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The basic rating is continuous current, the value of amperes
intended to flow through the device. (This is sometimes described
as "operational current" or "thermal current.") When we speak of a
"100 ampere fuse," for example, we mean that a load drawing 100
amperes continuous current is the maximum for which that fuse is
intended to provide fault protection. How long is "continuous"?
"More than three hours," according to one definition. Although we
tend to think of "inductive" and "resistive" circuits as two
separate entities, the distinction is only one of degree. Whether
a-c or d-c, all real circuits contain both resistance and
inductance. Opening a current-carrying circuit will therefore
always result in an arc across the opening contacts. How
intense-and therefore how damaging-that arc may be depends upon the
relationship between resistance and inductance, expressed by the
circuit power factor.
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