In electrical circuits, overloads and short circuits can cause currents larger than normal to flow. A power fuse is one of the protective components used to interrupt such overcurrents and protect circuits and equipment.

When the current exceeds specified conditions, the fuse element inside the fuse melts and opens the current path. Because current cannot flow again after the fuse operates, the fuse generally needs to be replaced.

Fuses are also used in PCS (Power Conditioning Systems), which are power conversion equipment, depending on the equipment configuration and what is being protected. In particular, when protecting power semiconductors, the ability of a fuse to operate in a short time is important.

2.1 Role of Power Fuses

The main role of a power fuse is to interrupt the circuit when an overcurrent occurs and keep the effects of the fault from spreading.

Under normal conditions, the fuse carries current. An overcurrent raises the temperature of the fuse element, and when the element melts, the fuse opens the circuit. In general, the larger the overcurrent, the shorter the time to melting.

Fuses include fast-acting types, which operate in a short time, and time-delay types, which withstand an overcurrent for a certain period. Time-delay types are used in circuits where inrush currents occur, such as at startup.

2.2 Basic Structure and Operation

An enclosed power fuse consists mainly of a fuse element, terminals, an insulating body, and a filler used for arc quenching.

The fuse element may have sections with a reduced cross-section so that current concentrates there and heats them more readily. The shape and material of the element affect its temperature distribution and melting time.

When an overcurrent melts the element, an arc forms at that point. In an enclosed current-limiting fuse, the filler and other components quench the arc and interrupt the fault current.

Fuse operation is not determined by the current value alone. The duration of current flow, the resistance and heat capacity of the element, the ambient temperature, the heat dissipation conditions, and other factors affect the temperature rise.

2.3 Time–Current Characteristics and I²t

The operating characteristics of a fuse can be checked with its time–current characteristic, which shows the relationship between the magnitude of the current and the operating time.

At currents close to the rated current, the temperature rises slowly, so melting may take a long time. With a large short-circuit current, on the other hand, the element heats rapidly and the fuse operates in a short time.

I²t, the time integral of the square of the current, is used as an index of the thermal operation of a fuse. In the short-time adiabatic region, where heat loss to the surroundings can be neglected, the I²t until the fuse operates is nearly constant.

When protecting power semiconductors, the operating time and I²t of the fuse must be coordinated with the range of fault current that the semiconductor to be protected can withstand.

2.4 Fuses Used in PCS

A PCS uses power semiconductors to convert between DC and AC. Fast-acting fuses may be used to protect the semiconductors and circuits from short-circuit faults inside the equipment.

IEC 60269-4 covers fuse-links used in equipment containing semiconductor devices. The nominal voltage of the circuits covered is 1,000 V AC or less, or 1,500 V DC or less.

IEC 60269-6, on the other hand, covers fuse-links that protect PV strings and PV arrays in solar photovoltaic systems. Fuses used on the PV side and fuses used inside a PCS for semiconductor protection must therefore be considered separately in terms of where they are used and what they protect.

In addition, not all PCS use fuses in the same positions or in the same way. The actual protection configuration depends on the circuit topology, voltage, expected fault current, semiconductor withstand capability, and other factors.

2.5 AC and DC Fuses

IEC 60269-1 covers enclosed current-limiting fuses with rated breaking capacities above a certain level that protect circuits of 1,000 V AC or less, or 1,500 V DC or less.

AC and DC circuits differ in the behavior of the current and the arc during interruption. Fuses must therefore be checked not only for rated voltage but also for whether they are intended for AC or DC.

In particular, when a fuse is used in the DC circuit of a PCS, the ratings and breaking performance specified for DC circuits must be checked. The ratings of an AC fuse cannot necessarily be applied as they are to a DC circuit.

2.6 Main Characteristics to Check

Power fuses should be selected considering not only the normal current of the circuit but also the fault conditions and the characteristics of the equipment being protected.

The main items to check are as follows.

• Rated voltage
• AC or DC classification
• Rated current
• Rated breaking capacity
• Time–current characteristic
• I²t
• Equipment to be protected and applicable standards
• Ambient temperature and heat dissipation conditions
• Terminal temperature
• Coordination with upstream and downstream protective devices

If a fuse is chosen on the basis of rated current alone, it may operate unnecessarily due to temporary currents during normal operation, or the protected equipment may be damaged before the fuse operates during a fault.

The heat generated by a fuse also affects the temperature of the terminals and surrounding parts, not only the element. For continuous operation, the temperature inside the panel, the mounting conditions, the cooling conditions, and other factors must also be checked.

Summary

A power fuse is a protective component that interrupts a circuit when an overcurrent melts its fuse element.

In a PCS, fuses may be used to protect power semiconductors and DC and AC circuits, but their positions and purposes vary with the equipment configuration.

To use fuses appropriately, it is necessary to check not only the rated voltage, rated current, and breaking capacity but also the time–current characteristic, I²t, AC/DC classification, ambient temperature, and coordination with the protected equipment.

About This Article

References

• “Numerical Analysis of Thermal Behaviour of DC Fuse,” Adrian Plesca
• “Modeling the Tripping Behavior of Fuses Based on Data Sheet Characteristics and Conductor Material Properties,” Manuel Seidenath, Martin Maerz
• “Transient Thermal Analysis of NH000 gG 100A Fuse Link Employing Finite Element Method,” Michał Szulborski et al.
• “Circuit Breakers in Low- and Medium-Voltage DC Microgrids for Protection against Short-Circuit Electrical Faults: Evolution and Future Challenges,” Bayron Perea-Mena et al.
• “A Remedial Control for Short-Circuit Fault in NPC/H-Bridge Inverters without Redundant Component,” Sajjad Ahmadi et al.

Sources

• IEC 60269-1:2024, Low-voltage fuses—Part 1: General requirements
• IEC 60269-4:2024, Low-voltage fuses—Part 4: Supplementary requirements for fuse-links for the protection of semiconductor devices
• IEC 60269-6:2010+AMD1:2021 CSV, Low-voltage fuses—Part 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems

(Last accessed: October 14, 2026)


*This article was compiled based on the author’s understanding, with reference to the above literature.