CC, that is why I check the supply voltage, not only do fuses go half bad, but sometimes connections fail, like splices inside cable ducts. No, there are not supposed to be splices in cable ducts on panels, but I have seen them and seen them fail. A bit tough to spot sometimes.
Years ago I ran into an intermittent problem. Of course I didn't check the fuse since there is no such thing as an intermittent fuse! During several rounds of troubleshooting, everything worked fine, but when replaced, the system would fail. We would again remove it, remove the cover, set it on the bench and all systems checked out. We spent hours looking for a loose wire or bad connection. Finally we pulled the fuse and it looked OK. More testing ensued. We pulled the fuse again and tested it and it was bad. I tapped it with a finger and one end of the fuse wire vibrated. I did it again and it shook but suddenly stopped vibrating. When tested, the fuse now showed good. After several repetitions, it was possible to orient the fuse so the broken wire would make contact, then rotate it, tap it, read open reorient and connect, and then test good. Apparently, the orientation when operating was the open state, and when on the bench, it was rotated to the fail state. Ever since then I not only test, I also tap it once or twice while reading the meter. Of course it's never happened again, but you never know.
Fuse failure from thermal cycling is why military vehicles hace circuit breakers instead. And does anybody remember the "battle switches" that were used to bypass the fuses in the much older military equipment? That was in the era before they had circuit breakers.
Now, we see that a lot of the higher reliability fuses have coiled elements and an inert powder to support the fusible element. The powder also serves to absorb the heat and cool the plasma when the fuse opens a high overcurrent fault.
On the way home one evening many years ago, my motorcycle quit running and I coasted into a parking lot to troubleshoot. It would crank over and try to fire, spit and sputter when cranking, but wouldn't run. Daylight was fading quckly. I checked the fuses and they appeared to be intact. I feared an engine failure. By then it was nearly dark, so I crossed the street to call home for a ride, planning to deal with it the next morning.
I was working at a motorcycle dealership at the time, so I called a coworker that lived near my home. He brought a trailer the next morning and we loaded and took it into the shop. One of the mechanics checked it while I got other things going for the day. He found the fuse element was broken but the ends of the element would touch until they were heated by current flow. Then they would distort and open the circuit.
The mechanics at the dealership joked about installing a 30 day (or some other time frame) fuse into a customer's bike so they would have to bring it back in for repair, not knowing it was just a fuse. Of course they didn't have any such fuses.
That was many years ago now and I've come across similar problems in automotive and industrial applications a number of times. It seems the element fatigues from repeated cycles of heating and cooling when conducting current. This expansion and contraction causes the element to break but the ends do not separate until heated by current flow. I've also seen similar problems with light bulb elements that test good with an ohmmeter but open when heated.
Contrarian wrote: "found an 1157 dual filament bulb that had a broken filament that was touching the adjacent filament"
Yup, seen that one before. Luckily was thinking about the problem at work, so forced to figure it out before spending any time with the car.
I had an 70 Opel GT. One of the fuses (10A, wide element) fatigue-cracked through. You couldn't see daylight through the element when it was under the dash, but it had the peculiar property that it would heal and carry a low current but open with higher current. When all current was removed it would heal again. Something like: you could play the radio but if you turned on the wipers both the radio and wipers would quit. If the ignition was turned off for a few seconds you could repeat the sequence. After several minutes I noticed that the fuse element wasn't flat; both ends of the waisted part of the element formed a V with vertex in the center. I removed the fuse from the clips and held it directly in front of my 100-watt trouble light and could see the hairline crack. Several minutes to find--less than a minute to fix.
Several decades ago I was working on a gas plasma laser power supply, on a tight shipping schedule. As we were trying to get the display system and the 2 lasers out the door on a Friday afternoon for a Saturday night show one of the lasers's power supplies would not strike the tube the interlocks wouldn't allow the water cooled series pass bank to energize. After an hour of poking around I discovered that someone had reversed the water flow (in to out and out to in) why they did such a thing I will never know but when we got it to fire up it popped two of the three the 35 amp fuses, So we sent some one off to the electric supply store before they closed for the weekend to get a box of 10 because we needed some for the shop anyway.
We replaced all three fuses, powered the unit up all three fuses popped as we brought the current up to opperating power. There were 30 transistors on the water cooled series pass bank so we started to pull them and test them because "they were usually the problem". An hour or three later after finding no bad transistors we powered up the unit with fresh fuses and again as we brought up the current control up all three fuses popped again.
We were now running out of time, so the owner called out to a friend and borrowed a power supply we kept trying to get the other power supply going to no avail blew all of the new fuses. The barrowed power supply arrived and worked, the show went on!!!
Saturday morning I went to the same electric supply house and picked up 2 boxes of 10 fuses, hooked up a different laser head to the supply and blew all of the fuses again. We needed the supply later in the week so I continued to trouble shoot the supply after all Saturday 12 -14 hours and all day and night Sunday I had blown all of the fuses But had cleaned and checked EVERY THING in that power supply at leased 3 or 4 times.
Monday morning on the way in to the shop I stoped by the electrical supply house to pick up another box of fuses. In talking to the counter man I discovered that they had been notified about a bad batch of fuses that morning by a call from the factory. Sure enough the boxes of fuses that I had spent all week end blowing/testing were in the bad batch!!! The electrical supply house went to a competitor and brought over a different brand of fuses the supply worked perfectly for several years to come. One lost weekend that I will never forget!!!
I have seen this a lot with dual filiment bulbs. Now whenever I have lighting issues and the system is working (not the fuse) but working erratically, the first thing I check is the dual filiment bulbs in the eratic circuit. Usuallyit is one of the primary bulbs in the function (brakes, turn signals etc.)
I had a "hidden" fuse break happen to me in high school; been there/done that!
This sounds like a common problem, so it's time to go to market with my Fuse Tester. A fuse clip, momentary switch, and AC plug all connected in series. Install the suspect fuse into the clip. Plug the tester into a wall outlet and press the switch. If you see a blue flash the fuse *was* good! :)
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