Fuse terms with cut-off characteristic sketch

Sketch of cut-off characteristic of a fuse

 i   prospective current
 │       ╱‾‾‾╲
 │      ╱     ╲
 │ cut-off     ╲
 │   ●──╲       ╲
 │  ╱    ╲       ╲
 │ ╱      ╲___    ╲___
 └┴────┴────┴─────────────► t
  0  melting  arc
     (pre-arc) extinction
 Without fuse, current follows the
 prospective curve; with fuse it is
 cut off at a lower peak.

(i) Minimum fusing current

The minimum value of current that will cause the fuse element to melt (and thus operate) under specified conditions. Below this value the fuse carries current indefinitely. It depends on element material, length, cross-section, enclosure and ambient temperature.

(ii) Rated current (current rating)

The current that the fuse element can carry continuously without overheating or melting. It is always less than the minimum fusing current. Example: a 32 A fuse can carry 32 A for an indefinite time.

(iii) Prospective current

The r.m.s. value of the fault current (first loop, symmetrical) that would flow in the circuit if the fuse were replaced by a link of negligible impedance. It depends only on the source and circuit impedance, not on the fuse.

(iv) Cut-off current

The maximum instantaneous current actually reached before the fuse element melts, when the prospective current is large enough. Because an HRC fuse melts before the first peak of the fault current, the cut-off current is much smaller than the prospective peak, limiting electromagnetic and thermal stress.

(v) Fusing factor

 Fusing factor = Minimum fusing current / Current rating

• It is always greater than 1.
• A value close to 1 gives closer protection, but too close causes nuisance blowing.
• Typical values: about 1.1–1.5 for HRC cartridge fuses, about 1.8–2 for semi-enclosed rewirable fuses.
• Example: a 100 A fuse with minimum fusing current 140 A has a fusing factor of 1.4.