Lief Characteristics of Insulated Wire Subjected to Short Time, High Current Faults

This paper reports a study of the damage done to insulated wire by high currents which act during the brief time necessary for protective devices to clear a bolted short circuit. The damage was measured by the decrease in dielectric strength related to various values of the time integral of the square of the current. This is called I2t. One major advantage of this approach is the definiteness with which significant damage is identified. Results indicate that the value of I2t for which the dielectric strength is reduced to half of its original value is substantially larger than it was previously thought to be. Also, some wires, identified as being suitable for higher temperature use, are more easily damaged by a high current fault than are wires of lower temperature rating. The study was limited to plastic covered conductors for general wiring.