I need help in designing an automatic on/off panel for my submersible pump. I live in an area where the yield rate of the ground water is low hence the pump is susceptible to the risk of "dry-running" in which it will pump out all the water in ground before it well has enough time to recharge - a situation that can cause damage to the expensive pump. Hence, I would like someone to help design a control panel for my pump with the following requirements:

(1) I want an electrical panel that will use a contactor for switching but will take 3 different inputs. First input is a pair of electrodes that will go down into the well to monitor the water level and send signal to the control panel when there is a differential in the water level (an indication that the well is about to be emptied). The second input is a float switch connected to an overhead tank that will send a signal if the water has reached the upper level and about to overflow (to prevent wastage), while the second signal is for when the water level in the overhead tank has reached the lower level (to initiate an automatic pumping signal to the controller). The third input is a timer (set at 3 hours but configurable) that will not allow the pump work for more than 3 hours (or any other preset time) no matter the signal from the other two inputs. The "truth table" of the operations of all the input combinations and the corresponding output is attached.

(2) Additionally, if any of the input is not needed (e.g the float switch), I should be able to "jumper" its terminals to allow for the other inputs to work.

(3) The outer casing of the panel shall have a knob switch with two options - "auto" and "manual". The "auto" shall incorporate all the three inputs while the "manual" shall be a manual override that bypasses the input and only connects to an independent on/off switch that will operate without the logic inputs or contactor output. Hence, the outer casing shall also have an independent "on/off" switch for the manual override.

(4) In the event of power outage, none of the inputs is reset and when power is restored, the system will continue to work as it was before the outage.

(5) Contactor to fail in an "off" position or open circuit position.

(6) The other components like the starting capacitor, thermal fuse etc shall also be accommodated and connected to the circuit.

 

I would think one of the solutions would be a Programmable relay, it is a miniature PLC with timers etc & programmable logic.
Max.

 

The control panel is not too bad to design and put together.
The harder part is sourcing the individual components, "catalogue engineering" as it's called.

But your solution is not ideal - best is to also have an AC power meter monitoring motor power. This tells you right away it's running dry. Or a flow switch. Or a current switch. Running it for 3 hours dry doesn't protect it. It needs to go into Lockout and not keep restarting, retrying as well.
Load Controls has pump controllers and I've used their power monitors. Many other brands out there.
If you just buy a bunch of components i.e. switches, contactor, timers, liquid sensors etc. you end up with something inflexible and usually a rat's nest of wires and difficult to troubleshoot.
I would say first look at dedicated pump controllers out there, or make your own with a low cost micro PLC, something programmable so you can easily add sensors/delays/logic if needed.

An example of drama is splash or tank slosh - so a level sensor gives out a false signal. Then you would add a timer, i.e. has to hit tank full sensor for solid 3-5 seconds to be considered full and turn off the pump. With a small PLC or something programmable, easy to add a delay but using only hardware have to add a time delay relay. So leave room in the enclosure for surprises.

 

The nice thing with the Smart Relay is the logic can be changed without alot of rewiring, also you can have a display monitoring, if needed.
Max.

 

I was just about to suggest a smart relay - then I scrolled down...
For the electrodes that measure the level in the well you need a special controller for this. DC across the probes will lead to corrosion. Something like this. Uses AC across the probes.
Also, your smart relay and external float switches need to be operated from a low voltage - isolated from the supply - for safety. I would suggest 24V DC from a small power supply in the control panel.

 

The design of a control panel depends on the complexity of the device. Here is an example to illustrate.