I’m looking for a solution to turn on/off a high load of a device (electronic Spa) with 230V / 40A. Right now I use the FI/RDC for that, but it needs to switched manually and I’m looking for a solution to let it switch reliable remotely. Are there any suggestions on that?
Use a regular lower load relay, for lighting or whatever, to control a fat 40A contactor.
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as rossko already wrote, in german it is called “installationsschütz”. i use this for switching my pool-pump. i use a quite normal actor that switches the “schütz”. but pay attention, there are a lot of different models on the market. i think you will need one with contacts that close when activated.
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I gave advice to a colleague who has little smart home experience to use ikea tradfri that drives power contactor, and he is happy with the result. You could also use solid state relay insead contactor. In one my applications I use rPi zero + 8 relay module to drive 6 sprinkler valves and I have connected one relay to a contactor to drive a pump.
i didn´t know solid state relais are available up to 40 ampere, but now i found them. so i would prefere solid state relais (not possible for 380volt but in your case 220volt should work).
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I’d say, a Tradfri won’t last long with a average Ampere per day (24/7) of 20 and a peak load of 40A.
“Normal” household appliances are at max able to cope with 10A for a maximum of 2hours! at least that’s the spec, the EU has in place for the 230V power outlets and so is all “normal” appliances.
So, having a Spa, which not only requires 40A, but also a separate FI/RCD to avoid triggering it, if the pump is starting with a sudden peak will cerainly fry everything I can buy at IKEA (no offense, please!). And the 8 relay modules I know have even less restrictions (and most are imported chinese stuff, which can easily power some Raspberry Pis, but I won’t trust them a coffee machine)
But yeah, a 40A contactor and having it switch via some “normal” relays sounds nice.
You’d use the Tradfri, or Chinese ethernet relay, or zwave switch, or whatever to work the contactor. The contactor does the heavy work.
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oh sorry, just now read thoroughly… 
But yes! Don’t do that at home, seriously!
That’s what trained electricians are for!
(and I won’t put my fingers on 230V, let alone with 40A! But I want to come up with some suggestions for my electrician, which is a cool guy, but not a home automation nerd)
(it is from the real jacuzzi installation mentioned above)
Or one of these…
I’ve seen as low as £70.
By the time you buy a contractor, housing, relay it’s not far off the cost. Plus you get the space saving / cosmetic advantage
Plus you get energy consumption
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Tempting… but I’ll need some zwave stick or gateway or what’s it called for that… 
Sure, but you haven’t given any clues about what communication technologies you are already using and/or have the reach to your chosen location.
You could look at the Shelly EM. It has a contactor output and CT inputs so you could use a solid state relay or contactor
you wont regret going down the zwave route… most realiable / stable connection in my system
Jasoc/GE makes one as well.
https://byjasco.com/products/ge-z-wave-plus-direct-wire-indooroutdoor-smart-switch-40a
I’m building one with a Sonoff and a 40A contactor. Then I can run Tasmota. Extending the button and LEDs to outside the enclosure.
It is hard to get the materials for this for under $100 USD.
I’m building two - one for a 220 (split phase) water heater and one for a well pump.
Nice, but as this is a metal case (at least it seems to be) I doubt that you will be happy with WLAN
Maybe it’s possible to build an external antenna.
You will need other sockets for 40 A 
Power contactors aren’t cheap. Bit googling might help reduce the price but the cheapest 40 amp contactor will cost 50+ euro. You could also try the aliexpress route by getting some cheap SSRs plus heat sink for 10 euro, but not sure how reliable those are.
Per link above the z-wave device is rated for 40A 240 US l1 l2 power. it took a few to verify it was supported. [The other link did not originally show when searching for 240V hence answering for future searchers as all others are meant for 120v us with l1 and N 240v being euro l1 N not us l1 l2 power which is NOT interchangable.]