Speed
The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuse selection depends on the load's characteristics. Semiconductor devices may use a fast or ultrafast fuse as semiconductor devices heat rapidly when excess current flows.
Definition : It has been design 3 types of fuse depending of the load of the circuit that they will protect. (IEC 60269). There are:
• General use fuse (gG – Black written). Protect against overload and short circuit. Often use in domestic installation.
• Motor accompaniment (aM – Green written). This Fuse is to protect against short circuit only. It is used with induction load such as motor or transformer.
• Ultra-Fast fuse (UR – Red written). This fuse is used to protect semi-conductors.
gG and aM fuse can be found in different form and size
Definition : The IEC 60127 standard defines also 4 types of fuse. Each type is defined by the time to open the circuit for a current equal at 10 times the rated current. There are:
• FF (very fast), less than 1 ms ;
• F (fast), from 1 to 10 ms ;
• T (slow blow), from 10 to 100 ms ;
• TT (Very slow acting), from 100 ms to 1 s.
Example : Cut-Off Characteristics for Class: aM Fuses
The I2t value
The I2t value defines the amount of energy required to blow the fuse. It defines for a particular current the time required to open the circuit and stop the overcurrent.
Breaking capacity
The breaking capacity is the maximum current that can safely be interrupted by the fuse. Some fuses are designated High Rupture Capacity (HRC) and are usually filled with sand or a similar material. Above this value, the fuse would be unable to cut the circuit.
Rated voltage
Voltage rating of the fuse must be greater than or equal to what would become the open circuit voltage. For example, a glass tube fuse rated at 32 volts would not reliably interrupt current from a voltage source of 120 or 230 V. If a 32 V fuse attempts to interrupt the 120 or 230 V source, an arc may result. Plasma inside that glass tube fuse may continue to conduct current until current eventually so diminishes that plasma reverts to an insulating gas.
3.7. Symbol & Size & Case
