Re: Fuses
This business of tandemed fuses is a very interesting one. Let's just take an imaginary example:-
You have a load, protected by (say) a 7Amp fuse. It's fed from a plug-top containing a 13Amp fuse. The load develops a fault, and the 7Amp fuse blows. The 13Amp fuse will be protected - right?
Answer: not necessarily. After you've fixed the fault and replaced the 7Amp fuse, you may still find that the 13Amp fuse has blown too. Why?
The answer turns out to be rather interesting (well, I think so, anyway...)
When a fuse blows, it absorbs energy. It requires a finite amount of energy to cause the fuse wire to melt: but on top of this, when the fuse opens an arc is started which dissipates a further amount of energy until the arc is extinguished.
Now here's the interesting bit. If the total amount of energy expended by the 7Amp fuse (blowing + arc) is greater than the amount of energy needed to melt the wire in the 13Amp fuse, then that latter fuse will melt and go open-circuit. There probably won't be an arc in that fuse, but at the end of the day, it'll be open-circuit anyway. Interesting, or what?
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