Do fuses wear out? 1

[jmbelectrical]

I'm working on a tenant improvement project at a commercial facility that was constructed approximately 20 years ago. All of the large (600A - 2,000A) 480/277V and 208/120V distribution equipment are comprised of fusible distribution switchboards. While I know that circuit breakers can wear out over time and require replacement, is the same true for fuses? Would it be prudent to replace all of the existing fuses on the switches that we intend to re-use?

 

[itsmoked]

It's a hard question to answer. In smaller fuses that protect devices, they definitely wear out. They're just a small calibrated wire. Every power-on stresses them both thermally and mechanically because they get longer due to heating then contract as soon as the starting transient ends. Estimating it is very hard to do because even tiny differences in the load profiles make huge differences in the stress cycling.

I once had a fuse blow in a big TV. I replaced it with the identical fuse then turned on the TV. I happened to be looking at the fuse when I turned the TV on and was in dim light. To my amazement I saw the fuse wire turn red and bow dramatically for the TVs inrush event. It promptly returned to it's original length and color after the event. All this took place in about a second and a half. I realized this was why the fuse had failed. It failed every few years so I just taped a box of fuses nearby.

With large power fuses you can have the same phenomenon especially if they're protecting something like a motor with huge starting currents that reach near the fuse's protection limit. If on the other hand the fuse is protecting a panel that has small loads that are always present then the large load excursions are not going to be present. More likely with large fuses is weathering. Corrosion with respect to their holders and loss of spring tension in their holders (due to the same heating phenomenon)is probably more likely than a fuse element fatigue failure.

In your case I'd probably look at each fuse. If they don't look particularly 'weary' I'd run with them. If one does look tired I'd replace it. Definitely check the holders for being sprung.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

I have had that experience with superfast semiconductor fuses in the 350 - 630 A range. The fuse element is embedded in quartz sand and seems to wear out if stressed near trip point many times. A guy in the know (fuse manufacturer's lab) told me that repeated cycling made the abrasive sand wear the fuse element out. I haven't heard that anywhere else. But it makes kind of sense when someone tells you about it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

I believe there can be some aging due to thermal cycling and heat. After 20 years, there may be a good number of mismatched fuses anyway, so replacement is probably a good idea.

 

[sunnysky]

There are Arhnenius chemistry effects that affect aging on conductors, semiconductors dielectrics, resistors and fuses alike with details stress factors that define the acclerated againg effects acoording to the design specs and chosen part. Inrush, temperature, vibration, shock levels all affect FIT or MTBF.

Ground Benign vs ground mobile vs Air vs aerospace enviro also affect stress rating factors/

Consult MIL-STD-HDBK-217E for stress factors, weighting. etc.

Tony
EE since '75

 

[Zogzog]

Use a microhmeter and compare fuse resistance to manufactures published values.

 

[jmbelectrical]

Great information all around! Thanks for the responses, everyone!

 

[oldfieldguy]

Zogzog-

Good advice if given to the right person. I'm saying this because I had a technician give a helper the job of doing a microohmmeter check of some fuses. Wasn't a bad idea for 300E, but using the 10-amp DLRO on some 0.5E fuses meant we paid for a new bunch of fuses from the expected profits on the project.

And next time I have premature fuse openings, I'm remembering "Arhnenius chemistry effects".

Actually, I seem to find high resistance fuse holders leading to thermal pre-loading to be a more common problem.

old field guy

 

[Zogzog]

Good point OFG, I was assuming common sense would prevail but should know better than that.

 

[TLCPA]

To add to dpc's comments, the fuse holder is also subject to thermal cycling. Thermal imaging of the equipment may show loose connections between the fuse and the fuse holder.

 

[TheBlacksmith]

I would think thermal imaging to be a very useful tool if available. Neither a good fuse or holder should generate significant heat.