What happens when a fuse is undersized

[FaradayFF]

Greetings,

In a scenario where a fuse is undersized and the short circuit through the fuse is higher than what the fuse is rated for, the fuse will still blow, agreed? If that is the case, why is it still important to have the rating on the fuse higher than the max. available short circuit current(other than NEC requirement, of course). I can see that for a thermal magnetic circuit breaker, if the short circuit duty is higher than the max. interrupting rating of the breaker, the breaker may fail to open and quench the arc. For a fuse, the current interrupting mechanism is different..

Thoughts?
Brother Sparky.

 

[MyCleveland]

Isn't a disconnect without a fuse only rated for 10k amps ?
You get a higher rating with an appropriately rated fuse to get you up to 50k or 100k etc.
Would you not expect the switch itself to fail or be destroyed or worse if it was exposed to a current that the fuse could not clear ?

 

[Eddie702]

I am assuming you mean if the available fault current exceeds the rating of the fuse or breaker? In the case of a fuse it will open but if the interrupting rating of the fuse isn't high enough current could arc through the fuse keeping the circuit live

 

[charlie b]

If you run too much current through a breaker, its contacts may melt and weld themselves together. The breaker would not be able to open.

I am guessing here: If you run too much current through a fuse, it could explode, possibly scattering molten metal around the enclosure.

 

[FaradayFF]

I am assuming you mean if the available fault current exceeds the rating of the fuse or breaker? In the case of a fuse it will open but if the interrupting rating of the fuse isn't high enough current could arc through the fuse keeping the circuit live

Wouldn't the high current melt the duse filament regardless of what the kAIC rating of the fuse is?

 

[charlie b]

Wouldn't the high current melt the duse filament regardless of what the kAIC rating of the fuse is?

The fuse element will certainly melt, and thereby open the circuit. But (here again this is mostly a guess) it may melt so quickly and violently that the glass containing the element could shatter, resulting in molten metal scattered like shrapnel.

 

[gadfly56]

The fuse element will certainly melt, and thereby open the circuit. But (here again this is mostly a guess) it may melt so quickly and violently that the glass containing the element could shatter, resulting in molten metal scattered like shrapnel.

I don't know, but I could see a case where the fuse element is vaporized and turns into a plasma without rupturing the cartridge. It might continue to conduct current that way.

 

[junkhound]

With high enough fault currents, can turn everything into slag all the way back to generator.
Varies greatly with configuration and proximity to the return path.

Have seen arcing to box cover after fuse blows and arc does not clear and arc attaches to enclosure and then a foot diameter hole burnt into cover before the gap was wide enough for the arc to extinguish.

 

[Jraef]

You said "undersized", I think you meant under rated for SC duty. Sizing has to do with the working current, Interrupting Capacity rating has to do with Short Circuit current.

The issue is that yes, the fuse element would still attempt to clear the fault, but in the process, the arc that forms inside of the fuse will have more electromagnetic energy in it than the fuse is rated to handle, and it becomes a bomb instead of safely interrupting the current flow.

This one is fairly mild, I've seen worse...

 

[bwat]

And with a breaker, yes, the contacts could weld and continue to conduct, but with both a fuse or breaker the device could essentially turn into a bomb of sorts and release energy and shrapnel violently. I seem to remember reading this both in a Schneider breaker manual and bussman fuse manual recently. If I can find I’ll post.