FUSE Size

[001]

Hi

I see different  MAINS fuses
20mm 30mm 38mm 60mm

How to choose correct size?

[Zero999]

Presumably you're designing a new project, rather than simply replacing a fuse?

What are your requirements? Generally physically larger fuses have higher breaking capacities, than smaller ones, but that's a generalisation.

[wraper]

Generally 5x20mm fuse is fine. If there will be current higher than 5A for prolonged time, it's better to use 6.3x30mm, If over 10A current for prolonged time, then 10x38mm fuse. This suggestion is valid if fuse is placed in socket. Due to heating, over time surface of fuse terminals and socket may oxidize severely and thus further increase heating (higher resistance/dissipated power) causing chain reaction. I've seen this happening in industrial furnaces. If the fuse has leads or high current flows only temporarily, not causing fuse running hot, smaller size can be used reliably. If high current is applied for prolonged time, fuse holder quality is essential as well. I've seen many of those failing spectacularly.

[wraper]

That was suggestion based on current rating. Breaking capacity Hero999 mentioned might be important as well. But in that case you can simply look into datasheet.

[001]

That was suggestion based on current rating. Breaking capacity Hero999 mentioned might be important as well. But in that case you can simply look into datasheet.

Is it for Arc prevention?

Why do You say "Current"?
May be You mean "Voltage"? 

[wraper]

Why do You say "Current"?

Because small fuse will overheat and oxidize if run for long time at high current even if it's rated for it. Resistance is not zero and fuses heat up quite a lot at high currents. Breaking capacity should be fine for any size if it's a mains fuse for home appliance. Voltage rating would be important for say multimeter fuse or anywhere else where higher voltages may be present.

[coppercone2]

Do ceramic fuses have this problem still?
Are there nonoxidizing fuses like inert filled hermatically sealed?

[wraper]

Do ceramic fuses have this problem still?
Are there nonoxidizing fuses like inert filled hermatically sealed?

Terminals do oxidize regardless if it's glass or ceramic fuse. Though generally ceramic fuses should have better heat dissipation over their body due to sand they are filled with.

[coppercone2]

Oh the terminals not the inside of the fuse. So you mean the junction between the fuse and the holder.

Those typical fuse holders are sheet metal bent bull shit. I am imagining something like the type of heat sink you slide over a round metal can style transistor but for holding a fuse...
I don't see why you can't solder cooling fins to a fuse holder lol

It sounds like a fuse that threads on with a star washer would be a good solution here?