Cut-off current characteristics of HRC fuse

An HRC (high rupturing capacity) fuse is current limiting: on a heavy short circuit it melts and clears the fault within the first quarter-cycle, before the current can reach its full prospective peak. The cut-off current is the highest instantaneous current reached before the fuse interrupts the circuit.

 i
 ^         prospective (would-be)
 |        .--.    current
 |      /      \
 |    /    .     \
 |   /   /  \     \
 |  /  /  ^  \     \
 | /  /   |   \     \
 |/  / cut-off  \     \
 +--+--------+-----------> t
 0  t1      t2
 0–t1 : pre-arcing (melting) time
 t1–t2: arcing time
 0–t2 : total operating time

Explanation

1. Pre-arcing time: the silver element heats and melts at its narrow sections. For a large fault this takes only a few milliseconds.
2. Arcing time: arcs form at the breaks. The arc energy melts the surrounding quartz sand, which absorbs heat and forms a high-resistance glassy material; the arc voltage rises and the current is forced to zero.
3. Cut-off current: the current at the instant of melting/peak is much lower than the prospective peak (for example, prospective peak 50 kA, cut-off about 10 kA).

Cut-off characteristic curve

It is drawn on log-log axes: cut-off current (peak) vs prospective current (r.m.s.), with one line per fuse rating. For small faults the fuse does not cut off (the line follows the prospective peak); above a certain fault level the line flattens, showing the current-limiting action.

Importance

• The let-through energy (I²t) and electromagnetic forces are greatly reduced, so cables, busbars and switchgear downstream do not need to withstand the full fault current.
• Allows use of HRC fuses as back-up protection for MCBs and contactors.
• Gives very high breaking capacity (80–120 kA) at low cost.