I use these blocks from Sparkfun: Solid State Relay - 40A (3-32V DC Input)

They have four screw terminals - two for 3VDC to control and two for 24V - 380V AC for the load. They have the CE certification.

I bolt them down inside a NEMA enclosure and use standard residential wiring techniques (buy a book or two!) to wire them to standard outlets, where I plug in the load.

The controller (Arduino etc.) is typically mounted in a small plastic box outside the NEMA enclosure with a terminal strip to attach sensors and so forth. It's powered by a wall wart.

This approach meets code and is perfectly safe. For level of competence needed - you should be capable of doing residential electrical wiring. That's a useful skill to learn.

You can use smart plugs like Sonoff S20[1] and flash your own firmware. They have the widely used ESP8266 inside. I use ESPurna[2] on them, which has many common integrations built-in (Alexa, MQTT, etc) and is very easy to extend further.

[1] https://www.itead.cc/smart-socket.html

[2] https://github.com/xoseperez/espurna

There are basically two options:

- learn how to and design a PCB with relays or FETs or whatever, following all the best practices for it to not catch fire.

- buy off-the-shelf rail-mounted relays/contactors, then design a driving circuit that will be all low voltage.

Assembling it all on a breadboard won't cut it (as in will blow up sooner or later), that's for sure.

I've been looking into this as well since the time/risk/insurance issues of diy are kind of putting me off. The Functional Devices RIB (Relay In a Box) devices for ~$10 look pretty slick, only downside is 12v control signal. I ordered a couple to play with but haven't had time to install them yet.