If you've ever had to replace a fuse, you may have realised that there's a few different fuse speeds. In simple terms, 'fuse speed' tends to refer to the time between the moment that an overcurrent begins and the moment the fuse arcs and opens the circuits.

Most fuses can be considered to be fast acting, however there are also slow acting varieties, also known as 'motor fuses', 'time delay', 'slow blow', 'delayed action', etc and semiconductor protection versions, also known as 'ultra-rapid', 'solid state', 'rectifier', etc.

When it comes to official classifications though, you may be surprised to know that fuses are not classified in terms of 'speed' but more in terms of 'range' and 'application'. All fuses that are covered by the IEC standard have an utilisation category, and when selecting fuses, it's very important to get this right.

Fuses are marked with symbols denoting their particular application or characteristic.

First Letter (lower case)

The first, lower-case letter indicates the breaking range of the fuse.

"g" stands for full-range breaking capacity, meaning that the fuse is capable of breaking all overcurrents from its minimum melting current up to its breaking capacity. These fuses can protect from overloads and short circuits. Full-range breaking capacity fuses can be used as sole protective devices.

"a" stands for partial-range breaking capacity, meaning that the fuse is capable of only breaking high currents of a multiple of its rated current. Partial-range breaking capacity fuses are designed for short-circuit protection only, and are therefore used in combination with other devices providing protection against overcurrents. They are also frequently used for back-up protection of other switching devices having a lower breaking capacity (e.g. contactors or circuit breakers).

Second Letter (upper case)

The second, upper-case letter indicates the characteristic. Refer below.

Current Utilisation Categories

Former Utilisation Categories