Assuming the 15 volt power supply is regulated:
500 milli-ohm resistor is the constant current load being driven.
2.2 ohm resistor is the current sense resistor
Circuit's initial state is relay off.
Pressing the button near the relay turns it on. If all is well, the output of the current sense amplifier will cause the comparator to output a high and keep the relay on.
Normally PNP controls the current, which is driven by an opamp. If the PNP fails shorted, (simulate with adjacent switch) the current sense opamp will hit maximum output as it tries to balance out its inputs. When the voltage exceeds 14 V, it causes the comparator to turn off, and the relay will deactivate. Note that if the button is held down, current will keep flowing. It is the user's job to use his or her brain. :)
