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The Graphs in Fig 3A4-6. What does the vertical part of the characteristic mean?

David Stone
There is a problem with these graphs. What is it and how should it be corrected?
  • 0
    The vertical bit means anything between those values - pretty useless really.


    I'd say ditch the graphs (they don't seem to be consistent with BS EN 60898 anyway - occasionally dangerously so) - and replace with tables of time/current (rather like the ones for RCDs) and energy let-though.

      - Andy.
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    AJJewsbury:

    they don't seem to be consistent with BS EN 60898 anyway - occasionally dangerously so

     


    Could you highlight where the inconsistencies are? Are you referring to Table 7?


  • 0
    OK  Andy, is that really what they mean? Please explain.
  • 0
    gkenyon:
    AJJewsbury:

    they don't seem to be consistent with BS EN 60898 anyway - occasionally dangerously so

     


    Could you highlight where the inconsistencies are? Are you referring to Table 7?




    Back to this old discussion - https://communities.theiet.org/discussions/viewtopic/1037/24502 - which seemed to conclude that BS 7671 was claiming that certain rating of certain types of MCB would open within 5s for a given current whereas both BS EN 60898 and manufacturer's data suggested it could take up to 8s to disconnect.


      - Andy.


  • 0
    davezawadi (David Stone):

    OK  Andy, is that really what they mean? Please explain.


    The vertical bits? - that's how I read them at least - there's a discontinuity between the thermal and magnetic parts - so when the current is right on the boundary between the two (the tiniest fraction below 5x In for a B-type for example) the thing could operate either in thermal mode and take over 10s to open, or it could be in magnetic mode and take (anything up to) 0.1s.


    I know some mathematicians who would say that the vertical bit shouldn't really be drawn at all when there's a discontinuity - better to just leave a gap.


    Given in BS 7671 land we're not really interested in disconnection times of over 5s, the whole graph is seems of limited use anyway.


       - Andy.


  • 0
    The vertical part is a mathematical error Andy, it should certainly not be there, see the spur thread. It certainly does not mean a range of values, and essentially should not be drawn like that. You are correct in that the graph is discontinuous and the mathematical reasons are as follows:

    The two axes on a graph are related to one another by an equation, which may be very curious and complex. However, there is a proviso that the slope may never be infinite, which means that the relation between the axes is no longer present. There is no equation available that gives a "range of values" as the answer, it is not possible without cheating. The error is that the magnetic part is completely separate from the thermal part and they may never join. True there is a point where one takes the place of the other, but this point is not really defined. I also do not see why the vertical lines stop at 0.1 seconds, this is completely arbitrary and does not reflect the real operation. The rest of the curve over 5 seconds is very useful, it shows (as do the fuse graphs) what happens when the fuse is operated at less than the disconnection time (which is normal operation). You depend on this in your house DNO supply every day, and probably the cutout fuse too, it is "normal" not during fault operation.
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    Your reference to the old post Andy is the same answer from OMS as I have been giving, with slightly differing minor details. You should note that this is not a graph but an area plot, conceptually very different.
  • 0
    davezawadi (David Stone):

     The rest of the curve over 5 seconds is very useful, it shows (as do the fuse graphs) what happens when the fuse is operated at less than the disconnection time


    Well, no it doesn't. In fact, none of the curves in Appendix 3, whether for fuses or mcb's, show that - quite simply because there's a range of operation (two bounds, lower and higher) that are described for most fuses and circuit breakers, either in, or as a result of, standard requirements, or from the required manufacturer's data. Both have their uses in design, and really need to be considered - for example, in selectivity studies and to prevent nuisance tripping due to inrush currents and similar. But I think you alluded to that in your reply to Andy in respect of circuit breakers.


    The purpose of the data in Appendices 3 and 4 is mainly aimed at the worst-case limiting conditions discussed in BS 7671 to comply with, for the most part, Chapters 41, 42 and 43 of BS 7671, for the majority of smaller installations. Outside this scope, you need more information (and more standards).


  • 0
    gkenyon:
    davezawadi (David Stone):

     The rest of the curve over 5 seconds is very useful, it shows (as do the fuse graphs) what happens when the fuse is operated at less than the disconnection time


    Well, no it doesn't. In fact, none of the curves in Appendix 3, whether for fuses or mcb's, show that - quite simply because there's a range of operation (two bounds, lower and higher) that are described for most fuses and circuit breakers, either in, or as a result of, standard requirements, or from the required manufacturer's data. Both have their uses in design, and really need to be considered - for example, in selectivity studies and to prevent nuisance tripping due to inrush currents and similar. But I think you alluded to that in your reply to Andy in respect of circuit breakers.


    The purpose of the data in Appendices 3 and 4 is mainly aimed at the worst-case limiting conditions discussed in BS 7671 to comply with, for the most part, Chapters 41, 42 and 43 of BS 7671, for the majority of smaller installations. Outside this scope, you need more information (and more standards).




    I agree with you, the Appendix 3 curves do not provide the information required to do a selectivity study.


    So what are they for then?  You are suggesting that they may be used for the adiabatic equation to determine energy let through or to look at disconnection times.

     

    The question is are they in the best format for the intended use.  Well given that the range of application has an upper bound of 5 second and a lower bound of 0.1 seconds presenting them as log log curves seem to me to be over complicating things.  It also makes getting accurate readings from the graphs very difficult as it is hard to estimate values between axis divisions on a logarithmic scale.


    The following comments only apply to fuses because, as David Stone has pointed out, the graphs have no real meaning for mcbs (or any other device that uses a magnetic trip for high fault current protection such as mccbs).


    Over the required range the five given points for times (5, 1, 0.4, 0.2, 0.1) can be plotted on linear scales.  This makes it much easier to determine the values between the given points.  I am not aware of any equation the accurately models the graph, I have tried various methods (Cubic Splines, BSplines and Polynomials together with some 'home brew' efforts) and whilst they work to some extent they are not perfect for all devices.


    Given that the values plotted have a significant uncertainty there is not much error in taking a value from a graph that simply links the points by straight lines.


    I have developed an application that calculates the disconnection time using various equations.  Taking as an example a BS 88-2 (E) 315 A Fuse, at a fault current of 2500A gives:


    Newton Difference    BSpline         Linear Interpolation  between 5 sec & 1 sec

             2.37                     3.13                                         3.1


    Try reading the value from the Appendix 3 graph - well it's somewhere between 2 and 3 - remember the scale between divisions is logarithmic!

     

    IMO the result from the relatively simple linear equation is adequate for most purposes.


    Regards

    Geoff Blackwell


  • 0
    Hi Geoff,

    I think I may have been slightly less than clear on the point, I meant current is less than causes fault type disconnection. Sorry if anyone was confused.


    There is a severe mathematical problem plotting things on logarithmically scaled graphs (and we do it a lot for all kinds of things) in that the reason to use these scales is that they in many applications allow us to plot a straight line, which we can the use. It doesn't work for fuses or MCBs does it? Whoever chose to use this scaling has not really been very helpful, they need to calculate the underlying function they wish to plot and scale the axis using a suitable function. There is no need for this to be a log, exponential, or any other easy curve. It may mean they need to make their own graph paper but this is trivial nowadays with computers.


    Now fuses, as excellently described by Mike follow an I²t heat characteristic whilst adiabatic and will need some heat loss correction at longer times. It would seem that the fuse plotted on log time against a log square law current might produce a fairly straight line. Log current is probably incorrect, and so the graphs are curved.


    The MCB also depends on heat, but well outside the adiabatic region, so the current axis will be a modified square law, probably a square plus a thermal loss part which will be probably connected to temperature to a fourth power and an operating environment offset in absolute temperature.  The magnetic part has a threshold, the ampere-turns to cause disconnection, and another factor of current controlling the acceleration of the contact mechanism, so energy let through.


    Whilst various kinds of curve fitting can improve the situation, a straightforward equation is the way to go for accuracy, but probably not for general electrical use. It is unlikely that a discrimination study needs more than the equation, after all, there are more factors, such as how many times a fuse has been stressed before, which will have an effect on the real performance. It is likely that we have these graphs because no one has thought it worth the effort to produce linear lines, and in fine detail, it might be quite difficult. Curve fitting is, in my view, a dangerous game unless the underlying characteristics are known, various kinds of regression analysis should be most accurate if the degree is chosen to match the underlying characteristic, but then determining this is difficult as I say above. B-splines or other spline types have all kinds of nasty end effects and become complicated to use when end connection slopes are taken into account. Linear interpolation of a curve is pretty poor, one takes one's pick depending on the effort and time available.

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