The purpose of using fuses is to cut off the line safely and correctly to protect discrete components or the whole line in case of circuit errors. The following are the necessary conditions to be considered when selecting fuses:
■ Ambient Temperature
Ambient temperature means the air temperature directly around the fuse, and should not be understood as the room temperature. In many application cases, the fuses are at rather high temperature as they are installed with supporting devices or bases in different structures and they are closed in the distributing or controlling boxes.
■ Derating
We recommend that the actual working current of a fuse should not exceed its rated current under the ambient temperature of 20℃. While selecting the fuses, environment and working conditions should be considered, such as the variation of situation of closing, air flow, wire sizes (length and section) and instantaneous peak value etc. The current load capability of fuse links are tested under the ambient temperature of 20℃, however the actual load capability is affected by the ambient temperature. The higher the ambient temperature, the higher the working temperature and the shorter the service life of a fuse will be. On the other hand, the service life of a fuse can be longer when working under a lower ambient temperature.
The following is the typical curve showing the affection to the current load capability by the ambient temperature.

Ambient Temperature-Load Capacity Curve Note：A：(gG) type for line protection; B：(aR) type for semi-conductor protection.

e.g. When gG type fuse of 63A rating is used under ambient temperature of 20℃, reduction in working current is necessary when the ambient temperature is changed to 70℃. The Ambient Temperature - Load Capacity Curve A shows that the rating should be 78% at 70℃, and the new rating should be determined as:

So fuse links of 80A rating should be selected for the new ambient temperature.
■ Malfunction
Malfunction is usually a result of incomplete analysis on the design of circuit. Special attention should be given to 1) Normal rated current, 3) Ambient temperature, and 6) Overload increment of FACTORS TO BE CONSIDERED FOR SELECTION OF FUSES listed below. For example, frequent reasons for malfunction under normal working conditions are insufficient consideration to the start current of motor circuit, surge current of capacitor circuit and the ambient temperature around the fuse link.
■ RATED BREAKING CAPACITY
Rated breaking capacity is the maximum short-circuit current allowed for the fuse link to cutout reliably under rated voltage. The instantaneous current loaded to the fuse link is much larger than the normal working current when short-circuit occurs. The fuse link is supposed to cutout the line in an undamaged condition i.e. without bursting. The rated breaking capacity of MIRO fuses is up to 120KA and the excellent current limiting characteristics
reliably protect the equipment from damages by electric power.
WAVE CURVE OF CURRENT LIMITING CHARACTERISTICS OF FUSE LINKS
Where:IS- Peak value of maximum asymmetric current
at 100KA perspective current Ip (The impulse
factor of a short circuit should be 1.5).
ID- The actual current at breaking(limiting current)
US- Arc voltage
U- Voltage
tL- Arcing time
tS- Melting time
α- Burning corner of arc after zero voltage
■ NORMAL WORKING CONDITIONS
The working current of the fuse link should not exceed the rated value when working under 20℃ to prevent
malfunctions. The rated voltage of the fuse link should always be higher than that of the working voltage of the line. However for fast-acting fuse links for semi-conductor protection, the selected rated voltage should not exceed one grade of the working voltage of the line (≤2UN) to prevent reverse breakdown of silicon parts due to excessive arc voltage.
■ FUSE SUPPORTER (FUSE BASE)
In many application cases, fuse links are installed on fuse supporters/fuse bases. They are not to be used as switches for connection and disconnection of the load.
■ FACTORS TO BE CONSIDERED FOR SELECTION OF FUSES
1. Normal working current
2. Working voltage
3. Ambient temperature
4. Overload current and cutout time
5. Possible malfunction current
6. Impulse current, surge current, starting current and transient value of the line
7. Sizes and dimensions, connection methods, indicators, etc.
■ THRESHOLD VALUES OF aM FUSES