power input for small transformer, good practice

[FlyingHacker]

I actually think the UK plugs are the safest design out there.

Until you stand on one while barefoot. Then they become the most painful plug design out there.

Hahah! Like the Danish foot torture device known as “LEGO.”

[C]

Cerebus

I understand what you stated in your post.
As the fault current becomes a greater % of supply you have a chance to have this kind of need.

But looking at total system changes things..

To get from 35A to a 1.6kA state you would need a very fast change. You would need a fast change just to get to 35A point.

The connecting wire inductance will reduce fast changes a lot. Each stage of breakers or fuses is  separated by more inductance.

Short of a lighting strike, I see little chance. And here it becomes a race against time for what gets blown.

The rare case of the glass exploding just shows a need for an enclosed fuse holder or use of fancy fuse that does not explode. That is selecting one of two options..
 
But there is a point where you are just selling numbers with little purpose.

You could make the same clam for breaking voltage. Above a point nothing or very little is gained.

This chain of posts started with a power supply that would have a cable that connects to an outlet. This is part of the facts.

[exe]

The earlier example was of a 1A load capacity fuse with a 1.6kA breaking capacity versus a 1A fuse with a 35A breaking capacity. If the current flowing in a fault exceeds the breaking capacity of a fuse, that fuse will fail to open the circuit and can even induce a higher current fault itself, tripping the next bit of protection upstream. So, an example for the latter case would be that 1A glass fuse with a breaking capacity of only 35A. It's very easy to generate a fault current of >35A with a simple accidental dead short - loose wire, loose screw, that kind of thing. The 1A HRC fuse would break that fault current, the 1A glass fuse wouldn't and might even become a lower impedance than the fault it was trying to isolate.

One thing that puzzles me is how this is possible. I shorted glass fuses several times, they blew instantly. Not even a spark was visible. The only way I see fuse don't open is when somehow evaporating wire inside the fuse turns into plasma or something. But I thought it's more danger for DC, not for AC as plasma disappears at zero crossing.

So, how can a glass fuse conduct under overcurrent condition?

[Cerebus]

Cerebus

I understand what you stated in your post.
As the fault current becomes a greater % of supply you have a chance to have this kind of need.

But looking at total system changes things..

To get from 35A to a 1.6kA state you would need a very fast change. You would need a fast change just to get to 35A point.

The connecting wire inductance will reduce fast changes a lot. Each stage of breakers or fuses is  separated by more inductance.

Short of a lighting strike, I see little chance. And here it becomes a race against time for what gets blown.

The rare case of the glass exploding just shows a need for an enclosed fuse holder or use of fancy fuse that does not explode. That is selecting one of two options..
 
But there is a point where you are just selling numbers with little purpose.

You could make the same clam for breaking voltage. Above a point nothing or very little is gained.

This chain of posts started with a power supply that would have a cable that connects to an outlet. This is part of the facts.

Really, you're either just arguing for the sake of it or out of ignorance. If it is hard to create a fault current in excess of 30 odd amps why have all of us experienced fuses/breakers of that kind of size tripping? Why do regulations, in my country at least, require minimum breaking capacities - 6kA for a domestic main breaker (BS EN 60898) and 6kA for a plug fuse (BS1363) if this is all fanciful and of no practical concern?

[Cerebus]

One thing that puzzles me is how this is possible. I shorted glass fuses several times, they blew instantly. Not even a spark was visible. The only way I see fuse don't open is when somehow evaporating wire inside the fuse turns into plasma or something. But I thought it's more danger for DC, not for AC as plasma disappears at zero crossing.

So, how can a glass fuse conduct under overcurrent condition?

By being a neat little glass tube full of metal vapour. If you consider how readily a column of non-conductive gas (Neon discharge tubes, xenon discharge tubes, lightning in air etc) will conduct electricity once you get it hot enough, how much more enthusiastic to carry current is the vapour phase of something that was already a conductor at room temperature going to be?

It's a misnomer that plasma discharges necessarily disappear at zero crossings. That only happens if there's enough off time passes that the plasma system cools to below the temperature that it is a plasma. Fluorescent discharge tubes run quite happily on 50/60Hz AC as we all know from experience.

Fortunately the 'perfect storm' necessary for a puny 5x20mm glass fuse to turn into a raging torrent of plasma is rare or we'd all have horror stories of equipment bursting into flames because of them. However, it can happen and as the manufacturer's data shows the guaranteed breaking capacity of 5x20mm glass fuses is low. That doesn't mean that they will fail to operate correctly at higher fault currents, just that it is a possibility - one that is easily avoided by paying 2p more for an HRC fuse.

Also bear in mind that the guaranteed breaking capacity is more prosaic than preventing the perfect storm. All a fuse has to do to fail dangerously is to leave equipment live that ought to have been isolated, even if that is under circumstances where a failed 'blown' fuse is simply carrying less current after failing than before failing - it ought not to be carrying any current at all. You can start a fire with a smouldering bit of insulation, it doesn't require a can of petrol and a flare or a 6kA plasma arc.