Use a 2500V insulation resistance measuring instrument (also called an insulation megger, including an insulation shaker) to measure the primary winding, each secondary winding, and the final screen. The non-tested winding (or final screen) and housing should be grounded during measurement. You can connect the primary short circuit to the L terminal of the meter, and the secondary short circuit to the ground terminal to connect to the E terminal of the meter. This is the "primary to secondary and ground" insulation of the measuring transformer (similar to the high-to-low-ground insulation of the measuring transformer). It is also possible to measure "secondary versus first ground". Generally, the insulation resistance to ground is not less than 1000 megohms.