In a three phase power supply with {HEAD ~ four wires}, the wires are coupled to {TAIL ~ three large star connected resistors} having different resistance values 
In a three phase power supply with { HEAD ~ four wires } , the wires are coupled to { TAIL ~ three large star connected resistors } having different resistance values
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH