HRC fuse operating characteristics and applications

The operating characteristics of an HRC fuse describe how quickly it operates and how much current it lets through for different overcurrents. They are given mainly by the time–current characteristic and the cut-off (current-limiting) characteristic, together with the I²t values.

1. Time–current characteristic

 time (s, log)
  │╲
  │ ╲  pre-arcing time
  │  ╲
  │   ╲   inverse region
  │    ╲
  │     ╲___
  │         ‾‾──____  total operating
  └──┬─────────────── current (log)
   min. fusing current

• Inverse: the higher the current, the faster the fuse melts.
• Below the minimum fusing current (≈ fusing factor × rated current, e.g. 1.25–1.6 In for gG) the fuse never operates; overloads just above it melt the fuse in minutes to hours.
• For heavy faults the operating time is a few milliseconds.
• Characteristic categories: gG (general purpose, full-range), aM (motor circuit, short-circuit only, withstands starting current), gR/aR (semiconductor, very fast).

2. Cut-off characteristic

 peak let-through (kA)
  │         ╱ prospective peak (≈ 2.5 × Irms)
  │        ╱
  │       ╱ ___ cut-off current
  │      ╱_─‾
  │   _─╱
  └──────────────── prospective current (kA rms)

For large prospective currents the element melts before the first peak; the current is limited to the cut-off current, which is much less than the prospective peak.

3. I²t characteristic

The pre-arcing I²t (fixed for an element) and the total I²t (which depends on voltage and power factor) define the energy let through.

Influence on applications

1. Cable and equipment protection: low cut-off current and I²t mean that cables, busbars and switches need to withstand only a small let-through energy, so smaller, cheaper equipment can be used.
2. Motor circuits: aM fuses or time-delay fuses are chosen so that the starting current (6–7 × In for 5–10 s) does not melt them; overload protection is left to a thermal relay.
3. Transformer circuits: the fuse must withstand magnetising inrush (10–12 × In for 0.1 s) yet clear secondary faults.
4. Semiconductor protection: very fast fuses with total I²t below the device rating.
5. Discrimination: fuses in series are selective if the total I²t of the minor fuse is less than the pre-arcing I²t of the major fuse (typically a 1.6:1 or 2:1 rating ratio for gG).
6. Back-up for breakers/contactors: their high breaking capacity allows HRC fuses to protect contactors and MCBs against currents beyond their breaking capacity.