In a four wire connection the clock pulses are formed from {HEAD ~ the a.c. control voltage} arising between the neutral point and {TAIL ~ the neutral line} 
In a four wire connection the clock pulses are formed from { HEAD ~ the a.c . control voltage } arising between the neutral point and { TAIL ~ the neutral line }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL