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Transformer Screening

 

 Definitions of ATL screens

 

Safety isolating earth screen

A metallic material (typically copper foil) between the primary and secondary windings offering isolation to ground between the two windings sometimes referred to as: Separation of dangerous active components by use of a conductive shield, which is located between the two parts and is connected to an external earth terminal.

 

 Static screen

A static screen is used 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 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 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 increase of attenuation up to 20dB can be reached at relatively low frequencies. The terminal is connected to the core.

 

 Magnetic screening

The magnetic screen 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 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