3.6.3 -  Fuses

Fuses operate because the fuse element is the 'weak link' in the circuit, so that overcurrent will melt it and break the circuit. The time taken for the fuse link to break the circuit (to 'blow') varies depending on the type of fuse and on the characteristic of the device. The time/current characteristic of a typical fuse is shown in {Fig 3.12(a)}. Curves for other types and ratings of fuses are shown in {Figs 3.13 to 3.15}. The figures are adapted from Appendix 3 of the BS 7671: 1992.

Where the current carried is very much greater than the rated value (which is usually associated with a fault rather than with an overload) operation is usually very fast. For small overloads, where the current is not much larger than the rated value, operation may take a very long time, as indicated.

Fig 3.13 Time/current characteristics of semi-enclosed fuses to BS 3036

A graph with linear axes would need to be very large indeed if the high current/short time and the low current/long time ends of the characteristic were to be used to read the time to operate for a given current. The problem is removed by using logarithmic scales, which open out the low current and short time portions of the scales, and compress the high current and long time portions.

This means that the space between two major lines on the axes of the graph represents a change of ten times that represented by the two adjacent lines. In other words, a very much increased range of values can be accommodated on a graph of a given size.

Fig 3.14 Time/current characteristics of cartridge fuses to BS 1361

Rewirable (semi-enclosed) fuses to BS 3036 may still be used, but as they can easily have the wrong fuse element (fuse wire) fitted and have low breaking capacity {3.7.2} they are not recommended for other than small installations. Where used, they are subject to the derating requirements which are explained in {4.3.8}. The diameter of copper wires for use as elements in such fuses is shown in {Table 3.2}.

Table 3.2 Sizes of tinned copper wire fuse elements (from [Table 53A] of BS 7671: 1992)
Fuse element rating (A)	Wire diameter (mm)
3	0.15
5	0.20
10	0.35
15	0.50
20	0.60
25	0.75
30	0.85
45	1.25
60	1.53

All fuses must be clearly labelled with the fuse rating to make replacement with the wrong fuse as unlikely as possible.  It must not be hazardous to make or break a circuit by insertion or removal of a fuse.