Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!caen!jameson.engin.umich.edu!ssave From: ssave@caen.engin.umich.edu (Shailendra Anant Save) Newsgroups: sci.electronics Subject: Re: Can fuses get "worn out"? Summary: Variable blow fuses. Message-ID: <1990Dec14.191104.28628@engin.umich.edu> Date: 14 Dec 90 19:11:04 GMT References: <1990Dec14.171020.3615@phri.nyu.edu> Sender: news@engin.umich.edu (CAEN Netnews) Organization: The University of Michigan, Ann Arbor Lines: 38 In article <1990Dec14.171020.3615@phri.nyu.edu>, roy@alanine.phri.nyu.edu (Roy Smith) writes: > replaced the motor, and yesterday, one of the 15A cartridge fuses protecting > it blew. We replaced the fuse and the A/C came back on, but we're worried > that there is more to it that just a chance fuse failure. > > The motor only draws about 6A (yes, I know about startup surges), so > I think the fuses are the right size. My current theory is that when the > previous motor died, it wasn't quite drawing enough current to blow the > fuse, but "damaged" it enough that two weeks later a normal startup surge > was enough to blow it completely. Is this possible? Do fuses act that way? Yes, fuses act this way. A very untechinical explanation would be that the repeated cooling and heating of the fuse would put enough stress on it that it would just plain break. I don't know what would happen if you left the motor running on continuously after it has been turned on, since then there would be no temp. cycling. I do know that my car fuses blow quite often, usually when I am in real bad weather. I haven't quite figured that out yet. Some related thoughts. Many applicatons nowadays no longer use fuses for engineered weak-points in the circuit. Most are now converting to circuit-breakers, which are solid-state devices, reasonably cheap when doing a lot of experimental work, where you are liable to blow a lot of fuses. These breakers do have reset switches which can do the equivalent of replacing a blown fuse. The other thing is that you said that you used a 15A fuse for taking startup surges into account on a normal 6A load. A better alternative is to use a 6A 'slow-blow' fuse. Meaning that it takes a longer time to melt and hence not blow during start-ups. I have found that to be safer. --Shailendra -- Physical: Shailendra Save, Logical: ssave@caen.engin.umich.edu 2303 Conger Baits II, UUCP: ...!umix!caen.engin.umich.edu!ssave Ann Arbor. MI 48109. Audible: 313-763-1627(H) 313-764-8033(O) ICBM: 42 33'W 83 71'N Fax: 313-747-1781 Eagles may soar, but weasels don't get sucked into jet engines. (For those who don't know, a weasel is a wolverine)