FARADAY SHIELD

In some applications, interwinding capacitances are very undesirable. They are completely eliminated by the use of a Faraday Shield between the windings. Sometimes called an Electrostatic shield, it generally takes the form of a thin sheet of copper foil placed between the windings. When connected to circuit ground, a Faraday Shield intercepts the capacitive current that would otherwise flow between transformer windings.
Faraday Shields are nearly always used in transformers designed to eliminate ground noise. In these applications, the transformer is intended to respond only to the voltage difference or signal across the primary and have no response to the noise that exists equally (or common-mode) at the terminals of its primary. A Faraday shield is used to prevent capacitive coupling, of this noise to the secondary.