Yes i mean ground it's engine body, but sometimes there are a potention between earth and engine body. 1) check potentional between eng body and ground. Must be 0 volts. 2) check configuration soft: 3phase 4 wire. according picture. or you have another AC system?
As i understand: you connected CT in star but disconnected 54-com from ground? Num: 1, 15-19, 7 must have good ground. May be you have some voltage between "-" battery and engine ground? I usually connect "-"bat and engine ground together - one point.