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Fuse Operating Class Explained

2022-07-08

Fuse operating class, aka fuse speed, is the time it takes for the fuse to open when a fault current occurs. When choosing a fuse or replacing it, it's vital to choose a fuse that will open quickly to protect the circuit from accidental overload but will not open under normal operating conditions. The speed of the fuse is also known as the time-current characteristic of the fuse.


For example, if a current of 60A was flowing through a fuse rated at 30A, a very fast-acting fuse may open in 100ms, a fast-acting fuse may open in 1s, whilst a slow-acting fuse may take 100s to open.


When it comes to official classifications, 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 covered by the IEC standard have a 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.


Very Fast Acting (Ultra Rapid Fuses, High Speed, Super Rapid, Ultra Rapid or Semiconductor Fuses)

aR Partial-range breaking capacity (short-circuit protection only) for the protection of Power Semiconductors. Typical applications include protection of semiconductors (diodes, thyristors, triacs, etc) used in power rectifiers, UPS, converters, motor drives (AC and DC), soft starters, solid state relays, photovoltaic inverters, welding inverters and any application where it is necessary to protect semiconductor devices.
gR Full-range breaking capacity (overload and short-circuit protection) for the protection of semiconductors as well as cables and all switchgear of installation. Typical applications include protection of semiconductors (diodes, thyristors, triacs, etc) used in power rectifiers, UPS, converters, motor drives (AC and DC), soft starters, solid state relays, photovoltaic inverters, welding inverters and any application where it is necessary to protect semiconductor devices.
gS Full-range breaking capacity (overload and short-circuit protection) for the protection of semiconductors as well as cables and all switchgear of installation. When compared with class gR fuses, gS fuses have lower power dissipation due to tighter melting gate values. The lower power dissipation in class gS fuses also result in lower fuse body temperatures. Typical applications include protection of semiconductors (diodes, thyristors, triacs, etc) used in power rectifiers, UPS, converters, motor drives, soft starters, solid state relays, photovoltaic inverters, welding inverters and any application where it is necessary to protect semiconductor devices.
gRL Same as gS.

Fast Acting (Fast Blow, General Purpose or General Application Fuses)

gG Full-range breaking capacity (overload and shortcircuit protection) for general applications.
gL Same as gG.
gF Same as gG.

Slow Acting (Slow Blow, Time Delay or Motor Start Fuses)

aM Partial-range breaking capacity (short-circuit protection only) for the protection of motor circuits.
gM Full-range breaking capacity (overload and shortcircuit protection) for the protection of motor circuits.

Special Purpose 

gPV Protection of solar photovoltaic arrays. They interrupt short circuits typically seen in PV systems and are designed for use in direct current circuits.
gB Full-range breaking capacity (overload and shortcircuit protection) robust for mining application.
gTr Full-range breaking capacity (overload and shortcircuit protection) for protection of transformers.
gN North American general purpose for protection of conductors.
gD North American general purpose, time delay.


If you are confused about the fuse operating class, don't hesitate to contact us. We are experienced and are always ready to help.


Part of the content is excerpted from Swe-Check