A 100 ampere fuse does not open instantly at 101 amperes, nor even at 200 amperes. The fuse opening time is dependent on the type of fuse and magnitude of the overcurrent. In fact, this delay may be desirable. An overload current condition may only be temporary in nature, and the current may subside to normal current conditions in short order. For example, a typical harmless current overload is encountered whenever most motors are started. The built-in fuse time delay permits the motor to start without unnecessarily blowing fuses.
Two broad fuse characteristic types are (1) dual-element, time-delay fuses and (2) non-time delay fuses. Each type has attributes suitable for specific applications. The dual-element, time-delay fuses are widely used in general purpose applications, motor circuits, transformers, and other circuits. The non-time delay current-limiting fuses are used where fast response with little or no overload delay is desired A typical application of these is for protecting circuit breakers for possible large short-circuit current levels.
The graph shows the difference between the melting curves of a typical 100 ampere dual-element time delay fuse and a non-time delay fuse. If we look at a comparison at 500 amperes, the dual-element fuse melts in about 10 seconds and the non-time delay fuse melts in .2 seconds. This is a time ratio of 50 to one. At a current of 200 amperes (twice the nominal rating) the time ratio is about 9 to one.