CONCEPT:There are 5 types of insulators used in transmission lines as overhead insulation:Pin InsulatorSuspension InsulatorStrain InsulatorStay InsulatorShackle InsulatorAs the figure is given below shows the plane of discs of the strain insulator is VERTICAL. Pin, Suspension, and Strain insulators are used in medium to high VOLTAGE systems. While Stay and Shackle Insulators are mainly used in low voltage applications.Strain type insulator used at dead ends and on a STRAIGHT line as suspension type for voltage 3.3 kV and above.A strain insulator must have CONSIDERABLE mechanical strength as well as the necessary electrical insulating properties. Explanation:So, the figure shows the plane of discs is vertical.