Question about the electrical drawing


Hi everybody,
I would like to make some modifications to my Robobrew for 2 reasons:

  1. I don’t find the temperature regulation very accurate (fluctuations around the set point)
  2. It is difficult to adjust the strength of the boil.
    As I have bought a PID PWM controller I decided to have a look at the internals of the device.
    I have made this electrical drawing just by looking at the various wires.

    On this drawing there are 3 manual switches that seems to include 2 NO contacts side by side (the contacts are isolated from one an other) and a control board with 2 blue relays (I don’t know how they are arranged) and I don’t know if their contact have on board connections).
    I can follow the circuit for the 1800W resistor starting from the red wire of the 240 V supply. It seems to go through R2, then through the manual switch, then through the resistor, then to the green wire of the supply.
    As for the other resistor (500W) I confess I am a bit lost. I guess the 2 terminals component in the middle of the resistors is a safety switch but I cannot understand how the 500W resistor can be powered.
    If we admit the red phase of the supply is connected (by the control board) to the upper terminal of the relay R1 then we can follow the circuit as we did for the 1800W resistor.

But what seems strange is that in this case, the supposed safety switch shunts the main supply.
Can somebody help me understanding?


After some checking and test, I eventually understood how it works.
I guess the thermal safety switch cuts the circuits of the relay coils as described in the drawing below.
I checked that disconnecting one wire of the safety switch prevented the relays from being powered… It also triggered a warning sound signal.
The connections of the relay terminals are probably what is shown in the drawing below.