Screening

A metallic insert between two windings reduces the transfer of disturbances. You have to differ between a static and a magnetic screening.

Safety screen
Separation of dangerous active components by use of a conductive shield which is located between the two parts and is connected to a terminal for an outer PE conductor.

Static screen
A static screen is done to attenuate line disturbances and voltage peaks in high frequency conditions (EMC-transformer). The static screen has a green/yellow terminal. To control operation the screen can be equipped with a terminal at the beginning and a terminal at the end. One terminal is cut off after the test.
A better attenuation can be achieved by installing two static screens. One screen is connected to the protection conductor and has a green/yellow terminal. The second screen has a secondary potential and a black connection. The most used material is copper foil. Though for EMC purposes high permeable foil suits better because the attenuation here is better at high frequencies compared to Cu-foil. With highly permeable foil an increase of attenuation up to 20 dB can be reached at relatively low frequencies. The terminal is connected to the core.

Magnetic screen
The magnetic screen lowers the outer magnetic stray field of the transformer. The screen is implemented by an encasement with highly permeable material. The material, e.g. mu-metal, can be built as a housing around the transformer or installed as a metal sheet. Magnetic screening usually doesn’t have a terminal.

Both ways of screening raise the dimensions of the transformer or lower the power at the same size.
Further terms to this topic:
EMC-transformer, mu-metal, mu metal pot, voltage disturbances, EMC-screen, special screen, capacitive screening, grid-to-plate capacity, permeability.