The paper describes how frequent flashovers on powerful ‘limit’ d.c. motors and generators [maximum power-speed product (kW × r.p.m.) = 1.6×106] for gas circulator drives, ascribed to copper drag, have been virtually eliminated, and gives an account of some unusual experiences with carbon brushes and types of brush box.Experience has shown that such machines operating at 600 V (20 V per segment average) can be run day and night continuously at full load with few unscheduled outages, provided that they are kept absolutely clean and maintained at periodic intervals, and copper drag is removed when seen. It is important, therefore, to select a brush grade which produces the minimum commutator wear and does not cause copper dragging.