Will a fuse limit current before blowing?

[Bigmac]

have a set of LED pods 40W each. been running them with no issues for quite a while. each pod came with its own harness and switch however i used a Y adapter to attach both to the same harness and switch.

today i flipped them on and no worky worky. checked fuse, sure enough it was blown but it seemed odd to me, looked as if it had blown slow if that makes sense. wire had distorted to the side and melted just a small portion through the side of the fuse. I noticed it was a 5 amp, which seemed puny to me for a nice set of lights.

did the math and sure enough both lights together should draw 6.6 amps. so this seems to be the cause of the fuse blowing. It just seemed odd to me it took so long to happen. I would expect it to blow when i installed them, not a full year later.

i understand the ratings on these fuses are not exactly laboratory grade exact.

Is it possible that not only was the fuse running hot the entire time, but that it may also have been limiting current and therefore light output? I doubt it would make much difference as 1.6 amps capacity short doesn't seem like a big difference, but i was just curious.

 

[Steel City 06]

have a set of LED pods 40W each. been running them with no issues for quite a while. each pod came with its own harness and switch however i used a Y adapter to attach both to the same harness and switch.

today i flipped them on and no worky worky. checked fuse, sure enough it was blown but it seemed odd to me, looked as if it had blown slow if that makes sense. wire had distorted to the side and melted just a small portion through the side of the fuse. I noticed it was a 5 amp, which seemed puny to me for a nice set of lights.

did the math and sure enough both lights together should draw 6.6 amps. so this seems to be the cause of the fuse blowing. It just seemed odd to me it took so long to happen. I would expect it to blow when i installed them, not a full year later.

i understand the ratings on these fuses are not exactly laboratory grade exact.

Is it possible that not only was the fuse running hot the entire time, but that it may also have been limiting current and therefore light output? I doubt it would make much difference as 1.6 amps capacity short doesn't seem like a big difference, but i was just curious.

A fuse is rated to its maximum continuous current. It is essentially a resistor. Once the heat reaches a certain threshold, the fuse begins to melt, and once fully melted, breaks the circuit. A fuse running slightly above its rating will blow very slowly, and one running way above its rating will blow very quickly.

This means that it blows in instances like short circuits where there is a sustained high power draw and significant risk of fire, but not in situations where there is only a high power draw for a very short period of time, like when you turn on a set of LED lights and the capacitors soak up charge. Time-delay fuses are specifically made to lengthen the amount of time an overload can be sustained.

All that heat is created by the current, meaning that the fuse has non-negligible resistance. So the partially blown fuse likely did reduce the voltage seen at the lights, but not significantly.

A good rule of thumb is that the fuse should be rated to about 125% of the maximum expected continuous current draw (or the maximum continuous current draw is ~80% of the fuse rating). So a pair of lamps with an 8 amp draw ideally would have a 10 amp fuse.

However, the fuse must NEVER exceed the maximum allowable current in any component. This typically ends up being the wiring or the connectors. The wire itself acts as a resistor and will heat up, and if the insulation melts off, can short out and start a fire.