In a three wire connection the clock pulses are formed from {HEAD ~ the a.c. control voltage} arising between {TAIL ~ the first and second neutral points} 
arising between
In a three wire connection the clock pulses are formed from { HEAD ~ the a.c . control voltage } arising between { TAIL ~ the first and second neutral points }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL