Electrostatically (faraday) sheilded transformers are designed to protect sensitive electrical and electronic devices and systems from high frequency voltages (electrical noise) or transients that occur due to switching and loading on distribution lines.

Electrical noise and transients are classified two ways:

Normal Mode - noise which appears between the hot & neutral current carrying conductors.
Common Mode - noise which appears between between the ground wire and hot and neutral current carrying conductors. Common mode noise is more prevalent and should be the key criterion for any noise suppression device.