Fuse Time-Current Characteristics and Proper Fuse Selection


A Fuse is a common temperature sensitive device that protects a circuit when an over-current condition such as an overload or short circuit occurs. It does so by melting due to the excessive temperature caused by too much current and opening the circuit, cutting off current flow through more sensitive devices. Knowing this, how can we determine how fast a given fuse will open under specific current conditions?

A fuse’s time-current characteristics can help. For example, Littelfuse’s 383 series - TR5® time-lag 300V rated fuses have time-current characteristics as shown by the below graph.

This time-current characteristic chart shows you how fast the fuse responds to different levels of overcurrent condition. All fuses have an inverse time/current characteristic; as overcurrent increases, time-to-open decreases. Put more simply, the fuse will open faster the more severe the overcurrent problem is.
Generally these charts are shown in log - log scale, as above.

We can use this information to account for how quickly our fuse will open under specific current conditions and ensure it does so fast enough to protect the sensitive, pricey electronics we’re using a fuse to protect in the first place. Alternatively, we can use this information to select a proper slow-blow fuse that won’t trip when subjected to inductive inrush currents.