Core Principle Of Fuse Selection: Load Characteristics Determine Protection Logic

The load characteristics in a power system or equipment fundamentally determine the appropriate response characteristics and breaking capacity of the drop out fuse. Ignoring this point will significantly reduce the effectiveness of the protection.

The starting current of a resistive load (such as a heating element or an incandescent lamp) is basically equal to the operating current, and the curve is stable. Inductive loads, such as motors, are quite different; the inrush current at startup can reach 5-7 times the rated current, and although the duration is short, it contains enormous energy. When a capacitive load (such as a large power supply filter) is switched on, it experiences a high-amplitude charging current surge that is close to a short circuit. Electronic circuits may be extremely sensitive to these very brief overcurrent anomalies.

Fuses designed for resistive loads have a fusible element heat accumulation model based on steady-state current. They cannot withstand the repeated impacts of the inrush current during motor startup and may experience abnormal melting. Conversely, time-delay fuses designed for motors, while having thermal inertia for inrush current, are sluggish in the face of the rapid response required by electronic circuits, potentially damaging precision components.