Differences Between Fast-Acting and Slow-Acting Fuses

The core differences between fast-acting and slow-acting fuses lie in their response speed and application scenarios: fast-acting fuses blow instantly to protect sensitive components, while slow-acting fuses delay blowing to withstand surge currents.

Analysis of the Main Differences

Blowing Characteristics and Response Speed.

Fast-Acting Fuses:

They have extremely fast response times, blowing within milliseconds (typically 0.1ms-5s) when overcurrent occurs. They are suitable for protecting precision electronic components such as IC chips and semiconductor devices. (UL standard).

They follow the Joule heating effect (Q = I²Rt), have a simple fuse design, and use narrow-section metal wire for rapid heat conduction.

They are sensitive to instantaneous currents and cannot withstand surge currents during power on/off or motor startup.

Differences Between Fast-Acting and Slow-Acting Fuses

Selection and application of fast-blow and slow-blow fuses

Applications of Fast-Acting and Slow-Acting Chip Fuses

Slow-Acting Fuses:

They withstand short-term overcurrents (e.g., 7 times the rated current for 0.5-3 seconds during motor startup). ‌‌
It features a time-delay function, taking 5 naar 10 seconds to fuse at 2 times the rated current, and can withstand short-term high-current surges (such as motor starting currents up to 7 times the rated current).
It has a high melting heat value, achieving delayed fuse opening through heat absorption in quartz sand or a spiral design.

Fast-blow applications:
Resistive load circuits (elektrische verwarmingstoestellen, LED lighting);
Protection of sensitive semiconductor devices (such as MOSFETs and lithium battery packs for short-circuit protection);
Resistive loads (waterkokers, rijstkokers);
Protection of sensitive circuits such as lithium battery packs and circuit boards;
Applications requiring rapid interruption of short-circuit currents.

Slow-blow applications:
Inductive/capacitive loads (motoren, schakelende voedingen);
Applications requiring surge protection (such as magnetizing surge protection for transformers over 100kVA);
Inductive/capacitive circuits such as motors, voedingen, and inverters;
Equipment subject to startup inrush current (such as switching power supplies and transformers);
Environmental environments requiring pulse current tolerance.

Protection function differences Fast-blow fuses: Provide short-circuit protection only and cannot distinguish between overload and transient pulses. ‌‌
Slow-blow fuses: Provide both overload and short-circuit protection, using the I²t value (the integral of the square of the current and the time) to determine energy. ‌‌

Key Parameters and Selection Key Points
‌I-T Curve Differences‌
Fast-blow fuses have a steeper curve, with a melting time of ≤0.1s at 2x the rated current; slow-blow fuses have a flatter curve, with a withstand time of ≥10s at 2x the rated current.
‌Interchange Risk‌
Replacing a slow-blow fuse with a fast-blow fuse may cause the device to fail to start; replacing a fast-blow fuse with a slow-blow fuse may increase the risk of damage to sensitive components.
‌Cost and Structure‌
Slow-blow fuses are more expensive due to their special alloys or complex structures.

‌Selection Considerations‌
‌Parameter Calculation Priority‌:
Verify that the I²t value of the circuit’s maximum surge is less than the fuse’s withstand value (Bijvoorbeeld, a power supply must pass a 15A/150ms surge test). ‌‌
The interrupting capacity must be higher than the system’s maximum short-circuit current (e.g., for a 35kA short-circuit, choose a 50kA interrupting capacity). ‌‌

Common Misconceptions‌:
High temperatures can cause the rated current of a slow-blow fuse to drop by 30%. ‌‌
Misusing a fast-blow fuse in a UPS can cause false tripping (one case resulted in losses of 1.8 million yuan). ‌‌
Experiments show that when a lithium battery short-circuits, the probability of thermal runaway caused by a slow-blow fuse is eight times higher than that of a fast-blow fuse.
In inverter testing, misusing a slow-blow fuse can increase the module damage rate from 1% naar 37%.