Fusing current and current carrying capacity

Current carrying capacity (current rating)

The current carrying capacity of a fuse is the r.m.s. current that the fuse element can carry continuously without melting and without its temperature exceeding the permitted limit. It is marked on the fuse as the rated current.

• It depends on element material, cross-section, length, contacts, enclosure and ambient temperature.
• The fuse rating is chosen slightly above the normal full-load current of the circuit.

Fusing current

The fusing current (minimum fusing current) is the minimum current at which the fuse element melts and opens the circuit within a specified (long) time, e.g. 1 hour for small fuses or 4 hours for large ones.

• It is always greater than the current rating.
• It depends on the material, length, diameter, contact condition and surroundings of the element.
• Preece's formula for a round wire: I = k·d^(3/2), where d = diameter and k a material constant (e.g. k ≈ 80 for copper with d in mm, I in A).

Fusing factor

 Fusing factor = Min. fusing current / Rating  (> 1)

Typical: 1.1–1.5 for HRC fuses, about 2 for rewirable fuses.

Example

• A lighting circuit draws 12 A. A fuse with current rating 16 A is chosen; it carries 16 A indefinitely.
• If its fusing factor is 1.45, the minimum fusing current = 1.45 × 16 = 23.2 A.
• Thus currents up to 16 A are carried continuously; at about 23 A or more (overload) the fuse melts after a long time, and at a short-circuit current of, say, 1000 A it blows in milliseconds.