Let me pose this question to you: What is the current rating of common household aluminium foil? Don’t know the answer? Well, nor do I and more importantly, none of us should know because it should never be used as a substitute for a fuse! The reason I even pondered this question was because a friend of mine came to see me yesterday with a sprinkler timer he has been having problems with recently and had now stopped working. I asked what the problem was and he told me that it kept blowing fuses. So I opened the front access panel up and saw a 1A 3AG fuse wrapped in aluminium foil sitting in the fuse holder. I asked what the hell he had done and was informed that he had run out of the correct size fuses, so he had improvised. He was actually rather impressed with his efforts and informed me that it worked for a little while longer than it had been. I then proceeded to open up the unit and inspect the board. What I found was quite impressive. The fuse had blown due to a short and by effectively bypassing the fuse the current draw caused by the short had heated up several components to the point that they not only failed, but lifted the tracks off the board. One resistor had actually caught on fire. All in all, given the chips on the board were quite probably damaged as well, he is up for a whole new unit. All this could have been prevented. So, perhaps the question should be what are fuses and how do they work?