Perfectly normal looking fuse

[WayneG]

I was out at the 38th Woodstock reunion with my MH and I was getting weird voltage readings in the house circuit. I had my converter on and it was putting out 13 volts or so, but my house voltage was 11 and change.

I went to the fuse panel and found that one side of this fuse had the 13+ and the other 11+ and even though the load was not very high, the fuse was warm.
I swapped it out with a spare and both sides showed 13+ volts.

So a new lesson learned, even if a fuse looks good, it may not be

[TG Twinkie]

Yes. They tend to cook slowly, then become detached from the end cap. What you see is the interim result.
It this kind of thing that causes problems in furnaces. The motor won't get up to speed and set the sail switch.
It's a good idea to replace these old glass fuses from time to time. OR upgrade to the blade type.

[uncle_bob]

Hi

If things are acting weird, don't trust a visual check. Use a multimeter.

Bob

[WayneG]

Originally Posted by uncle_bob

Hi

If things are acting weird, don't trust a visual check. Use a multimeter.

Bob

Ohmmeter showed 0 ohms, (that was when cold) The voltmeter told me the story when in circuit.
If I get a chance I will slice the fuse to see what is going on inside

[rmkrum]

And make sure you let the multimeter settle...had a fuse that looked ok, but when I really measured its resistance, it was much higher than normal..if I just tapped it with the probes, the meter headed toward zero ohms, but did not make it all the way...

if both sides of a fuse is accessible, a voltage drop check across the fuse in its holder is the best indicator of trouble. If there is a significant voltage drop, it needs fixing--a couple volt drop across a 12 volt fuse is a bad sign...

Typically glass body fuses have a drop of solder inside the end caps to connect to the fuse element/link. The solder crystallizes because of vibration, increases in resistance, and starts dropping voltage across the fuse, which leads to more heat, and more weird voltage-related issues in the rest of the circuit....

Blade-type fuses tend to have less of this silliness...

[Wingeezer]

Good point - I have also encountered this and now as a matter of routine always check continuity even if the fuse looks good - t'aint always so!

[Wayne&Sam]

Woodstock was 48 years ago. We're you in Bethel?

[WayneG]

Originally Posted by Wayne&Sam

Woodstock was 48 years ago. We're you in Bethel?

Yes we were!, had some folks play on my front porch again this year.
Only one rainy day this time, but it aint Woodstock unless you get some red mud on ya!

[WayneG]

I took apart the fuse and found one end was slightly loose in the end cap.
After some gentle wiggling, the end of the fuse link came out of the solder in the end cap.
For now I ordered gold plated fuses, but if I was going to redo the panel I would probably use aircraft circuit breakers like I did for my solar gear

[66Overlander]

Originally Posted by WayneG

Ohmmeter showed 0 ohms, (that was when cold) The voltmeter told me the story when in circuit.
If I get a chance I will slice the fuse to see what is going on inside

A standard ohm meter is not accurate enough to test for anything more than a completely open fuse. For example if your total 12V load current was 10A (mainly to charge the 12V battery), you'd get a 2.0V drop across the fuse if the resistance of the fuse was only 0.2 ohm, which would round to 0 when checked with an ohm meter, but which would be a bad fuse. The proper fuse resistance should only be a few milliohms (thousandths of an ohm).

The poor solder joint could have easily added a few tenths of an ohm resistance to the fuse, so I think you likely found the source of the problem.

[WayneG]

Originally Posted by 66Overlander

A standard ohm meter is not accurate enough to test for anything more than a completely open fuse. For example if your total 12V load current was 10A (mainly to charge the 12V battery), you'd get a 2.0V drop across the fuse if the resistance of the fuse was only 0.2 ohm, which would round to 0 when checked with an ohm meter, but which would be a bad fuse. The proper fuse resistance should only be a few milliohms (thousandths of an ohm).

The poor solder joint could have easily added a few tenths of an ohm resistance to the fuse, so I think you likely found the source of the problem.

Really need a MHO meter for those resistance levels, that is bench top stuff, reading the voltage across the fuse under load is the easiest test of a problem.
Also add heat to the equation and if the fuse solder joint has a PTC response to a high current load, that will increase the voltage problem. Worse case scenario; The solder starts to melt and I am guessing that you could get a lead vapor arc develop inside the fuse!

[Lingtodt]

Usually, not always with the blades. I've learned not to trust my eyes. I've learned to follow this advice also when looking in the mirror.

Best way to check is with a new fuse that you know is good.