Insulator Leakage Distance Dimensioning in Areas of Winter Contamination using Cold-fog Test Results

Electrical utilities have adapted insulator leakage distances to a wide range of contamination environments. This has led to development of guidelines for unified specific creepage distance, for example in IEC 60815. Under freezing conditions, a number of factors change. The surface conductivity of ice is much lower than that of water, but there is the countervailing possibility of accumulating a significant amount of ice. One of the crossover points between applications occurs when a very light layer of hoar frost or freezing fog, simulated in a "cold fog" test, fully wets a pre-contaminated insulator. The leakage distance requirements for these conditions are compared with those suggested for conventional clean-fog requirements. The possibility of treating fog accretion as a form of non-soluble deposit density is explored.