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

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

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