Melted fuse?!?!

[Rob]

I was trying to find out where the 20mA drain on the batteries is today and I found this:

The fuse still has continuity and it was the correct rating for the application. It was one of the headlight fuses.

How is it possible for a fuse to melt before it it fails? The only thing I can think of that could have caused this is arcing. Any other ideas?

The worrying thing is that this is not the only fuse with these symptoms. One of the 15A fuses from the fuse box under the steering wheel is slightly melted too, its difficult to show it on a photo so no pics. No idea where the other one came from as I removed all the fuses before I realised it was melted. My 80 has been suffering from a battery drain which will drain the batteries after 4-5 weeks which I can't find, not sure if this has anything to do with it.

 

[adrianr]

Ouch - cheap fuse perhaps?

 

[Rob]

Maybe, but it still does not make much sense, if it was just a cheap fuse would it not melt from the middle where the fuse wire is and not the side?

 

[Chas]

It's obviously melted at the top of one blade, so maybe it was arcing as the blade that was arcing would get hooter than the other.

EDIT;
or even hotter

 

[Julian Voelcker]

I would be surprised if the fuse was getting hot enough to melt the plastic, but not melt the wire and at 12volts it wouldn't be arking.

Were there any relays beside it that might have heated up?

 

[Rob]

The green relay right next to seemed fine. No damage to the fuse box at all. I have seen arcing on the multimeter leads when I was testing for battery drain last year when the preheater relay failed, was only 0.5A but was still enough to cause visible arcing.

 

[MuckyPup]

Not terribly unusual. That circuit is drawing close to 15A continuous with no sudden spikes.
It takes close to 40A to actually blow a 15A
You've either got higher rated bulbs or a high resistance in the circuit (same thing as far as the fuse is concerned)
Check your bulb holder terminals and the earth connection nearby.

 

[Dave Docwra]

Looks like slow burn syndrome, check to see if your headlight bulb has failed (but still working) by causing the bulb filament to arc across from dip to main at the same time...

 

[Towpack]

It takes close to 40A to actually blow a 15A

Not necessarily. Fuses also have time ratings and a standard (as used in vehicles) fuse can take up to twice it's rating to blow in 1 second but will still blow at a much lower current draw over a longer time period.

Rob

As Cossack has pointed out it looks like one blade of the fuse has heated so that blade/connection must be offering a higher resistance to current flow than the circuit it's protecting, due to corrosion, dirty contact surfaces, poor fuse construction of whatever. Ideally any connection, be it a fuse holder or plug/socket etc., should offer as near as dammit zero resistance across it's terminals. If there is exccesive resistance to current flow it will behave like a heating element.

 

[Chris]

Now I am NOT an auto electrician, but worth checking the plastic plugs on the back of the lamps. Especially if you have higher rated lamps in your headlights. I have owned more Toyotas than any other make and I have to say that in my experience, it's quite common for the plastic plug to basically carbonise leading to high resistance. I can't say if this could be a cause of the melted fuse, but it's certainly been the cause of problems for me in the past such as very dim lights etc. Something leads to them becoming hot and after a while the plastic turns from being an insulator into being a conductor. My usual cure is to snip out the old ones and bullet connect in new block. I actually keep a set ready in my spares box it's happened that many times.

Chris

 

[Rob]

Hmm, starting to make more sense. I will check the bulbs and connections and clean them all.

 

[Jon Wildsmith]

Probably a cheap and badly made fuse operating close to its limits and turning into a heating element like Towpack explained.

 

[frank rabbets]

I've seen this on other 12 volt connections. Where there is a poor connection heat can rise at very low currents and a fuse will not fail. So looks like a poor connection inside the fuse where the fail wire joins the blade or perhaps a poor blade contact to fuse socket or a poor wire to socket contact on the other side of the fuseboard.

The worst example was when I was driving at night in my Audi and the headlights went out. When I took the stalk switch apart there was a faulty rivet connection and the surrounding plasic part of the swith had turhed to "charcoal". This had obviously been happening for years [1989 car in 2008]. So this is how electrical fires start in cars. The fuses do not always protect. You probably have a similar situation in your fuse. The exact location of a poor connection in the circuit is irrelevant. It still heats up at low or operating current. A "cheap" car will catch fire easier otbe because electrical components are not fire resistant.

Frank

 

[TonyP]

Had a similar looking fuse on my LC. It's the main 35Amp fuse that supplies 4 x 5A Brittax spots, 4 x 5A Xeng lights, 2 x 5A rear work lights, and also a carputer prob drawing another 5A. (They all do have separate fuses too) So in total that comes to 5A. Now of couse I don't run with all that on at the same time as that will blow the fuse, but one evening greenlaning I had the 8 spots and computer on at the same time, and was working fine for about 10-15mins, when it got dark all of a sudden (LC's main lights still on though).

Pulled the main fuse and it was hot and melted... Needless to say, I don't do that again

 

[Julian Voelcker]

Putting higher wattage bulbs in to the headlights could melt a fuse like this.