Thread: Fuses
View Single Post
Old 4th Oct 2009, 6:49 pm   #30
ppppenguin's Avatar
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: Fuses

Originally Posted by jamesinnewcastl View Post
If you know your equipment will draw 20A for 2ms after switch on, then settle to 0.4A, the only fuse that is guarenteed not to nuisence trip is 20.1A.
This is an oversimplification. As indicated by Peter Munro in an earlier post the really important characteristic of a fuse under surge or short circuit conditions is its "let through" energy. Commonly written as "I squared t". This will be larger for a higher rated fuse and also for a slow blow fuse. The fuse should be chosen to not let through hazardous amounts of energy under these conditions yet not blow under normal surges. Long term heating effects are not important here as the fuse should blow pretty fast if there is this kind of fault.

The normal overload characteristics are rather different. It is assumed that thermal equilibrium is attained and the fuse should blow before anything else overheats dangerously. This can give very real problems, since fuses will take a long time to blow on small overloads (up to 50% or sometimes even more) during which time things can get a bit too hot.
ppppenguin is offline