III — Insulation material aids extinction of cable arcs

A CLOSE STUDY of the type of arcs occurring with faults in low-voltage a-c. networks and the method by which they are extinguished has brought out several striking conclusions. It has been found that short arcs between copper electrodes and not adjacent to insulation are inadequate to meet the fault-clearing requirements of low-voltage a-c. networks of more than 130 volts. Actual low-voltage networks of voltage higher than 130 depend for extinction of arcs upon the strongly deionizing action of gas blasts coming from decomposing adjacent insulation. As suggested by this result, it has been found also that the extinction of arcs in cables is strongly affected by the closeness of the insulation to the arc. Also, inorganic insulating materials giving off non-combustible gases have been found to be effective in aiding arc extinction, and of the various inorganic materials tried, boric acid was the best. Further, where organic insulation has been charred, it may be expected to lose its property of aiding arc extinction.