5
\$\begingroup\$

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?

\$\endgroup\$
5
  • 1
    \$\begingroup\$ Just a thought: if it was some kind of issue, wouldn't you like, be able to read everywhere about it? \$\endgroup\$
    – PlasmaHH
    Commented Aug 8, 2016 at 13:19
  • \$\begingroup\$ I can see it now. "Fuse Insurance" \$\endgroup\$
    – Rob
    Commented Aug 8, 2016 at 13:38
  • 1
    \$\begingroup\$ Related: electronics.stackexchange.com/questions/209255/… \$\endgroup\$
    – sweber
    Commented Aug 8, 2016 at 14:14
  • \$\begingroup\$ Bussman say its not an issue with modern fuses. link \$\endgroup\$
    – Steve G
    Commented Aug 8, 2016 at 14:50
  • \$\begingroup\$ I don't know what "fuse fatigue" means. But I have had problems in production designs where fuses blew because of inrush. These were battery powered consumer devices, so there was an inrush event every time the consumer changed batteries. We were able to simply use a larger fuse without causing any additional problems. After that, I learned how to interpret the datasheet repetitive pulse ratings for fuses. \$\endgroup\$
    – user57037
    Commented Aug 8, 2016 at 15:20

3 Answers 3

4
\$\begingroup\$

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.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ +1 for the useful rule-of-thumb. Toroidal mains transformers are particularly nasty if no PTC is used- depending on how the core was left when turned off they can draw a massive current surge at the next turn-on. \$\endgroup\$ Commented Aug 9, 2016 at 15:12
5
\$\begingroup\$

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.

\$\endgroup\$
3
  • \$\begingroup\$ I have a 30+ year old microwave and about 15 years ago the fuse blew. Oh no, my micrwave is bad! No, I replaced the fuse and it's been fine ever since. Microwaves get cycled a lot and the fuse heats up and cools down everytime. The original fuse was sand-filled and probably not as suseptable to fatigue. \$\endgroup\$ Commented Aug 8, 2016 at 17:00
  • \$\begingroup\$ The sand filled (ABC) types break faster than glass AGC types- less arcing I think. \$\endgroup\$ Commented Aug 9, 2016 at 15:13
  • \$\begingroup\$ @RobertEndl My twelve-year-old microwave's fuse blew about ten years ago. I replaced the fuse and it's been fine ever since. \$\endgroup\$
    – Kaz
    Commented Aug 9, 2016 at 15:22
5
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.