High power relays with safety certifications are typically used in mains boxes available commercially. In fact, I'm using the ones from my old mains box. Even

High power relays with safety certifications are typically used in mains boxes available commercially. In fact, I'm using the ones from my old mains box. Even theoretically, the only way a relay could catch fire if the coil inside overheats or if you pass very high voltages to the coil. But, with a well designed circuit, it should be fine.