How does surface contamination influence insulator performance under humid conditions?

• A. Contaminants form insulating surface paths; moisture reduces leakage currents.
• B. Contaminants form conductive surface paths; moisture increases leakage currents and lowers flashover voltage.
• C. Contaminants have no effect on leakage currents even with moisture.
• D. Contaminants remove surface moisture increasing insulation.

Answer: (C)

Surface contamination changes how current travels across an insulator under humidity. When contaminants sit on the surface and moisture is present, a thin conductive film forms along that surface. This film acts as a leakage path, so the resistance along the surface drops and current can flow more easily between the energized part and the grounded part of the system. Because the surface is now conducting, leakage currents rise, and the voltage at which the surface breaks down (flashover) is reduced. The effect is especially strong with salty or mineral contaminants that dissolve in the moisture to form an electrolyte, making the surface highly conducting. In dry conditions the film may be less conductive, but humidity activates it, so the insulator’s ability to withstand high voltage is degraded.