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There has been alot of confusion and misinformation on the fuses and breakers. The equation stated before for heating in an electrical circuit is not right. The temperature in a circuit is related to the current (I) squared times the resistance of the circuit or device in the circuit or I^2 * R. Someone wrote about the different fuse voltage ratings and thinking that the higher voltage rating ment that you had to have that voltage and current to cause the fuse to blow. Not true!!! the voltage ratings show what type of fuse is needed to ensure that when the fuse operates and that the circuit opens. Whenever a fuse opens, an arc is created across the opening element. If the gap is not large enough then the circuit can actually stay energized instead of opening. This gap that is required is determined by the system's voltage. So whether you have a 30 amp automotive fuse ( rated at 12 V ) or a household fuse at 30 amp ( rated at 120 or 220 V ) they both blow at 30 amp. It is just that the automotive fuse is smaller than the household fuse because it only has to ensure an open circuit at 12 V instead of 120 V or 220 V. A fuse is a relatively instantaneous device. Up to about 102 % of its rating a fuse will work. Above that, the element inside will rapidly melt and separate, opening the circuit. A breaker, on the other hand does not instantly open when its rating is exceeded. The most common breaker that we see is the Bimetalic element. It works by having two different metals that heat diffently. As they heat they move and cause the circuit to open circuit. As the metals cool they move back and the circuit can be made again. When a breakers rating is exceeded, the breakers opertate on what is known a an I^2 T curve. That is current vs. Time curve. The higher the current the faster the breaker trips until it is actully an instantaneous device also. The same thing applies the the breaker elements. Because the temerature of the element is I^2*R, it does not matter what the voltage rating of the breaker. It will still go at the same current. There are different types of breakers and fuses. Some are "quick blow" and some are "slow blow". depending on your application you might need to experiment with differnt types to find on that provides for turning the component on and still provide running protection. If you have any questions please respond. I work in the electrical testing and maintenance field and do this for a living. We have a high current machine that we use to test 480 V breakers. This machine only puts out .5 to 2.0 V while providing up to 50,000 amps. So we trip breakers every day with low voltage but the proper amount of current. Dave Howard |