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Discussion Starter · #1 ·
Hey you guys, just wanted to know your feedback on this current situation that I have. I have two pumps in my plant, each pump must have a LOW LEVEL FLOAT so the pump will stop in low level. The panel that I have has only one input for one float. I had the idea to add a selector switch for LOW LEVEL FLOAT 1 and LOW LEVEL FLOAT 2 and connect it to that input and pick witch LOW LEVEL FLOAT I want to use. The problem comes when I want to use both pumps, is there a kind of switch that I can use for LOW LEVEL FLOAT 1, LOW LEVEL FLOAT 2 and BOTH in case I want to use both pumps and still want the control panel to stop when one of the Level is low? Se attached picture of idea. Thanks!
 

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It depends. you really probably can't do as you envision without extra work because you only have one input. If both pumps can be controlled by one float then use a two position switch and label one position A & A+B and the other position B and connect the primary float to A. If they need separate floats you need a controller or a relay system to disconnect one float when the other is activated. Not too difficult to construct but it will tell the system that both are down if one quits.
 

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I'm confused - IMO you need to provide more information about the process.

Is this a situation where low level indicates an alarm or hazardous condition, in which case the logic behind this would be A OR B = ALARM

or is this a case where the pumps are pumping the same liquid from different tanks and when they are both out of product, you want the panel to shut down the process because A AND B = SHUT DOWN SEQUENCE

(in either case the logic could be as simple as twisting the correct wires, or adding an additional relay(s) or switch.)
 

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Discussion Starter · #5 ·
It depends. you really probably can't do as you envision without extra work because you only have one input. If both pumps can be controlled by one float then use a two position switch and label one position A & A+B and the other position B and connect the primary float to A. If they need separate floats you need a controller or a relay system to disconnect one float when the other is activated. Not too difficult to construct but it will tell the system that both are down if one quits.

Thanks! But how can I prevent the backfeed?
 

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If your running out of input or something for your float you could always make your contactor coil pass through the float contact but even so i really recommend using a timer relay for the float when your tank get full and you got some wave inside that tank your pump will go crazy.

for the selector switch i dunno... you should be able to always have your float active without switch.
 

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Use a 3 position switch with 2 Normally closed contacts. I - O - I. Have it so when they select pump A, the switch is actually opening up pump B contacts and when they select pump B the switch is opening up Pump A contacts. When the switch is in the OFF position both A and B contacts would be closed.
 

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If both pumps are doing the same thing, like filling one tank, then yes you can use one input to control both pumps.
But both floats need to be in parallel with the input terminal.
Of course we need to know exactly what you are trying to accomplish.

If both pumps do something different or fill or empty their own tanks, then your one input will not distinguish between the two pumps.
You can get a warning, shut down or start from either one, but you will not get individual control of both unless they both are doing the exact same job. (meaning both pumps start, and run simultaneously.
What type of PLC and can you add another input card? Or is it a fixed amount of physical inputs and outputs?

Dorian is correct. Make a simple drawing and make sure what you want to do will work on paper before you try it in a real world operation.

You can make the drawing and post it. Then you can get much more help.

Or send Dorian the scope of operation and for a marginal fee, he can draw it for you. LOL!
 
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