Used to monitor the level of conductive liquids in tanks and wells. It connects with hanging-type electrodes. Its operation is based on measuring the resistance of the liquid between the electrodes. The relay activates when the liquid level reaches the top electrode and deactivates when the level drops below the middle electrode. The lowest of the three electrodes serves as a reference and can be replaced by ground when the tank is metallic. The use of a safe low alternating voltage between the electrodes prevents electrolysis phenomena. The complex resistance between the electrodes should be less than 20kΩ.