The effect of the fuse enclosure temperature rise on low voltage overhead conductor protection

This work examines aspects of low voltage distribution line protection through fuses. More specifically, the effect of the increased ambient temperature inside a fuse-box, due to joule losses, on the current rating of the fuses is examined in conjunction with the ampacity of the overhead conductors. The temperature rise of the fuse-box is calculated according to IEC 60890 with some improvements so as to cope with external installation of the switchboard, i.e. the heat gain due to solar irradiance. Finally, whether the protection offered by the fuses is sufficient, is examined, as fuse operation at high ambient temperatures causes a shift of the time current curve towards shorter times. Through this work, the exact relation between conductor ampacity and fuse rating can be ascertained, thus enabling protection engineers to identify any possible cases of conductor over-loading, under-utilization, or nuisance tripping of the feeder.