If the switch fails open, {HEAD ~ the potential} at {TAIL ~ the load side} of the switch will drop, and the second diode will allow current to flow to the load through the circuit 
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If the switch fails open , { HEAD ~ the potential } at { TAIL ~ the load side } of the switch will drop , and the second diode will allow current to flow to the load through the circuit
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH