NS156 / NS159 Series NCC National Controls Corp Liquid Level Boards
As liquid is removed from a tank and contact with a probe is interrupted, a fixed time delay is started. At the end of the time delay, the output is activated until the level rises and contacts the probe.
On the Model NS159, when the output is turned on, a fault timer is started. If the output is still on at the end of the fault timer, the fault timer will shut the output off until voltage is removed and re-applied to the control system. The purpose of the fault timer is to eliminate overflow due to an open probe.
If the tank is non-conductive, a second (common) probe must be installed at a level where it will always be submerged.