In {TAIL ~ a four wire configuration}, {HEAD ~ the control voltage} is the voltage between a neutral point formed by three unequal resistors and the neutral line 
In { TAIL ~ a four wire configuration } , { HEAD ~ the control voltage } is the voltage between a neutral point formed by three unequal resistors and the neutral line
REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH