What would cause an inline fuse to Melt? - Defender Source
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  #1  
Old April 14th, 2007, 04:01 PM
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What would cause an inline fuse to Melt?

My rocklights stopped working. I just went to fix them and the problem is both the inline fuses (the plastic case around them), melted. Both fuses were fine, but they melted completely apart. What causes this? The wire guage is fine for the load. Everything is installed correctly too. Thanks
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  #2  
Old April 14th, 2007, 05:11 PM
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I'd guess and say sounds like a heat source nearby. Wouldnt an electric spike pop the fuse before melting plastic? Are either fuses near something that would heat up that much (assuming on both sides of the truck?)? Is the wide casing one either side of the fuse melted too or just the fuse housing?

Could the fuse casing been bad somehow that the electricity was bypassing the fuse and generating heat rather than blowing the fuse? But I guess that should pop the light before it melts something right?
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Old April 14th, 2007, 07:52 PM
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I am pretty confused on this one. There is no external heat source that could do it. Both separate fuses did the same thing too. It could be a really bad fuse design, but something had to generate all that heat. The fuses were fine too. They were cheap lights, maybe they went too cheap on something.
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Old April 14th, 2007, 07:57 PM
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If the wire casing on either side of the fuse assembly is fine, I'd suspect the design too. Got a pic of where each is and whats around it?
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Old April 14th, 2007, 08:01 PM
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Quote:
Originally Posted by chrisvonc
If the wire casing on either side of the fuse assembly is fine, I'd suspect the design too. Got a pic of where each is and whats around it?
No, I didn't think to do that before I removed them. They are both mounted together with all the other wires that run into the firewall along with both my airlines. There is no other heat source where they were mounted. The plastic casing melted away completely so the wires just fell apart. Very odd.
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Old April 14th, 2007, 08:39 PM
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I've read a number of instances of fuse holders melting, particularly those given in wiring kits for lighting. Not sure if it's poor wiring inside the holder, or a stupidly low melting temp for the plastic.

Were they the white plastic type that breaks in half to change the fuse? Never did care for them myself.

I've been using some of the Hella fuse holders for a number of projects. Good selection at rallylights.com and they use the ATO style blade fuses, with some multi-fuse boxes that hold 4, 6, 8, 12 or 16 fuses. The relay holders are pretty nice too, and many of them can dovetail together to make a nice solid unit.

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Old April 14th, 2007, 09:01 PM
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Yep,

They were to crappy white plastic that snap together with a half turn. Sounds like it might just be the design. I just wired them without the a fuse for the time being. I did the same thing with my hellas actually, and the time being ended up being 2 years and counting. But, I appreciate the info as I am going to rewire the entire truck next summer.
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  #8  
Old April 14th, 2007, 11:54 PM
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I've had many problems with that POS fuse holder design. NAPA sells replacement fuse holders that take the blade style fuses fot around $3.
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  #9  
Old April 16th, 2007, 05:09 PM
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Heat from flowing electricity... that's what melted 'em

Follow-up Post:

The heat came from restisance in the fuse holder.... crappy connection, corroded contact or the like. It warns up poor plastic melts and there you go. The Headlight connectors in Discovery 1's do the same thing.
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Old April 17th, 2007, 09:30 AM
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Fuses get hot when operated close to their current rating (otherwise they wouldn't blow when they exceed it). Poor quality fuseholders and undersized wire make the problem worse by not carrying the heat away, and by generating heat at bad connections.

- Depending on what you have already installed, consider a larger rating fuse.
- Change to a better fuse type (I like to ATO type as other have suggested)
- Uprate the wiring size - not because it's undersized, but because it will carry the heat away better.
- Make sure the connections are good (clean and apply petroleum jelly)

It's not uncommon to see ATO fuses where the plastic fuse body melts, due to operation close to current rating. Remember that a circuit with an 8A load *can* probably have a 10A fuse, but a 20A will usually give adequate protection (blowing before the wiring is damaged by a short).

All this assumes you don't have an undiscovered fault further downstream...
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Old April 17th, 2007, 02:31 PM
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Yep that summs it up
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