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Comprehensive Guide on High Speed Fuse Protection


Power semiconductor devices are the heart and muscle of modern power electronics. However, this muscle is sensitive and must be protected to maintain its lifespan.

Power semiconductors, which have high voltage and current ratings, are used in high-power circuits. Power semiconductor devices apply power to loads and switch it off whenever they are sent a control signal. They must do this rapidly and with precision.

Power semiconductor devices that are rated for higher-power applications are typically affected by two failure modes: thermal overstress and electrical overstress. Thermal overstress is caused by oxidation on the terminals and cooling component failure. Electrical overstress is often due to current and voltage transients, or insulation failure. Power semiconductors combine high-power handling and fast switching capability in a small package size. These devices generate excessive heat during their normal operation and have a low thermal withstand capacity. Any reduction in size impacts the device’s ability to withstand overcurrent and overvoltage.

An important part of developing quality overcurrent protection is understanding the system’s needs as well as overcurrent protective device fundamentals.


Overcurrent Condition

An overcurrent is any current larger than what the equipment, conductor or device is rated to carry under specified conditions. Unless removed in time, even moderate overcurrents can quickly overheat system components, which in turn, can damage insulation, conductors and equipment. Large overcurrents can melt conductors and vaporize insulation.

Very high currents produce magnetic forces that can bend and twist bus bars. These high currents can pull cables from their terminals and crack insulators and spacers. Uncontrolled overcurrents are prone to cause fires, explosions, and poisonous fumes. This not only damages electrical systems and equipment, but can cause injuries or fatalities to nearby personnel.


Overcurrent Types

The term overcurrent includes two types of fault conditions:

  • Overload fault condition
  • Short-circuit fault condition

Overload fault condition: An overcurrent that is confined to the normal current path, which if allowed to persist in the circuit, will likely damage the equipment and any connected wiring.

Overcurrent protective devices must disconnect circuits and equipment experiencing continuous overloads before any overheating occurs. Even moderate insulation overheating can seriously reduce the life of the components and the equipment involved.

Overcurrents that are 600 % or less rated current of the device or application are considered an overload fault current. Overload conditions often arise in applications when temporary surge currents persist in the system due to mechanical obstruction or jammed equipment conditions.

Short-circuit fault condition: An overcurrent that flows outside its normal current path in the circuit. A short-circuit fault is most commonly caused by an insulation breakdown or a faulty connection.

When a short-circuit fault occurs, the current bypasses the normal load and takes a shorter path, hence the term short circuit.

Short-circuit faults are typically divided into three categories: bolted faults, arcing faults and ground faults.

Overcurrents that are greater than 600 % rated current of the device or application are considered to be a short-circuit fault current. Short-circuit conditions are often due to human error, dropped tools, misapplication, and insulation breakdown.


Power Semiconductor Device Protection

The various stresses—including electrical, mechanical, thermal, and environmental stresses—that power semiconductor devices endure during their operation impact their ability to perform.

When these stress levels exceed their withstanding limits, the devices often fail. Thermal stress is the leading cause for semiconductor failure, which can lead to ruptures, fires and explosions.


What are High-Speed Fuses?

High-speed fuses are thermal, current-controlled devices used for semiconductor electrical circuit protection. Their element profile and body construction are specifically designed to have the short-circuit characteristics necessary for protecting semiconductor devices: low energy let-through (l2t), low peak currents (IPEAK), low-arc voltage and high heat dissipation. High-speed fuses are the only type of circuit protection device that can protect them from overcurrent faults due to their low energy let-through characteristics.

High-speed fuses consist of one or more current carrying elements that are enclosed within a chamber. The chamber is fitted with contacts (also known as blade/end-bells or terminations) so that the fuse may be readily inserted into and removed from an electrical circuit. Unlike general industrial fuses, high-speed fuses do not have intentional time-delay features.

This application guide takes an in-depth look at high-speed fuse protection. To download the full guide and learn more about high-speed fuseology, sizing guidelines, application and installation guidelines, fill out the form below to download the full guide.