I replaced a MCB with a RCBO today for socket circuit in small factory unit. I tested it all and it was fine. After i left there was a bang in the CU and the power went down, blowing two of the three main fuses (3Phase). Upon investigation there seems to be soot on the new RCBO which is red phase and yellow phase next to it which is spare. Any suggestions what could of caused this as i've checked all connections and turned the main switch back on and it seems to be sp[arking, so ive isolated and left off for now