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

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).

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).