Fuse time-current factors and coordination

Factors governing time–current characteristic of a fuse link

The time–current characteristic shows the operating time of a fuse for various currents (inverse curve). It depends on:

1. Element material: melting point, resistivity, specific heat and thermal conductivity (silver, copper, tin, zinc, lead). Silver gives stable, predictable operation.
2. Cross-section and shape of element: thinner sections melt faster; notches (reduced sections) concentrate heating and decide pre-arcing time.
3. Length of element: a longer element has more resistance and less heat lost to end caps.
4. Number of parallel strips: affects current sharing and speed.
5. M-effect (tin/solder bead): lowers melting temperature at overload, giving time delay at small overloads.
6. Filler and enclosure: silica sand, ceramic body and its thermal conductivity determine heat dissipation and arcing time.
7. Ambient temperature and previous loading: a hot fuse operates faster.
8. Contact resistance and mounting: poor contacts add heat.
9. Circuit voltage and power factor: affect the arcing time (and thus total operating time) for high currents.
10. Ageing / oxidation of the element in non-HRC fuses.

Fuse coordination (discrimination)

Fuse coordination means selecting fuses in series (e.g. main fuse upstream, branch fuse downstream) so that for any fault on a branch, only the branch fuse nearest the fault blows and the upstream (major) fuse remains intact, keeping other branches in service.

 Supply ──[F1 major 200 A]──┬──[F2 minor 63 A]── load 1
                            └──[F3 minor 63 A]── load 2
 Fault on load 1 → only F2 should blow

Conditions:

• The T–I curves of the two fuses must not intersect over the whole range of fault current.
• For short-time (high) faults, the total I²t of the minor fuse must be less than the pre-arcing I²t of the major fuse.
• A rating ratio of about 1.6:1 to 2:1 between successive gG fuses usually ensures discrimination (as per manufacturer tables).

Coordination is also needed between a fuse and a circuit breaker or overcurrent relay (fuse curve below the relay curve with a time margin).