12V Fuses - How Long Should They Last and Do They Wear Out Over Time?

We had a problem with one of the fuses in the HWH 310 manual leveling system in our MH.  While retracting the leveling jacks, the system just quit retracting the jacks, leaving two of them semi-retracted.  We couldn't lower or retract the jacks from that point on and we couldn't pry them up.  We suspected the fuses but didn't know exactly where they were.  The HWH control panel was lighted and appeared to be working.  We then followed the clicking noise when hitting the 'store' button and found the HWH control box in a floor cabinet near the front door.  There were about 7 fuses on the back of that control box but they all checked out OK with my fuse tester. 

Then we located a couple more individual fuses in the outside front of the motorhome, in where the generator and the HWH hydraulic cylinders are.  One was a 15 Amp fuse and the other was a 40 Amp fuse.  The 15 Amp fuse tested OK but the 40 Amp fuse tested bad so I attempted to pull it out with my fingers.  When I pulled it out, it busted into several pieces as shown in the pictures below.  I replaced the 40 Amp fuse with a 30 Amp fuse (that's the highest I had) and the jacks then retractred successfully.  (We'll get a 40 Amp fuse before we use the jacks again.)

This brings me to my question.  The metal legs in the bad 40 Amp fuse looked fatigued and thin (not necessarily melted) and I'm wondering if the heat caused that when it burned out or if it somehow wore out and was weakened from normal use and 'weathering'.  Do 12V fuses go bad?  Do they wear out over time?  They were covered with the rubber or plastic fuse holders but I imagine being exposed somewhat to the open air there, moisture could still get in as I don't think they make an airtight or a real tight connection.  Any ideas or opinions?

Looks just plain corroded to me - I suspect water (or something worse) got into the fuse socket and it rusted away.

Fuses don't really wear out unless perhaps they are run continuously right at the edge of blowing. The constant heat of near-high amperage (which is what blows the fuse anyway) will eventually cause it to trip at a lower amperage than when it was new.

Low voltage on a circuit feeding a motor will drive the amps up, causing a fuse to blow when it would have been ok at normal voltage (12.6-13.6 dcv). And corrosion on the fuse blades inhibits voltage.

I'm with Gary.  It sure looks like moisture corrosion. It may have started with arcing in a loose fuse holder, causing heat and melting the plastic, but rust has eaten the fuse leg away.

I think Gary hit the nail on the head, that fuse is very corroded.  Indeed the corrosion goes up the one blade under the red plastic.  The swelling of the blade due to corrosion might have ruptured/weakened the plastic.  Looks like the thin fuse element was also getting corroded.  Your "40 amp" fuse might also have been less than the rated value when it blew.  Really should try and find where moisture is coming from a fix it.  Otherwise eventually the whole fuse box might look like that and need replacing.

Really should try and find where moisture is coming from a fix it.  Otherwise eventually the whole fuse box might look like that and need replacing.

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There is no fuse box involved.  The front compartment is open at the bottom so that's where the moisture is coming from.  Those two fuses, 40 Amp and 15 Amp, are independent fuses consisting of a wire in to the fuse and a wire out of the fuse.  These wires and the fuse aren't shielded in a harness or anything, but the fuse (sockets?) are completely covered.  I suppose it wouldn't hurt to pull the other fuse (the 15 Amp fuse) and see if that is corroded too. 

I'll echo corrosion as the cause as well.  

I would also be concerned about the real possibility of corrosion also existing in the fuse holder.  Dirty contacts and a new fuse can still cause problems.  Cleaning that corrosion may be hard.  I'd try some small pieces of sandpaper to clean the contacts, or equal.  After all parts are clean and the dirt removed, install the new fuse with a layer of grease on the fuse legs to help seal water out.  That will help to resist future problems.  In the extreme case where the fuse holder is too badly corroded, it will have to be replaced too.

I would also be concerned about the real possibility of corrosion also existing in the fuse holder.

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I don't think the fuse holder was corroded.  The fuse broke as I pulled it out but it was one piece when I initially tried to pull it out with my fingers.  There was some plastic or rubber stuck in one of the slots where the fuse legs go after I pulled the broken fuse out, but I pulled that out and one of the now broken fuse legs with a pliers.  But I didn't notice any other real corrosion anywhere except in the fuse itself.  Also, I didn't notice whether the the fuse cover was tight when I pulled it off initially or whether it was looser than it should have been.

"Do 12V fuses go bad?"

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Yes, just like any other component, a fuse can fail from age even when used well within its rating. But with most fuses, you can tell by looking if it failed by age or overload.

A fuse that fails from age will have a hairline crack type of break and clear class (or plastic). A fuse that blows from overload will have a much wider crack, usually with enough spark when it blows to make part of the glass (or plastic) of the fuse black or very dark.

Of course, the lower the current through the fuse (compared to its rating) the longer it will last, even if it fails only from age.

Even within the fuse's rating, there's heat very slowly eating away the fuse material.

The biggest problem with any small wirein a DC Circuit is metal migration.  It can be severe in some metals and the soft fuze wire is one of them.  Tungsten filaments in light bulbs, although hard, fail some what the same way but with a different mechanism.  Don's comment about the amount of current with respect to the fuse rating is a reflection of the migration problem as it is directly related to the current density in the wire.

Long answer to will age affect a fuse, the short answer is yes it will.

Fuses trip when the fusible link melts.  Corrosion at the junction of two metal pieces (the fuseholder and the fuse blade) produces heat when current flows through it.  Judging by the condition of the right hand terminal, that piece got pretty hot.  Also notice the fusible link failed right where it exits that piece.

Take another look at the contacts in that fuseholder.  I can't imagine how the fuse terminal could get in that bad of shape without also damaging the contact surface it plugs into.  If there's corrosion or roughness in the socket, a new fuse will only make partial contact and you'll start the process all over again.

Take another look at the contacts in that fuseholder.

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I'll try to do that when I get around to buying a 40 Amp fuse (the proper size fuse) to replace the 30 Amp fuse that I put in there temporarily.  Maybe I can even take a picture of it.  I was thinking that maybe the fuseholder is made out of a different type of metal (steel?) than the fuse itself and didn't corrode because of that.  I don't really know how the fuse got degraded so badly or if it just got worn down by heat.  Maybe the actual condition of the fuseholder will shed some light on that.

Another posibility is a bad fuse socket,,

Now.. I do not know if the fuse socket was bad and caused the corrosion and pitting and other damage you see. or if the corrosion caused a poor connection which caused the socket and fuse to overheat  (kind of which came first, chicken or egg)

But.. You need to replace the fuse socket.. That I do know

As to how long fuses last

I have had them last 10 years or more.. I've had 'em last 10 days or less, and no reason I could find for the fuse to fail (And the fuse element was NOT blown, it had failed at one of the ends)

As a general rule fuses do not "wear out". Your problem, in my estimation, is a bad socket. The clips in the socket can lose their tension. Losing tension could be caused by vibration or a loading problem. A general rule of thumb that I had been living with is that a fuse should operate efficiently up to 80% of rated load. The fuse has a time current charerestic and will operat at 100% of rated current for X amount of time and it could be for hours. If you run the fuse at anything over 100% of rated current the time it takes to blow quickly shortens. The clips can be subject to losing tension if you overheat them due to being close to the the rated current (40 amps in your case) and operating like that for long periods of time.. As has been suggested it could also be a corrosion problem too. To be on the safe side I would want to check the current draw when you get the system fixed and as I mentioned a good rule of thumb is the 80% rule.

OK, we got a 40 Amp fuse to replace the one that failed and while doing that I took some pictures of the fuse socket and fuseholder.  You can see something has affected the top half of the fuseholder but the bottom half appears OK.  It looks more charred than corroded to me.  I didn't attempt to clean it up before taking the pictures so this is what it looks like after taking out the busted fuse. We put the new fuse in and lowered the jacks and everything is running OK so far.

Rolf, like I said initially, I don't think there is any question that the fuse has been very hot.  The top connectionb is probably the loosest and was arcing, causing the heat.  The fuse would have simply blown if the current alone was causing the heat.  JMHO

Lou, yes, arcing apparently caused the heat which explains the charring seen in and around the top half of the fuseholder.  But I don't see any real evidence of corrosion in the fuseholder itself.  Maybe when I get around to cleaning it up and can see around the fuseholder better, I will see some evidence of corrosion caused by moisture.  Right now, I'd have to conclude that the corrosion seen on the fuse was not necessarily caused by moisture.

Rolf, I'm not sure just what you can conclude from a further study of the corrosion.  The fuse holder contacts would probably be made of some low grade spring steel (for tension on the fuse leg), and the fuse leg would be made of some softer metal to ensure good electrical contact and current flow.  They would most certainly react to any corrosive environment or stimulant differently.

Add heat, plastic, rubber, electricity and possibly moisture into the mix, and who knows what would result.

Well summarized, Lou, I agree with everything you posted.  But.....is there a chance that this fuse could have been made in China?  That might help to explain the unusual corrosion, maybe.  Just a shot in the dark.

Rolf, hang onto the evidence and I'll check to see if we can get a government grant to study the origin of the fuse.

BTW - Destroy any packaging that might have come with the replacement fuse.  We wouldn't want to prejudice or contaminate our research.