Self-immolating fuses

[jeremy996]

My heater fan stopped working, so after cursing it for a few days, I finally went to investigate. 

My '89 110CSW was originally a 19J and now has a 200Tdi; it has an early Defender Autosparks loom with blade fuses. It had melted and glued itself to the fuse-holder. I have cleaned up the manky spade terminals pressed into the plastic holder and replaced the fuse, making sure it is making a good contact with the spades.

Any suggestions for a better solution?  (Without rebuilding the truck, it's 10 years from the last one), 

[smallfry]

Looking at the intact fuse blade, and the burrs on it, I would say they were cheap fuses. Looks like the fuse was at fault rather than the connection to the fuseholder. but thats just my opinion.

However, I would say either guy genuine Durite fuses, OR (which is what I do) go to a breakers yard and help yourself to some OE ones from a scrapper. Vauxhall have good fuses IMO.

Dont buy these cheap selection boxes from the bay. I have noticed the blades on the ones I bought were a bit thinner than my originals.

[FridgeFreezer]

As Smallfry said - only two possibilities, bad holder or bad fuse!

If the new fuse went in firmly and made good contact you should be good to go - a loose or dirty contact will always create some heat under load.

[TSD]

There are more than two possibilites....

Blade fuses rely to some extent on the connecting wires to dissipate heat, so if they are undersized (and they often are) then the fuses run hotter than they should when operating close to rated current (Littelfuse datasheets specify the wire size for specified performance)

Wrong fuse entirely? Red fuses are usually 10A, but I think Defender fan fuse is usually 15A? Even 15A may be on the small side for a fan thats getting on a bit, and could benefit from a clean and some oil? I've measured defender fan motors drawing more than that.

The real point here is that a 10A fuse will run very hot at 12A or more, but likely not blow. A 15A fuse will run significantly cooler.

Sizing fuses is a complex business, but since a wiring short would likely give a current well over 30A (and that's the sort of fault fuses are supoosed to trap) then I'd happily put 15A in there.

(Actually, I'd put 20, but I'm not telling anyone else to do that - I know what my wiring loom looks like ).

 

Oh, and Genuine Durite is a bit like Britpart OEM in my book... but that's another argument entirely. As you say, much better than random ebay versions though.

 

EDIT : Just dug out my notebook, the last Defender fan motor I tested (300tdi, good condition) was drawing 13A at full chat on the bench. It'll most likely never reach that with the voltage drop in a LR wiring loom, but still 15A would be the smallest fuse I would fit.

Edited August 10, 2021 by TSD
Added info

[jeremy996]

Thanks everyone,

Confirmation I am not going mad; a better fuse and good contacts.

[FridgeFreezer]

On 8/10/2021 at 11:07 PM, TSD said:

There are more than two possibilites....

I should have known

And I should have remembered the fan experiments - I thought one of them got up to 17A but that could've been a different unit (SJ or Puma?) or just abuse on the lab power supply

[TSD]

Well remembered - the Puma fan hit 17.2A at 14V, while only delivering about 15% more airflow than the Gates unit from the 300tdi era. I suspect the test rig didn't present enough back pressure to let the Puma fan shine though, it seemed to blow up a storm when I tested it on the Defender dash in 2Bex. (Shame the Puma fan is not an easy retrofit to the standard heater box really.)

We only tested the SJ fan up to 11V (10.4A) for some reason I don't recall - maybe because it's a smaller fan and housing (and a tiny motor), the airspeed is much higher, it was probably making a hell of a racket

 

 

[ThreePointFive]

I've been putting much lower-rated fuses than the wiring would allow on things I've wired up, I thought it would catch any issues (like whatever it is drawing more current than I built the wiring for, though I've been quite careful to spec wiring at least equivalent) and blow before something more serious happened.

Now I'm worried. I'll need to go through every fuse.

[smallfry]

I saw the fuse ........ didnt think of the value ! 

Early this year I was testing the fan motor in my old (04) Vauxhall Combo, and that drew 10.7 amps out of the housing. It was the speed resistor pack that was faulty in the end, but however, the factory fuse value is 30 amps ! 

Today I tried a 10 amp fuse in it, just out of interest. It didnt blow on full speed, even after 5 minutes, but it DID feel warm to the touch.

[TSD]

Quoted 'typical' performance from one manufacturer is that blade fuses should blow within 5 seconds at 200% rated current. That is a short enough time to be confident the fuse blows before anything too bad happens, but probably long enough to survive switch-on surge, and not run so hot as to start melting things. I'd take that as a starting value, and select the next larger fuse.

(At 135% rated load, the fuse might take up to 10 minutes to blow, and below that it might never blow at all!)

@smallfry When I measured Defender fans, the current almost doubled when fitted into it's housing (of course a Defender heater box is probably more restrictive than most). So it's easy to imagine your Vauxhall fan drawing 15A when installed, making 30A sound like a good choice for a fuse.