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How To Use a Float Switch for Back Up Pump Control

Back up pump control is a critical part of any pump control application, ensuring that failure does not result in a potentially hazardous overflow or the unwanted emptying of a tank. Ideally, back up sensors work independently from the control system, so that failure of the primary controls will not disable them.

This means basic control features such as hysteresis must be built into the sensor. Float switches, a popular choice for back up control, rarely have the features necessary to work independent of the control system. Despite feature deficiencies, float switches remain popular due to the low cost.

Two of the float switches that can work independent of a control system are the KA-2H (2H) and the KA-2L (2L) cable suspended float switches. So we wanted to show you how to set one up for back up control.

The 2H is designed for high alarm and pump down applications, while the 2L has a low alarm and pump up features. Each have mechanical hysteresis built it, and can be used without a controller or PLC. In this fill control example, the 2L is the primary sensor, and the 2H will back it up.

Note: The contacts in the KA-2L and KA-2H are high performance miniature switches.  Because the floats will be immersed in liquid, it is not recommended to switch pump motor currents directly through the KA-2L or KA-2H.  Rather use a heavy-duty relay or contactor respectively between the 2L, 2H contacts and the pump.

All wires on the 2L and 2H are black with the conductor number printed in white on the wire jacket.

Back up pump control wiring diagram

 

Wiring the KA-2L and KA-2H

  1. Connect conductor # 1 from each float to the Line Power.
  2. Connect conductor # 2 to from each float the switch side of relay/contactor (Relay 1 for KA-2L and Relay 2 for KA-2H).
  3. Connect the power side of each relay/contactor (Relay 1and Relay 2) to the Line Power.
  4. Connect power side of R1A contact (NO) and R2A contact (NO) to Line Power
  5. Connect switch side R1A contact (NO) in series with power side R2B contact (NC)
  6. Connect the relay/contactor power contacts of R2A (NO) and R2B (NC) to the pump power.

 

 Note:  If the pumps use three phase power, use heavy duty three pole switch contacts.  Each contact should be rated to carry the voltage and current required by the pump’s horsepower rating.

The 2H is an example of a float switch that, when used for back up pump control, will prevent a tank from overflowing and shut the pump off with the liquid at a safe level. The float works by reversing the pump until a safe level has been reached. The built-in hysteresis allows the pump to reverse at the high level and stay on until at the lower level.

Let us know if you have any questions about how this works, or how to properly wire the switches and the pump. Our application engineers are ready and willing to help!

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