Cut-off and time-current characteristics of fuse

Time–current characteristic of a fuse

The time–current (T–I) characteristic shows the time taken by a fuse to operate for different values of current. It is plotted on log–log scales.

 t (s, log)
 10⁴│╲
    │ ╲
 10²│  ╲   pre-arcing curve
    │   ╲  ╲ total clearing curve
  1 │    ╲  ╲
    │     ╲__╲__
 0.01│         ‾‾‾──____
    └──┬──────────────────► I (log)
      I_min fusing

• It is an inverse characteristic: small overcurrent → long time; large current → very short time.
• Below the minimum fusing current the fuse does not operate at all.
• Two curves are usually given: pre-arcing (melting) time and total operating time (pre-arcing + arcing). The gap between them is the arcing time; it is important only for short times.
• The characteristic depends on element material, cross-section, shape (notches), the filler and the ambient temperature.
• It is used for coordination: the T–I curves of fuses, relays and breakers in series must not cross.

Cut-off characteristic of a fuse

When the prospective fault current is large, an HRC fuse melts before the current reaches its first peak. The maximum instantaneous current actually reached is the cut-off current.

 i
 │      ╱‾‾╲  prospective current
 │     ╱    ╲
 │  ●╲╱ cut-off
 │  ╱ ╲
 │ ╱   ╲___ actual (limited)
 └┴──┴───┴──────► t
   t_p  t_a

The cut-off characteristic is a graph of cut-off current (peak) versus prospective current (r.m.s.):

 I_cut-off (kA peak)
  │             ╱ no cut-off line
  │           ╱   (peak = 2.5 × Irms)
  │         ╱   ___ 200 A fuse
  │       ╱ __─‾ ___ 100 A fuse
  │     ╱_─‾_──‾
  └─────────────────► prospective (kA rms)

• Each fuse rating has a curve; above a certain prospective current, cut-off occurs and the curve falls below the "no cut-off" line.
• Smaller fuse ratings give lower cut-off currents.

Importance: it gives the maximum peak current (electromagnetic force) and let-through energy that the downstream cables, busbars and switches must withstand, and allows smaller equipment and cheaper switchgear to be used.