Deals with phase to phase insulation coordination, completing the principles and rules laid down in Publication 71-1, as well as the Application Guide proposed in Publication 71-2. Having specified the general principles, it gives the standard phase to phase insulation levels for ranges A (1kV to 52kV), B (52kV to 300kV) and C (above 300kV) and C (above 300kV). An application guide deals with voltage stresses in service and clearances in air.