Screening or screen winding

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 screening 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 operativeness, the screen can be equipped with a terminal at the beginning and a terminal at the end. One terminals is cut off after the test. A better attenution 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 mostly 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 increasement of attenuation up to 20dB can be reached at relatively low frequencies. The terminal is connected to the core.

Magnetic screening

The magnetic screening lowers the outer magnetic stray field of the transformer. The screening is implemented by an encasement with highly permeable material. The material, e.g. mu-metal, can be built as a housing around the transforemr or isntalled 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