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I am building heater controller system for 8 heaters using PIDs and contactors. Heaters are divided into 4 groups i.e. 4 pairs of heaters. Each group has its seperate thermocoupler, PID and contactor. We can set desired temperature for any group and respective PID will control heater contactor to toggle heater. Both heater in a group will toggle at same time, since using one PID and one contactor for each group.

If its confusing, it looks something like this:- Desired Configuration

As an input for overall controller, we have three phases denoted as R, Y and B. Now to figure out how load is distributed between phases I am referencing one premade controller available in market. But the problem is that somethings I cannot figure out by looking into it.

Please checkout the pictures of premade controller:- Front Back Inside Whole

I know these pictures are not very helping but my problem doesn't require that much details. Let me explain what it does.

  1. R, Y, B indicators indicate the input 3 phases
  2. 4 red indicators indicate if heater group is ON or not
  3. 4 amp meters show the current usage of each heater group
  4. 4 Rotary switch knobs manually turns off heater group, if required
  5. 4 PIDs to control each heater group based on thermocoupler for each group
  6. 4 contactors(one missing in picture) to control each heater group
  7. 4 Pole MCB (R,Y,B,N) for whole system

From this controller I can't figure out why do they use 3 pole contactors. And if in each group they are trying to provide each heater with different phase(which seems to be a good practice though), how are they managing to show ampere reading in single amp meter for a group.

And if they are using single phase for each group, then why they want 3 pole contactor!

Please help me out figuring this circuit or please suggest a circuit with balanced load.

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