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