Pumping Down a Sump Using One Level Switch with an Off-Delay Timer

This level control application uses only one level switch to control automatic pumping down of a sump.

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When the level rises to point “A”, the pump turns on. The pump continues to run, emptying the tank, until the timer turns it off.

The pump motor starter is activated by 220VAC control voltage at the Level Switch #1 relay contacts. Wiring terminations are made between Common (C1) and and Normally Closed (NC1). The switch is 

set at “Fail-Safe High”, which means that it will energize the relay output upon absence of liquid at the probe, and de-energize the relay upon presence of material at the probe. In addition, the Timer that is built in to the Level Switch electronics is set for OFF delay for one minute.

The Level Switch will de-energize (closing the NC contact) as the level rises and touches the probe. To keep the pump running as the level falls, the Off-delay timer keeps the Level Switch relay de-energized until the preset time is reached. In this example, we chose one minute to pump down. The time range is selectable from 1/8 second to 2 hours.

Level Switch “B” is set for “Fail-Safe High” (energize upon absence of material at the probe) with the wiring terminations made between Common (C1) and Normally Open (NO1) on the #1 relay contacts. 

Level Switch “B” must energize the relay output to enable the pump to run. When the level rises to point “B”, Level Switch “B” will de-energize, and shut off the pump motor.

The second set of contacts on the Level Switch relay, C2 and NC2, are used to provide remote indication that the Pump should be running.

This Fail-Safe logic is used to prevent accidental overfill if Level Switch B’s wiring is cut or relay contacts are burned.


  • Simple, reliable operation – no moving parts.
  • Minimal time to install and set up.
  • Minimal maintenance.
  • Calibration can be done without disturbing the process.

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