Electrical breakdown by tracking

When an electrically stressed insulating material is exposed to conditions which contaminate its surfaces, the tendency for the resultant transient leakage currents to form conducting tracks on insulator surfaces is of recognized importance. This form of failure is known as tracking and the processes leading to its development are very dependent on the ambient conditions. The work reported here uses a contaminant internationally accepted for assessing the degree of resistance of insulation to tracking and indicates some of the factors important in causing breakdown under these conditions. Above a certain voltage the time to failure depends on the power input to small arcs on the surface of insulation. At low voltages the dependence on power input is not so marked and failure is more dependent on chemical processes. The electric field necessary to produce tracking in track-resistant materials falls off much more slowly with increase in electrode spacing than is the case for materials with poor resistance to tracking. It is suggested that classification of materials by their changed susceptibility to tracking with change in electrode spacing may be advantageous. The values of direct voltage required to produce tracking are sometimes substantially greater than those for alternating voltage. If, however, reversal of polarity occurs the direct-voltage values are drastically reduced.