Fast blow fuses

[AdrianUK]

Here's a fuse that didn't take 35A to blow. 35A would have blown it instantly. One of the commenters said the fuse would blow at the rated current when given time. I am of this sentiment. Going beyond the rated current will merely speed up the blow.

I think you are still not clear on the terminology. Nobody is saying it takes 35A to blow. What we are saying is 35A is the MAXIMUM current that particular style of fuse can interrupt safely. If it is subjected to a fault current greater than 35A that it may fail in a catastrophic way and/or it may fail to break the circuit.

The rating of a fuse is the current that the fuse will carry indefinitely without failing. The higher the current is above that rating then the faster it will open BUT there is a max current that the designer has said is safe for that given fuse - its breaking capacity. In the case of a glass fuse this is usually 35A. Subject it to a fault current bigger than 35A and it may not play nicely.

 

[RandomGrinch]

Here's a fuse that didn't take 35A to blow.

Your selected video shows a non-typical fault scenario for a fuse.
Rarely would the current be slowly ramped up until the fuse blew.

A more typical scenario may be an instantaneous short circuit.

And as V=IR, an instantaneous short circuit may generate a massive over current fault.

...and back to the benefits of ceramic fuses over glass -

35A would have blown it instantly

- not if this exceeded the fuses breaking capacity. As @bernardgreen put it so well,

A fuse on the mains input may have to break well over 300 Amps, vapourised fuse wire inside an otherwise empty glass tube creates the ideal conditions for a plasma arc to form and be sustained until the glass eventually shatters or melts. Doesn't happen very often. The sand in a mains or other high rupture current fuse quenches the arc before it can damage the tube.

Basically, the fuse rating is the minimum current value the fuse will blow at.
It's breaking capacity is the maximum current value the fuse can safely interrupt.

Edit:
Beaten to it by @AdrianUK !

 

[DiyNutJob]

an instantaneous short circuit may generate a massive over current fault.

Or, it may not. You are just into tall tales to scare children.

That fuse was blown from short circuit multiple times owing to service monkeys trying to earn call out pay by playing with the conductor jackets. They put in a ceramic in place of the original glass. Hence, I was wondering what the deal was. I could not tell what rating fuse they put in as it had no obvious markings on it. I will put back a glass.

 

[RandomGrinch]

And as V=IR, an instantaneous short circuit may generate a massive over current fault.

Or, it may not. You are just into tall tales to scare children.

So, you're arguing against V=IR now!
Ok, I don't think I can win this one!

 

[bernardgreen]

Or, it may not. You are just into tall tales to scare children.

A failing motor staggered on rotating for several seconds on the current passing through a plasma arc in the fuse after the over current from the failing motor had blown the fuse. Now that really was scary for the maintenance people.

 

[DiyNutJob]

Ok, I don't think I can win this one!

You can win still. I won't be the only one reading. I am sure plenty of others will be taken in.

A failing motor staggered on rotating for several seconds on the current passing through a plasma arc in the fuse after the over current from the failing motor had blown the fuse. Now that really was scary for the maintenance people.

No motor is involved, just a low current electro-mechanical device being fed and controlled by ordinary relay switching. The jacket on the feed wires were tampered with, with the hope of short circuiting at indeterminate moments. Since the rating of the fuse is unknown and appeared to be unmarked, it could potentially kill whatever is down the wire. Anyway, the reason for the previous short circuits is known and corrected, and the clowns were fired. All is now good and the appliance should go for a lifetime.