Section 1: The Working Principle of a Circuit Breaker
When the fuse is energized, the electrical energy converts into heat, raising the temperature of the fusible element. During normal operating current or permissible overload current, the generated heat is radiated to the surrounding environment through the fusible element and housing, gradually reaching a balance with the heat produced by convection and conduction. If the generated heat exceeds the dissipated heat, the excess heat accumulates on the fusible element, causing its temperature to rise. When the temperature reaches and exceeds the melting point of the fusible element, it melts, breaks, and interrupts the current, thus serving as a safety protection for the circuit.
Section 2: Circuit Breaker Terminology
• Rated Current: The nominal operating current of the fuse tube (the current at which the fuse can maintain normal operation over a long period under normal conditions).
• Rated Voltage: The nominal working voltage of the fuse (the voltage the fuse can safely withstand when it opens). When selecting a fuse, the rated voltage of the chosen fuse should be greater than the input voltage of the protected circuit.
• Breaking Capacity: When there is a large overload current in the circuit (such as a severe short circuit), the fuse can safely interrupt the circuit's current. It is an important safety indicator for fuses. Safe breaking refers to the absence of explosions during the interruption of the circuit, which could cause splashes, fires, or other hazards that could endanger surrounding components, devices, or even personal safety.
• Overload Capacity (Load Carrying Capacity): A fuse can maintain operation under overload current for a specified period of time. When the current passing through the fuse exceeds the rated current, the temperature of the fusible element will gradually rise and eventually melt after a period.
UL standards specify that fuses must maintain operation for over 4 hours and the non-fusing current is 110% of the rated current (for miniature fuse tubes, it's 100%).
IEC standards stipulate that circuit breakers must maintain operation for over one hour with currents not exceeding 150% of their rated current.
• Fuse Characteristics (IT): The relationship between the load current applied to the fuse and the fuse's melting time.
The Interrupting Time Curve (IT Curve): It represents the average interruption time of a fuse at different load currents, plotted in a logarithmic coordinate system with load current as the X-axis and interruption time as the Y-axis. Each type of fuse has a corresponding curve that represents its interruption characteristics, effectively reflecting the fuse's overload performance for reference when selecting a fuse.
Blowout Characteristics Table: A table consisting of several specific representative load current values and the corresponding blowout time ranges. All safety standards clearly state that this is an important basis for the acceptance of fuses.
