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