Using this electronic circuit can be check if there is between two points or not galvanic connection. Continuity indicator can give an answer in case high resistance. The circuit indicate continuity when measured resistance is between 0 and 5 MOhm, over 5 Mohm resistances are shown as interruptions.
Since a FET gate current not practically exist, it is controlled by the voltage gate. If on this way there is a power interruption, the gate voltage is around 3 V and T1 conducts.
The source voltage is close to the supply voltage and T2 controlled and D1 LED lights.
If the resistance is less than 5 Mohm the gate voltage decreases and the FET behaves as a high resistance. This decreases voltage source and T2 blocked. T3 receives a current in base an start to conduct and therefore D2 light, D1 is off.