melting fuses

[macca81]

there is another topic on this but it went way off the track so i thought id start my own.

i have an upgraded headlight loom, and my high beams are 130w each, so 260w total. now i figure this to draw 21.6 amps, so i put in a 30amp fuse. now, 8 months down the track i had all my lights go out at 110kmph on a country road with no light from moon/stars, bloody scary.

popped the bonnet and found my 30amp fuse had blown, but before blowing it had melted both the green plastic that makes up part of the fuse, and the rubber that is the inline fuse holder. i replaced it with another 30amp fuse and it has been fine thus far...

BUT! why would it get hot enuf to melt but not blow, when it should be drawing ~8amps under its rating? if it was drawing say 28amps on a 30amp fuse i would understand it, but i cant accept that 21amps should get a 30amp fuse so hot... ideas??

[cooki_monsta]

30A is a spike current rating, they also have a temp co efficient, to avoid this you may have to seperate your lhs and rhs headlights

[bugshifter]

I had the same thing happen but the lights didnt blow. I put a 40amp fuse in it and have had not trouble since. But it was strange seeing all the melted green plastic the first time

[macca81]

they also have a temp co efficient,

elaborate please??

to avoid this you may have to seperate your lhs and rhs headlights

well thats a bit poo...

[its aford not a nissan]

the reason for the melting is a bad connection , probly in the fuse holder , so if your fuse is rated at say 30 amps and there is a bad connection which will only pass say 20 amps , anything over 20 amps will generate heat at the bad connection as this is now trying to act like a fuse ( weak link ) but isnt designed to blow like a fuse is

[murcod]

Macca fuses work by having a small amount of resistance in the narrow fusible part. The resistance generates heat proprortional to the current flow and once enough heat is generated the metal will melt.

My guess would be that the fuse is still heating up enough to cause long term heat damage. Don't forget they're not exactly a high precision device- you will find they will melt at around the rated current. If you run them close to the rating then they will die a slow death.

I'd replace it with a 40Amp fuse - provided the rest of your wiring is up to it.

Your lights will also cause a large current spike when first turned on which wouldn't be helping. The wattage rating is what they will roughly draw once the filament is hot.

[DamTriton]

Lights also have a cold resistance that is far less than the hot resistance and will pull well in excess of double the normal current in the half to one second that it takes to get to full output. This would have "stressed" the fuse without actually blowing it over time, and may have weakened it internally enough to cause a higher resistance than normal (as described by murcod above).