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Hi, I am trying to build a little control system for a washing system. It comprises of 2 tanks which both have 2 x 9kW heaters in so a potential of 36kW. However the generator to run this will only allow 2 heaters to be operated at any one time. I have 2 X 3 position selector switches on the control panel. They are labelled TANK1 and TANK 2 respectively. They have a mid OFF position then a HEATER 1 ON and BOTH HEATERS ON position. I need to be able to interlock the heaters and tanks somehow so that only 2 heaters can be selected ant any one time. Unfortunately the customer wants the option to have 1 heater on in each tank also which is the part I cannot get my head around at the moment being a newbie to ladder logic. Can anyone point me in the right direction please? I am also looking for a PLC programmer in the South of England if anyone knows of a good one? Thanks Duncan

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What I would do is this: If both tanks are in an "ON" position, and at least one of them is in the "Both Heaters" position, just turn on one heater for each tank, ignoring the two tanks command. You'll need to indicate to the operator that something isn't quite right. If you have an HMI, then great, you can raise an alarm. If you only have a console, put a an indicator that will flash when there's a problem. Hopefully this gets you pointed in the right direction?

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So each of those two switches can drive 2 inputs on the PLC. Call them tank_1, tank_2, 1_heater, and 2_heater. I'd write the logic so that if tank_1 AND tank_2 are off, and 2_heater is ON, then turn on 1 heater in each tank, plus flash a light or whatever warning you think is necessary, based on what other feedback is available to the operator.

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If you are new to programming, try to remember to structure your logic like Logic. IF | | Switch 1 is in 2 heaters position THEN (Tank 1 heater 1) AND |/| switch 2 is NOT in 1 heater position OR |/| switch 2 is NOT in 2 heater position THEN (Tank 1 heater 2) repeat and rinse....