The Boric Acid Fuse

Recognition of the part played by gas blasts in interrupting arcs in oil circuit breakers and expulsion fuses has opened new avenues for improvements in these devices. The present paper deals with a new type of fuse in which greatly improved performance is obtained by more effective use of the self-generated gas blast. In this fuse a form of construction is used which permits the interruption of the smaller ranges of current within a device that is capable of interrupting also heavy short circuits up to 20,000 amperes or more. A special feature of this design is that the lower ranges of current are automatically transferred to, and interrupted in, a small auxiliary fuse bore. While the construction may be used to advantage with any gas generating material, the present design embodies a compressed boric acid liner which has been found to be much more effective than fiber in extinguishing arcs. Since boric acid evolves water vapor, a condensible gas, it becomes possible to construct a totally enclosed fuse in which the gas blast is discharged into a surface condenser. With such construction, currents of from 5 to 20,000 amperes at 13,000 volts have been interrupted with an electrode separation of less than six inches, using a relatively small condenser. Currents of more than 20,000 amperes have been interrupted by similar fuses of open type. This type of fuse may readily be designed for various current and voltage ratings.