Why Neutral Busbar CT & transformer neutral CT connected
with phase CT in Restricted Earth Fault?

Answer Posted / pintu

THIS IS CALLED 5 CT CONFIGURATION (3 PH. CT + 1 X'MER NCT +
1 BB N CT)OF REF PROTECTION. ALL THESE FIVE CTS ARE TO BE
CONNECTED IN PARALLEL. TRANSFORMER NCT SHOULD BE LOCATED
AFTER NEUTRAL BIFERCATION (I.E. PATH TOWARDS EARTH) AS THIS
IS DEDICTAED FOR DETECTING THE RESIDUAL CURRENT IN CASE OF
GROUND FAULT. BUT NEUTRAL BUSBAR CT IS NOT FOR DETECTING
THE FAULT CUURENT BUT FOR THE UNBALANCE CURRENT IN THE
SYSTEM. IT SHOULD NOT BE TRIPPED DURING THE EARTH FAULT.

IN 4 CTS CONFIGURATION OF REF PROTECTION, NEUTRAL BUSBAR CT
CONNECTION WILL NOT BE THERE. YOU SHOULD DECIDE THE 5 OR 4
CT CONFIGURATION AS PER YOUR SYSTEM DESIGN.

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