When is fuse fatigue a concern?

Question

I've heard fuse fatigue mentioned as a possible cause for unexpected fuse failure. I've seen some documents claim fuse fatigue isn't an issue on certain modern fuses, but I'm not clear on exactly the details. In what circumstances, or with what fuses, is fuse fatigue a possibility?

Answer 1

Fuse fatigue is deterioration due to repetitive thermal stress caused by short-term overcurrent pulses that are part of the normal operating cycle of the fused system. For example, with full-voltage starting of an induction motor, there is a short period of starting and accelerating current that greatly exceeds the normal operating current. Fuse manufacturers provide application guidance for a variety of such situations, but there may be a concern that that information does not cover every eventuality.

The harmonic content of the input currents to power electronic equipment may be another factor that could cause fuse fatigue. In addition, power electronic equipment often has electronic protection that "instantaneously" shuts off the output in the event of a short circuit, but may let through a very short high fault current pulse. The electronic circuit may be designed to withstand those situations, but it could still cause fuse deterioration. As a result, many electronic designers have concluded that electric protection should be designed to protect the equipment from output short-circuits, overloads and most other abuse, but fuses should be selected to prevent fire, not to prevent the failure of electronic components.

Whenever the normal operating current contains pulses or repetitive surges, the guidance of the fuse manufacturer needs to be carefully applied. If the guidance doesn't adequately cover the situation, there could be a risk of a fuse fatigue problem.

Answer 2

Charles Cowie's answer is spot on, but I thought I'd include the only case of fuse fatigue I've run across.

One day, my 10-year-old Saab died on the highway. No fuss, no drama, the engine just quit and I pulled off on the shoulder. The starter worked, and the dash was normal, but the engine would not fire. I took off the cover to the fuse panel, and all the fuse elements were good. After a great deal of head-scratching I looked more closely at the fuses, and the one for the fuel injectors was...different. The exposed conductor was bowed outward and clearly longer than the others. Out of curiosity I touched it, and the element crumbled to dust.

Well, that pretty much identified the problem, and I raided one of the others for a replacement, and the car started right up.

It was clear that normal operating cycles had heated and cooled the fusible element, gradually changing its structure. When hot the element got longer and bowed out, and eventually it took a set. Presumably the metallic structure of the conductor also changed, and eventually a microscopic crack developed, and the fuse failed open.

Enclosed fuses are presumably made of a different material, but it seems reasonable that the same principles should apply.

Answer 3

Fuse fatigue is a factor due to current pulses that cause heating/cooling (expansion/contraction) cycles of the fuse element. A fuse not only has a current rating, it has a current pulse rating, known as the I²t rating -- this rating determines the amount of heat energy required to melt the fuse element in a single pulse of current before any cooling can take place -- for through-hole and clip type electronic fuses, this time is 8ms.

The fatigue limit on I²t, if you will, depends on the fuse element material -- zinc-based elements have a high coefficient of expansion and a low melting point, so they are more prone to fatigue failure than the silver or copper elements used in today's large fuses as per this Bussmann note. For smaller fuses, which likely still use the zinc alloy elements for cost reasons, a max repetitive pulse I²t of 20% of the fuse's rated I²t is recommended by Littelfuse.