Besides Normal Wear / Loosening, which is listed as a possible cause of high mica?

• A. Heavy Slot Bar Marking
• B. Light Brush Pressure
• C. Abrasive Brush
• D. Airborne Contaminants

Answer: (A)

High mica shows up when the insulating mica between the copper bars is raised or exposed on the surface. Besides normal wear or loosening, heavy slot bar marking is listed as a cause because the slot bars can gouge or score the mica as the brush contacts move across the commutator. Those deep marks can lift the mica slightly, increasing the measured mica on inspection. Light brush pressure wouldn’t typically cause this lifting and might even reduce wear, while an abrasive brush tends to remove material rather than create raised mica. Airborne contaminants can foul surfaces but don’t inherently lift the mica to create a high-mica condition.