Because it sparked, my guess is the switch you're using is not big enough. The pull-cord style double-pole switch at 45A is foreign to me (I have never seen anything like it in north america), but the specs on the Mira site say to use a switch with at least 3mm of contact separation.
This separation is important, because at these currents there is going to be arcing every time the power is toggled, which can do several things that negatively impact any type of switch/relay:
- "Burn" the terminal pads, which scorches them with the same black you see on the burnt wires outside, and over time, increases the resistance and thus increases the current draw through the pads, leading to even worse arcing
- Weld the terminals together
My guess with the info provided is both of the above happened in this case. The burn on the wires is probably caused by the arching inside the switch: maybe there is inadequate mechanical separation between the contact pads of the switch and the outside terminals?
My inclination, if I were to ever install something like this here, is that 45A is too much for any type of user-facing switch that will be used on a regular basis. I would install a contactor (basically, a big relay) separate from the switch, which is rated at at least 60A (since as Nial C pointed out, depending on the exact voltage, you could be seeing as much as 52A). Then I'd run a separate circuit to the pull-switch (or whatever you're using) that simply controls the power to the contactor coil. This circuit would only have a minimal current in it (<1A) required to run the contactor coil.
Here's a quick diagram hacked together based on the Mira install manual:
Note though, I do not know if this would comply with EU plumbing/electrical codes! It seems a whole lot safer to me as it keeps a human from having to interact directly with a high current switch, and it's likely easier to replace the contactor if needed (though I'd guess it's less likely to be needed, as contactors are very commonly used in industrial systems to control high-current loads for many years without issue).