as a loop from the field to cause it to operate once operated it will hold on the loop only an example of a ground start circuit is a paystation the presence of a proper coin condition e g inserting a coin causes a ground through a coin control relay to be placed on the line ground start circuitry 106 of the present invention automatically senses the absence of the central office ground and placed a ground on the line for purposes of testing as will be noted the ground start circuitry 106 can only be activated when the verify half tap button 60 is depressed causing contact c of switch k5 60 to disconnect from the common input point 80 and connect to the line 108 voltage to activate the ground start circuitry 106 enters line 108 and passes through a first diode 110 to a second diode 112 diode 112 is a 33 volt zener diode that is it cannot conduct unless 33 volts or greater is present on the anode under normal conditions i e non ground start circuits the presence of the ground at the central office will cause the voltage at the anode of zener diode 112 to be half of the applied voltage because the anode of zener diode 112 is applied to the line circuit at midpoint however if no ground is present the voltage at zener diode 112 will be in excess of 33 volts and zener diode 112 will start charging capacitor 114 which is connected from the.