A New Three-Element Current-Limiting Power Fuse

In order to meet the requirements for the protection of high-voltage motor starters and for protecting small loads and apparatus connected to those parts of electric systems having extremely high short-circuit currents, it has been necessary to increase the current range of the three-element current-limiting fuse. This fuse, which was announced in 1940, has been in commercial use for over two years in the lower ampere ratings. The development of higher current ratings in a fuse of suitable dimensions produced problems and solutions totally different from those previously encountered. A new type of fusible element has a time-current characteristic particularly suited to current-limiting fuses. Extremely fast melting times on high currents to give correct current-limiting action are combined with a continuous current-carrying capacity and normal melting time of an element having five times the area previously associated with the current-limiting characteristic. The association of this element, with gas-evolving material and quartz sand to give quick and positive current limitation while the metal vapors are absorbed, results in an entirely new fuse construction. The combination of this current-limiting element with a noninductive resistor of special characteristics and a totally enclosed boric-acid ``clean-up´´ fuse has resulted in a current-limiting fuse of more compact construction than has been available heretofore in the ratings involved. Exhaustive tests have demonstrated the suitability of the new fuse for application on high-capacity systems to interrupt the full range of fault currents with minimum system disturbance.