As the brushes ride on the commutator of a DC generator, eventually mica ridges will form between the bars causing sparking and chipping of the brushes. When this happens the mica must be undercut. To what depth is the mica undercut?

Correct Answer: Option C — The mica slots are undercut to a depth approximately equal to the width of the slot.

IEEE 45 maintenance standards specify undercutting mica insulation between commutator bars to a depth approximately equal to the slot width. This prevents mica high-ridges from lifting brushes, which causes severe arcing, sparking, and mechanical chipping.