Providing primary protection against overvoltage events
Overvoltages take place during events such as switching operations, electrostatic discharges as well as lightning discharges. Be they direct or indirect, they are introduced by galvanic, inductive or capacitive means to electrical lines, with the potential to create devastating effects.
Pulse-shaped load currents are a common circuit occurrence and a typical factor for consideration when designing in primary fuses.
Pulses can arise individually (Surge), or recurrently (e.g. timed circuits).
With individual pulses, the I²t value of the fuse wire becomes quite important.
This is because the higher the I²t value the greater the pulse tolerance; with pulse-shaped continuous currents, the calculation of the rms value is crucial, and generally the displacement of the rated current should be taken into account as a result of a possible increase in ageing (diffusion).