Which contaminant, especially when moisture is present, can destroy the interrupting capabilities of a dielectric medium?

• A. Dirt
• B. Oil
• C. Gas
• D. Salt

Answer: (A)

The main idea is that a dielectric’s ability to interrupt relies on keeping surfaces clean and dry so that there are no easy leakage paths. Dirt on insulating surfaces, when moisture is present, creates a conducting film or slurry. This adds leakage paths and surface tracking between conductors, so the dielectric can’t hold off the arc or extinguish it as effectively. The moisture makes the dirt conductive by dissolving minerals in it, turning the dirty film into an electrolyte-like layer that carries current. With these conductive paths, the arc can bridge the gaps more readily, and the interrupter’s job of breaking and quenching the current is compromised. Oil and gas are inherently insulating fluids, and salt would only pose a problem when moisture provides a conductive solution; dirt contaminated with moisture, however, provides a readily available, widespread leakage path that most strongly undermines the dielectric’s interrupting capability.