preventing groundloops while handling a breaker with an mcu

i want to turn on a machine (i dont quite know what it is) on a 5V mcu signal

but i have two boards... three boards

1) power supply
2) mcu board
3) "power interface"

so the power supply has 5V for my mcu board and 12V for the "power interface"

the "power interface" has a transistor to drive a relay which handles the 220V breaker coil (resistors and diodes included). that part seems fine

but if i drive the transistor directly from the mcu board i believe i'm gonna cause a ground loop

that's why i want to place an optocoupler, to isolate one board from the other and have only the power supply ground as reference

but once i have it, the optocoupler transistor driving the relay transistor will look like a darlington array. is that ok or should i remove the transistor driving the relay?



thank you