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