In real-world power system operation, many technicians encounter a puzzling phenomenon: a transformer’s load remains well below its rated limit, yet the high-voltage fuse blows during a scorching afternoon. This is often not due to overcurrent, but because the impact of ambient temperature on the fuse’s thermal tolerance was underestimated. According to IEEE C37.48.1 Section 6.2.4, ambient temperature is a critical factor determining fuse longevity and reliability.

1. Why Do Fuses “Fear Heat”?

A fuse operates on the principle of thermal equilibrium. Its melting depends on the balance between Joule heating (I²t ) and heat dissipated to the surrounding environment.

• Standard reference point: Most high-voltage fuses are rated at 25°C or 40°C ambient temperature.

• Physical effect: As ambient temperature rises, the fuse’s ability to shed heat diminishes. To maintain equilibrium, the maximum current the fuse can safely carry decreases. Running a fuse at its standard rated current in high temperatures risks thermal fatigue and nuisance tripping, even under normal load conditions.

2. Derating: A Fundamental Design Principle

IEEE C37.48.1 Section 6.2.4 emphasizes derating fuses when ambient temperature exceeds the standard rating, typically 40°C.

• Ambient temperature definition: Not just outdoor air temperature. Crucially, it includes the actual temperature around the fuse. For example, a fuse installed in a sealed metal enclosure may experience temperatures 20°C higher than the external environment due to heat from the transformer or limited airflow.

• Derating coefficient: Manufacturers provide specific derating curves. As a rule of thumb, for every 10°C increase, the continuous current rating may decrease by 5–10%.

3. The “Greenhouse Effect” in Enclosed Spaces

Section 6.2.1 highlights the risks of enclosed installations:

• Restricted cooling: Limited air circulation causes heat accumulation.

• Adjacent heat sources: Transformer cores and windings transfer additional heat via insulation oil or air to the fuse.

Result: Without a 10–20% derating margin, fuses in enclosed cabinets are prone to premature operation or even physical damage.

4. Selection Strategy: Current Is Not Enough—Consider the Season

When designing and selecting fuses, engineers should follow these steps:

1. Determine maximum ambient temperature: Include both extreme summer conditions and equipment self-heating.

2. Consult manufacturer derating curves: Match the calculated operating temperature to the correct derating factor.

3. Back-calculate the rated current: For instance, if the actual load requires 10A and the derating factor is 0.9, choose a fuse with at least

   $$10÷0.9\approx 11.1\text{A}$$

   rated current to ensure safe operation.

5. Summary

High-voltage fuses are not mere switches; they are precision thermal devices. Ignoring ambient temperature is equivalent to overdrawing on the device’s reliability reserve.

Professional guidance: During summer peak load inspections, pay attention to fuses in direct sunlight or poorly ventilated enclosures. IEEE C37.48.1’s advice is clear: never let environmental heat become the invisible killer of your fuses.