Dear Mr. Hamid, Good morning. I read your topics about RCD which is really helpful. But my question about the opearting principle of 4 pole RCD. Normaly the RCD will work by the resultant current i.e. vector sum of currents of all the three phases. But if the load is unbalanced then the vector sum of three phase currents will not be zero which cuase the 4 pole RCD to trip. Another thing if the 3rd harmonics present in the current, then 3 times phase current will flow to the neutral which cuase the RCD to trip. Please advise me how I wil install the 4 pole RCD if 3rd harmonics are present in load. Thanks Tapas Barua.