I created a Raspberry Pico based PLC board which worked perfectly alone, however once I placed it into a distribution box and started switching a fairly large (7.5kW) contactor, the on-board seven segment display started glitching by displaying random digits and eventually turning off. This happens every time I power on the system and must restart it manually once the display turns off.
The contactor is switched on 230V mains by a +5V relay and the relay has flyback diode across its coil, so it should not create much noise theoretically on the low voltage side.
So far I tried removing all unnecessary connections and upgrading the +24V 2A PSU (which feeds the on-board +5V switching regulator) to a 10A one, but the problem still persists.
I did some research and realized that I probably should have added a ground plane to the PCB to reduce EMI, however I already ordered them and look for a solution that does not require redesigning and reordering them.
Any suggestions how to minimize the noise?