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Choosing the right fuse speed

Choosing the right fuse speed


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

 

Utilisation Category

Application (Characterstic)

gG

Full range breaking capacity fuse for general application, mainly
used for cable and line protection.

aM

Partial range breaking capacity fuse for the short circuit protection of motor circuits.

gR

Full range breaking capacity fuse for the protection of
semiconductor devices (quicker than gS).

gS

Full range breaking capacity fuse for the protection of
semiconductor devices for increased line utilisation.
Combines gR and gG performance. Semiconductor
protection against short circuits via the gR characteristic
combined with cable protection against overload via the gG
characteristic.

aR

Partial range breaking capacity fuse for the short circuit
protection of semiconductor devices.

gB

Full range breaking capacity fuse for mining applications.

gTr

Full range breaking capacity fuse for transformer protection,
rated in transformer apparent power (kVA) instead of rated
current (A).

gM 

Full range breaking capacity fuse for motor circuit
protection, dual rated (widely used in the UK, Australia
and South Africa.

gN

North American general application fuse, mainly for cable
and line protection.

gD

North American general application time-delay fuse, mainly
for motor circuit protection.

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Former Utilisation Catagories
Utilisation Category

Application (Characteristic)

gl

Former IEC time-delay fuse, replaced by gG.

gll

Former IEC fast-acting fuse, replaced by gG.

gL

Former VDE fuse for conductor protectionm replaced by gG.

gRL

Manufacturer specific designation meanwhile standardised as gS.

gGR

Manufacturer specific designation meanwhile standardised as gS 

gT

Former VDE time-delay fuse, replaced by gG

gF

Former VDE utilisation category, replaced by gG

gTF Former VDE time-delay / fast-acting fuse, replaced by gB