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