The purpose of the outermost screen of a transformer bushing is to be grounded through the porcelain sleeve, with the entire capacitive screen immersed in insulating oil. The outermost screen allows for measuring the capacitance and dielectric loss of the capacitive screen, helping to assess its insulation condition and understand its performance, which is why it's also called a measuring terminal. To distribute the electric field evenly, the bushing consists of multiple metal screens (main) separated by insulation layers. The outermost screen is the last screen, used for connecting to ground at equal potential; during operation, it is grounded, and during preventive testing, it can be disconnected to measure the bushing’s capacitance and dielectric loss.  

Measuring terminals through the outermost screen can effectively detect defects like moisture in the main and outermost screens, deteriorated insulating oil, or open and short circuits between capacitive screens. However, if the outermost screen is open during operation, it will generate high voltage and can easily cause equipment damage.  

That's an introduction to the purpose of the transformer bushing grading screen. We hope it helps you.