Why equalizing wire is used in Current transformer?

Well! The equalizing wire is generally used in MV Switchboard mounted Ring Type or Bar Primary Type CTs. In such CTs, the inner wall of the CT Mould is at line potential and the outer wall at earth potential. Thus, there is a potential difference between these two. The insulation of the CT forms the dielectric. The inner bar (primary of the CT) acts as one plate of a capacitor and the air surrounding the CT acts as another plate. Thus, a capacitive coupling is formed and a small leakage current starts flowing thro the insulation of the CT, thus heating & damaging the CT insulation over a period of time. To avoid this, a small wire is looped from the inner busbar of the CT to a screw embedded in the outer wall of the CT. Now, the inner and outer walls of the CT are at the same potential and no leakage current can flow thro the insulation. Thus, any periodic insulation degradation is avoided.