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