Many alternating-current rotating electric machines are designed to have multiturn formwound stator coils. In these cases, the winding has two separate but interrelated insulating barriers: one (turn insulation) between the various turns and the second (ground insulation) between the turns and ground. Failure of either of these barriers will prematurely terminate the service life of the machine. A test level, twice rated voltage plus 1 kV, for the ground insulation has been in existence for many years. This guide suggests methods and test levels for the turn insulation.