Physicopoeia

φυσικοποιΐα
THINGS I WISH I'D KNOWN WHEN I STARTED TEACHING PHYSICS

WHAT CURRENT DOES A 13A FUSE MELT AT?

 

You're probably thinking 'just a bit above 13 Amps'. Obvious, right?

Well, that's what I thought and taught for quite a while, until I found this fascinating and surprising graph on this website:

This shows that a normal UK 13A plug fuse must be able to pass a current of 20 Amps indefinitely without melting. For it to melt in less than 1 second, there would need to be 80 Amps flowing through it!

This underlines the fact that fuses are primarily fire-prevention devices: 80A is unlikely to cause a fire if it flows through an appliance for less than a second. By themselves, fuses are not meant to prevent fatal electric shocks: 80A flowing through a human for one second would be more or less guaranteed to kill them.

It does, of course, complicate the issue of exactly what to teach pupils. Personally, I take the slightly weaselly approach of implying that a 13A fuse melts at just over 13A but stop short of explicitly stating it. If it comes up in discussion then I'll give them the detail above, but otherwise I don't think it does much harm to keep it simple.