Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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 Posted: Wed Sep 05, 2012 4:43 am Post subject: Fusing |
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Much has been written about fusing, and yes, the approach is to "fuse to protect the wire and not the end equipment. But there is room for what is called "Intrinsically Safe Wiring" that requires no fuse by definition.
A circuit can be "Intrinsically Safe" if that circuit can never provide enough short-circuit current, or even spark energy to ignite the hazard of interest (ethylene for most of us).
Now this is not so esoteric, since in many circuits, including sensors, memories small devices and motors, very bright LEDs, the "intrinsically-safe" current levels are quite usable. For simple LEDs the current limiting means would typically be at the bus.
See this for some background:
http://www.mtl-inst.com/images/uploads/datasheets/App_Notes/AN9003.pdf
For your boy's science project:
How these standard were developed might be of interest (old manuals showed this): Make a tiny grounded container (like a metal bottle cap) with a few drops of gasoline (or whatever material is of interest). Then at a range of temperatures, at a range of voltages and currents, touch the + lead to spark on the rim of the tiny container. Below certain limits, The intrinsically safe limits, nothing happens.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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