Recently we were doing some maintenance at a solar farm, investigating some blown fuses.
Per the normal process we found some nasties with our thermography analysis.
Both of these combiner boxes were blowing fuses.
In the end it was found that the fuse holders were not creating good connections.
We observed a classic THERMAL RUNAWAY.
Put simply the fuse generates heat, the fuse element gets hotter, resistance increases with temperature, increase in temperature makes more resistance than more heat. More heat makes more resistance, and so on.
This busbar section was running at 64.9degC, hot enough to damage the plastics of the insulation.
These connections were running at 117degC - hot enough to start a fire! Dangerous stuff!
What is a thermal runaway?
Thermal runaway with fuses happens when a fuse, instead of cleanly blowing at its rated current, gets stuck in a cycle where heat builds up faster than it can be dissipated. Here’s how it works step by step:
- Fuse Basics
- A fuse is designed to protect a circuit by melting its internal element when too much current flows.
- The melting point is determined by current, time, and the ability of the fuse body to dissipate heat.
- Heat Generation
- Electrical current produces heat in the fuse element (I²R losses).
- If the current is just under the fuse’s blow rating, the fuse element runs hot but doesn’t melt immediately.
- Poor Heat Dissipation
- If the fuse is in a hot environment, has poor ventilation, or is undersized, it may not cool effectively.
- This causes the element temperature to creep upward.
- Resistance Rise
- As the fuse element gets hotter, its electrical resistance increases.
- Higher resistance produces even more heat for the same current (since P = I²R).
- Self-Accelerating Loop (Thermal Runaway)
- More heat → higher resistance → more heat.
- The fuse may either:
- Blow prematurely, even though the current is within spec.
- Or, in some cases, not blow cleanly and instead partially melt, arc, or carbonise, leading to dangerous overheating.
- Consequences
- Surrounding insulation or components may overheat.
- The fuse body can discolour, crack, or even ignite.
- Instead of protecting the circuit, the fuse itself becomes a hazard.