Prospective current, cut-off current, time delay fuse

(i) Prospective current

The prospective current is the r.m.s. value of the fault current that would flow in a circuit if the fuse were replaced by a link of negligible impedance. It is decided by the source voltage and the impedance up to the fault, not by the fuse.

 I_prospective = V_phase / Z_fault-loop
 e.g. 230 V / 0.023 Ω = 10 kA (rms)

With DC offset the first peak can be up to about 2.5 × I_rms (≈ 25 kA in this example).

(ii) Cut-off current

The cut-off current is the maximum instantaneous value of current actually reached before the fuse element melts, for a large prospective current. An HRC fuse melts in the first quarter cycle, so the current is "cut off" before the prospective peak.

 i   prospective peak
 │      ╱‾‾╲
 │  cut-off
 │   ●╲╱    ╲
 │  ╱  ╲     ╲
 │ ╱    ╲__   ╲
 └┴─────┴───────► t
   pre-arc  arc

• It reduces electromagnetic forces (∝ i²) and heating (I²t) in cables, busbars and switches.
• The cut-off characteristic gives cut-off current vs prospective current for each fuse rating; a lower fuse rating gives a lower cut-off.

(iii) Time-delay fuse

A time-delay (slow-blow, anti-surge) fuse is designed to carry short-time, harmless overcurrents (motor starting current, transformer inrush, capacitor charging) without melting, while still operating quickly on short circuits.

Construction:

• A dual-element design: a short-circuit element (silver strip with notches) in series with an overload element (a heat-absorbing mass, solder joint or spring-loaded link) that melts only after sustained overload.
• Or an element with an "M-effect" tin bead to delay melting at moderate overloads.

Characteristic: at 5–6 × rated current it may take 5–10 s to blow (allowing motor starting), but at 20 × rated current it operates in milliseconds.

Applications: motor circuits (aM fuses), transformer primaries, capacitor banks, and circuits with lamp or heater switch-on surges. It allows a fuse rating closer to full-load current, giving better overload protection.