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