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