The fuses are intended for the protection of electrical and electronic equipment.

Normal blowing characteristic A fuses are intended for general circuit protection when only nominal short circuit currents are available. Where higher interrupting capacity is desirable, use characteristic C fuses.

Normal blowing characteristic C fuses are intended for general circuit protection where there is a possibility of extremely high short circuit currents. Characteristic C fuses can be substituted for characteristic A fuses, however, the reverse is not true.

Time delay characteristic B fuses are intended for use in circuits containing motors and other circuits where provision must be made for momentary surges. The fuses should be used only when nominal short circuit currents are available. Consideration should be given to using characteristic BR fuses in lieu of characteristic B fuses wherever possible. Characteristic B fuses cannot be used in rejection type fuseclips intended for characteristic BR fuses.

Time delay characteristic BR fuses are intended for use in circuits containing motors and in other circuits where provision must be made for momentary surges. These fuses are capable of interrupting extremely high short circuit currents. They have a

"rejection" feature which when used with "rejection" type fuseclips will prevent fuses such as the characteristic A, B, or C from being substituted. Characteristic BE fuses can be substituted for characteristic B fuses in all applications even when standard fuseclips are use.

The following steps should apply in the selection of a fuse for any application:

a. Step 1. Select a fuse with a voltage rating equal to or in excess of the circuit voltage (see 6.1.5.1).

b. Step 2. The short circuit interrupting rating of the fuse should be checked to determine that it is adequate to protect against the short circuit currents that the particular equipment or circuit can be exposed to.

c. Step 3. The ambient temperature in which the fuse is to be used should be compared to the curves shown on figure 1 or 2 to determine the effect the ambient will have on the current rating. For example, if a characteristic A fuse is to be used in a 60 °C ambient, it will only carry 95 percent of its normal rating a fuse which is used in a minus 20 °C ambient will carry 107 percent of its nominal rating.

d. Step 4. A fuse rating should be selected on the basis of equipment ampere load with compensation made for ambient temperature (see figures 1 and 2). Characteristics A and C fuses should not be loaded to more than 80 percent of the fuse rating (or 80 percent of the adjusted rating based on ambient temperature consideration) to avoid nuisance openings caused by inrush currents and other harmless overloads. Time lag (characteristic B or BR) fuses can be loaded to a higher level because of the ability to override harmless overloads, transients, and inrush currents of short duration.

Problems can arise when fuses are used at voltages considerably lower than their rated voltage. Due to the increase of the voltage drop when the element of a fuse approaches its melting point, care should be taken to ensure that there is sufficient circuit voltage available to cause the fuse to interrupt the current when an electrical fault occurs. Furthermore, fuses of the same type and rating may, due to difference in design or element material, have different voltage drops and may therefore not be interchangeable in practice when used in applications with low circuit voltages. A minimum low voltage test (3.10.1) is specified (see 3.1) for those fuses which are most likely to be effected when used in very low voltage circuits. However caution should be used in applying a fuse in any circuit with a voltage considerably lower than the fuse voltage rating.

Equipment designers should refer to MIL-STD-1360 for a selection of fuse types preferred for use in new equipment design.