at least 4 - which I actually doubt is the case with the curves we are looking at. To illustrate, an example ...
Sorry, where does 300 come from?
Oh, I'm sorry, too! It was just an arbitrary round number used for convenience for the purpose of illustration - you could substitute any other current you like (and then double that current in the two subsequent calculations.
Yes, I did look at the graphs, but seemingly not carefully enough, since I didn’t notice/realise that they are very different from the one I posted last night. Also, in terms of the calculations I recently presented, the Hager curves indicate that a 20% increase in current ( from 5*In to 6*In) results in a disproportionately large (50%) decrease in disconnection time, much more change than I had expected.
I can’t disagree with that, provided those Hager curves (which, as I said, are very different from the one in the IET Design Guide) are correct and generally applicable. So I suppose I am essentially ‘conceding’. I suppose the fact that I can now do that is not actually inconsistent with what I have been saying all along - that if we had decent data on how MCBs behave below 0.1s, we would be able to work out the true answer.
That's simple enough to answer. In terms of 'my table' which, as I indicated when I presented it, assumed a disconnection time of 0.1s, as you say, a Zs of 0.26 would clearly not be 'safe'.
Fair enough. I look forward to hearing anything useful you discover. I would be particularly interested to see any other t/I curves for MCBs you may come across since, given the pretty dramatic difference between Hager's and the IET's, we really need a 'decider' or three!
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