Thread: Fuse rating help

Wattage is not relevant for a fuse - the fuse doesn't know about the voltage, just the current (until it blows, when the interrupted voltage had better be below the rated voltage of the fuse). A fuse should not get hot, let alone melt, within its rating, but I'm wondering if the fuse holder rather than the fuse exceeded its rating? Otherwise heating will have been due to a poor contact, probably the result of corrosion - i.e. "waterproof" isn't!

If the fuse-holder is the type fitted with a spring and the spring has rusted and collapsed then there will be at least a high resistance joint or possibly sparking. I would replace the fuse-holder unless its only corrosion that can be cleaned.

I bet the fuse holder was an inline blade type fuse holder...

the rusty spring contact would imply a 3 or 5 ag fuse which is glass and if you're melting them please dont turn on the power until I can get there with a camera, Its been a while since youtube had a good van burn down video.

Back to inline blade.

the contact points to the fuse are too small for the fuse size, in the holder will be 2 spade type connectors which make contact on a very small area of the fuse. when you start punching the current through you dont have enough surface area so they start to get a touch warm and try to act like a fuse but as they are not fuseable material they just get hot. If you measure the resistance without a load onthem they will appear to have 0 Ohms but if you measure the voltage drop as the current goes up your fuse will begin to act more and more like a resistor.

Inline blade fuses should only ever really be relied upon for about 10 Amps of constant current, a 5 minute surge of 15 amps and a peak of 20.

"but Dave, My (XYZ) has 30 amp fuses in a very similar sort of holder"

True, BUT...

the fuse is to protect the wiring not the item and you'll probably find that whatever its powering has a very high startup current that only lasts a few seconds and then drops back and if its in a car then its got an open plastic box with airflow around it.

So whats the fix?

Well, maxi fuses are a good start BUT maxi fuses have a much higher "overdraw" before they blow.

As for oversizing your fuses, NO... if your charger can only push out 40Amps you fit a 40 amp fuse and I'm quietly confident that a 35A fuse will hold what your charger will put out if its a blade fuse. Average Bade fuses typically dont let go inside the 10 second mark until about 125-130% of their rated capacity and wont instantly blow until about 150%.