MELTING FUSES

The first type of fuse developed was of a melting kind. The word fuse comes from the Latin word to melt, “fusus”. As Thomas Edison developed his electric light circuit in the late 1870’s, he needed a protective mechanism “to provide against accidental crossing of the conductors leading from the mains”. Edison used a simple wire fuse in a wood-block holder. In 1890 Edison patented an enclosed fuse “to prevent or diminish the liability to surface creeping of lightning or other powerful current”.

The general idea of a melting fuse is (as the name reveals) to melt and make a galvanic break. A metal wire with smaller cross-section compared to the surrounding conduction line is placed in series with the conductor, see Figure 2.1.

Figure 2.1: Cross section of a melting fuse

If the current increases, the resistance in the small wire results in excessive power dissipation. This energy will eventually cause the smaller wire to melt. As the melting starts, the current in the line will create an arc in the gap between the not yet melted parts. Due to the heat dissipation in the arc it will continue to grow in length as the fuse wire melts. The gap will increase until the distance is too large for the energy that created the arc.

Not until the arc has been extinct, the fuse will stop to conduct. If the current, ergo the energy is too high, the arc will continue to conduct and the current will rush. Because of this, there is an upper limit for how high over-current that can be allowed.

When using an alternating current (AC), the reversion of current (zero passage) benefits the extinction of the arc. To be able to protect both from transients and constant over currents, there are many varieties and layout’s of melting fuses. Different thickness of the wire and solder joints that melts at different fault currents can help to design a melting fuse that fits the needs of a specific circuit.

Selecting the proper fuse can be very difficult and when designing a fuse several important parameters have to be considered. Here follows a section to give a basic understanding of important fuse characteristics.