When rubber-insulated cables operate at higher than 1,500 volts potential, conditions may be favorable to the production of corona discharge over all or a portion of the outer surface of the insulation, or its braid covering. The cable may be on an inner metal conduit surface, or lie on a grounded flat metal surface, all of these conditions being favorable to surface corona at sufficiently high voltages. Corona discharge is known to be objectionable because of the chemical effect of the actinic ozone produced, and because of inductive and radiation effects, notably interference with radio reception.