I'm wondering if there is information out there on stability of fuse elements over time. Is there a "shelf life" on fuses that have been in storage or in service for 5,10,20,40+ years? How do we really know?

Manufacturers will say the elements are stable if kept within the parameters of their design (mainly ambient temperature/humidity ranges, etc.). But how do we know this to be true? Of course testing fuses is not feasible for a customer. When we're talking about MV and other specialty fuses, those can get expensive to replace just for the comfort of having shiny new fuses out of the box.

Thoughts? Thanks for the input!

Typically fuse use shelf life / life expectancy is 10 years.

I've got to think a very large percentage of fuses in use at the moment are more than ten years old.

After about 20 years I've found that fuses tend to trip low. I've never seen one trip "high" personally although I have heard of a few cases of failure to trip with the boric acid (expulsion) types but it's very rare. I've also seen fuses that get hot spits (IR scans) when they get old but it's not really the fuse...it's that the fuse clip metal gets a bit weak over time and the spring loses tension. A lot of folks just use some electrically conductive grease and grease the fuse to get better contact but ultimately the permanent fix is changing the fuse clips.

I'd email some of the manufacturers and ask. I've had pretty good luck with folks like Littelfuse as well as Bussmann and Mersen. All have been responsive to my questions.

For MOST common fuses (RK-5's, etc.) think about what the fuse is...it's a piece of solid copper metal with a special pattern cut into it surrounded by silica sand packed into a container. Sand and copper are mostly chemically inert and only the copper can actually break down (corrode) over time. So there's really not much in the way of anything that can degrade other than mechanically external things to the fuse. With the boric acid fuses the boric acid/salt is hygroscopic (sucks up water) so if the seals go the fuse can swell and break over time. But more commonly I've seen them with a fiberglass housing where the resin starts to break down in sunlight which compromises the fuse enclosure. With any of these failure mechanisms, simple tests like low ohm bridges, visual inspections, and IR scans are really all you need to test them.

One word of warning...a lot of fuses have some kind of "trip indicator" such as a small piece of metal that is supposed to pop up or a spot on the side that is supposed to change color. Don't believe them for a second. I've seen plenty of these indicators fail...either it fails to trigger when the fuse triggers, or it triggers when in fact the fuse is still operational. This is not limited to a single manufacturer....they're all suspect.

The metal elements do degrade under load through successive thermal cycles such that they effectively thin out and "trip low" as PaulEngr suggested. But this is from duty under load, not sitting on a shelf.

Similarly, fuse connectors degrade under load and load cycling in the same way all connectors do, and may overheat to the point that the fuse catches fire just from high contact resistance, the same way some individuals in a population of similar connectors (by current class) would be expected to overheat after long duty. Again, this does not happen on a shelf.

The insulating materials of fuses, though, degrade on their own, and more so with exposure to the environment. Composite materials become brittle, particularly with exposure to heat and UV. I've seen outdoor mounted fibreglass fuses that have suffered so much epoxy degradation that the glass strands are sticking out and they look like they need a haircut. In this condition, the fuse housing is unlikely to withstand a fault and could explode when called upon to operate. If this is a current limiting fuse, the sand would be expelled and the current would no longer be limited and the fuse may not interrupt at all. Example in photo attached.

A little clarity on the topic:

1. As was stated, the fuse element is basically inert and has no known shelf life. The body on modern current-limiting fuses are also basically inert and shelf life is not an issue. Most of the comments that follow pertain to 600V class fuses.


2. As far as cycling fatigue, this is due to incorrect fuse sizing, if you size the fuse so that the the inrush current does not exceed a line that is drawn at 60% of the minimum melt TCC, you will not have element/crystalline fatigue.


3. The biggest problem is loading a Class J or Class R fuse more than 80%, this is also true of a circuit breaker. Basically, terminal temps (fuse or breaker) will exceed approx 167F (in a 70 deg ambient) if loaded more than 80%.


4. 100% devices such as some circuit breakers and Class L fuses are only 100% rated when the enclosure and fuse are tested/mated together.


5. For those doing arc flash studies, here are some fuse tips that dramatically reduce the incident energy:

A. Class RK1 or Class J are the fuses of choice to reduce the incident energy, these are a max size of 600 amperes and 600 volts. Class J is best used on new projects, the Class R is best where there is already an installed base of Class R fuses.

The fuses that follow will typically get the incident energy close to, or below 1.2 cal/sq cm.

B. Class RK1 - 600 Volts and 70 through 200 amperes: Mersen (Ferraz) A6DR + amp rating. Bussmann LPSR + amp rating

C. Class RK1 - 600 volts and 225 through 600 amperes: Mersen (Ferraz) A6KR + amp rating. Bussmann KTSR + amp rating.

D. Class J - 600 Volts and 70 through 200 amperes: Mersen (Ferraz) AJT + amp rating. Bussmann LPJ + amp rating

E. Class J - 600 volts and 225 through 600 amperes: Mersen (Ferraz) A4J + amp rating. Bussmann JKS + amp rating.


6. Cases above 600 Amperes at 600 volts or less:

A. For applications above 600 amperes the Mersen A4BQ or the Bussmann KTU can be used to reduce the incident energy. In most cases they will not bring the energy to below 1.2 cal/sq cm.

B. When trying to reduce the incident(above 600 amps) to below 1.2 cal/sq cm, it is possible to use Mersen A50QS + ampere rating fuse types. These are semiconductor fuses used for the protection of solid state power conversion equipment. Boltswitch manufactures Class L switches that will except these fuses through 1600 amperes.


7. For applications where a primary fuse is used to reduce the incident energy on the secondary side of a 15 kV class transformer. This is typically done to reduce the incident energy on the 480V side of a 15 kV class transformer.

A. Conventional 15 kV Class fuses: Mersen HMH (9F60HMH). These are available in 65E, 80E and 100E, therefore, work through 1500 kVA.

B. If the incident energy must be reduced further, in lieu of the HMH fuse, the new Mersen Medium Voltage Controllable Fuse can be use. This is a hybrid fuse that is relay controlled in the arc flash mode.

I always see recommendations from fuse manufacturers to use Class J fuses instead of the old RK series and indeed they are much smaller and lower priced packages, and the curves are much sharper. However the fuse holders are always a pain in the rump to get, prices are higher, and drive delivery times up whenever I specify them. Using a classic RK fuse eliminates all these problems at the expense of slightly more expensive fuses and since the fuse cartridge is not necessarily all that important you can of course get upgraded RK fuses (RK1's or the newer fancy ones) that have curves similar to class J's anyways.

1. RK1 and Class J have the same exact characteristic/curve. If the curves are different, the comparison is between different vintages.

2. They are very close to the same price.

3. If the switch is a UL98 heavy duty switch, the load side fuse clip/base can be moved towards the line side clip to convert the R/K/H dimension fuse into a Class J. There are actually three sets of tapped holes, Class J, 250V Class R and 600V Class R.

4. You are correct though, if you have an installed base of Class R fuses, it is best to stay with the Class RK1 on expansions. If the project is a stand-alone or new site, the Class J is best. The Mersen AJT and the Bussmann LPJ are more available than the Class RK1 fuses (A6D/LPS)

That may have been true with paper body fuses from the 70's and 80's, that definitely is not true with a modern current limiting fuse. The bodies are non-hydroscopic and the the elements are either silver or copper, therefore, a better answer would be "unlimited shelf life".