external inline fuse holder getting warm
I did an electrical load test to see if I had any bad connections that might heat up.
Wired up two PCB heating pads in parallel, resistance is 2.2 ohms, connected to 24V PSU through a bunch of stuff. Test took place outside in +12C weather
A smartphone with a thermal imaging camera was used to detect hotspots.
None of the terminals got warm. Good...
What did get warm is the 15A ATC fuse. It's inside an inline fuse holder (not on a PCB). The fuse holder showed 35C on the outside. Opening the fuse holder, the fuse body showed 40C.
The fuse showed 50mV across the terminal. The current consumption is estimated at about 10A. This means the fuse was dissipating 0.5 watts of heat. This gives me an estimated resistance of 5 milliohms. I don't have a 4-wire resistance measurement setup to verify.
So with those estimates, does 40C sound normal?
Bringing the machine indoors would probably mean another 13C of temperature rise. I know definitely the plastic of the fuse holder can survive at least 80C or else it can't be classified as "automotive". The fuse holder will be hidden away from human touch.
Can I confirm that all is safe and continue? Or should I suspect poor contact inside the fuse holder? I can't let it degrade and suddenly fail catastrophically.
Remove the fuse, go from one terminal to the end of the lead and ohm it, do this for the other side and then ohm the entire length with the fuse installed.
Each joint in a wire should be no more than .1 ohms per connection, so you technically have 4 connections in that fuse holder, two on each leg so a possible drop of .4 ohms.
If you have a considerable difference between one leg slot and the other, it would indicate a bad connection, though if it is only warm to the touch, you should be fine, fuse filaments are thin and will generate a little bit of heat.
5mΩ is absolutely normal for a 15A ATC fuse. A fuse is by its nature a resistor.