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Can a fuse degrade over time?


Mar 2, 2010
pacific northwest
I am running a 9hp three phase shaper and just blew a fuse on startup- there is a disconnect switch with three 30 amp time delay fuses. Probably 30 years old.
There was no apparent difference between starting it this time and any time before. Stuck in a new fuse and it works fine.
Just wondering if time or cycling can degrade a fuse.
Fuses degrade with time and will eventually fail. A blown fuse does not always mean that there is something wrong with the equipment. If the fuse does not operate then it will typically last 20 - 30 years as long as it is operating within its design limitations. If it is in a higher cyclic operating temperature i.e. very cold or very hot, then this can reduce its life.
Imho yes, when the fuse carries current it heats, cyclic heating and cooling can change the structure of metals, there’s got to be oxide changes and such, if it gets very hot then electrons got slung off like vapour deposition and probably stuff that will be the basis of someone’s PhD dissertation.
I’ve had old ones just fail, perhaps the initial start surge was bigger as the parent machine ages too.
However if a fuse fails you are duty bound to check stuff out, earth bonding, insulation test at 500 and 1000, megger if you will, a pat test machine is a useful test, tests everything and prompts you what to do, if the meter fails the appliance you can bet there’s something wrong.
Are fuses diamonds? No. Are they the Earth? The sky? No. Therefore, science tells us they don't last forever.
The RK5 dual element time delay fuses have a starting cycle life.
If you disassemble the failed fuse you will find that the spring loaded portion has opened up. There is a risk that a 3 phase motor will continue running on the remaining two leads and become overloaded.

You may find that it is more cost effective to buy a surplus 3 phase circuit breaker with a motor starting 100 to 600% over current adjustment than it is to replace time delay fuses on motors with frequent start/stop cycles.

Eaton has a illustration showing the construction of the dual elements.

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A fuse that lasts 20 or more years of daily use is not significantly under-rated. The same fuse rating and type from same maker should work fine for as many years into the future.

You can actually determine how many starts a fuse will last through, if you know the start current and time duration of the start.

Every fuse has an "I^2 * T" rating (current squared x time). You may not be told that rating, though. Some manufacturers always give it, others not.

That rating is the energy it will take to blow. Depending on how close to that energy your start is, it may blow in under 10 starts, or last many hundreds of thousands of starts.

With smaller glass type fuses, you can see the fuse element "squirm" under the inrush current. The fuse can open from a melting effect, after the wire has been thinned by partial melting, Or, it may fail from a mechanical flexing due to thermal expansion. If it partly melts (extreme cases) you can sometimes see the element has "sweated out" tiny globs of material.

The bad thing is that the very same nominal rated fuse from different manufacturers may have very different values for the "energy rating". A higher current fuse may actually have an energy rating the same as a lower current fuse (or even less).

It just has to do with the material in the fuse, melting point, thermal mass, etc. Figuring that out is why the fuse companies get the big bucks (at least for the good companies, import copies are different).

If the spring loaded portion opens, that is more from the "time" part of the formula. The start was not an excessivly high current, but it lasted a long time.

If the fusible section opens, that is more from the "current squared" part, the peak current was too high during the start. That guides what is needed to keep the replacement fuse from blowing.
European car fuses used to have an exposed element you could see with the fuse installed. Older ones got a gray lead corrosion with time. American fuses were in a glass tube, sealed?, and did not noticeably corrode.
This was all 12 volt stuff.
I talked my brother on how to replace a bad home screw in fuse. It checked good with a meter but under load it would open. I just had him replace one at a time. Started at the top feed and worked down to which sub panel. Then just random until it no longer failed. Probably internal corrosion from being outside in the weather in a external fuse box for 40-50 years.
Bill D
OK- you guys are freaking well of knowledge!
I will assume that the fuse was likely degraded over time and use, and replace the other two with new ones also.
Best practice is routine inspection of fuse, looking for discolouration, thinning and maybe shooting with a temp gun. It's called EPM, electrical preventative maintenance. Recommended intervals are 3 monthly for inspection and 3 years for test and maintenance. They are codified in NFPA and will probably be making their way into NEC and other electrical codes. Suggest you have spare fuses on hand when you are testing them.
I have seen fuses that developed a slight resistance, enough to cause a voltage drop across the fuse and caused intermittent failures in the circuit it protected. I have also found where the the internal solder joint was not complete which caused interesting failures intermittently. Fuses can be glass enclosed up to a voltage that I cannot remember. Above that voltage the case will be an unbreakable substance. The case has to be able to contain and vent the gas pressure generated by the arc when the fuse blows. Usually, the higher the voltage, the longer the fuse body.

Thinking about it, the shaper did not always start exactly the same.
Sometimes it just instantly started up, and sometimes it would chatter just for a fraction of a second while starting. I have not tried but once since replacing the fuses.