A second sensor located externally of the vessel and within a coupling in fluid communication with the vessel serves as a backup safety device and is connected to a second relay coil also having contacts interrupting the flow of electrical energy to the heating element when {HEAD ~ the water level} is below {TAIL ~ the outside sensor} 
is below
A second sensor located externally of the vessel and within a coupling in fluid communication with the vessel serves as a backup safety device and is connected to a second relay coil also having contacts interrupting the flow of electrical energy to the heating element when { HEAD ~ the water level } is below { TAIL ~ the outside sensor }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL