Electrical installation & energy efficiency

I'm not sure I understand your question. I assume that by electrical earthing you are referring to your AC power source ground or earthing. This is a fundamental part of your power distribution and should always be available to your equipment. For electronic earthing, I assume you are talking about the input and output sections of the PLC. If you are using AC modules, then your neutral should be very close to the earth potential. You may, however, be able to measure a small voltage (less than 5VAC) between neutral and earth/ground depending on several factors.

If your I/O modules use DC power, then you may not want to use any earthing connection because your DC power is usually generated from AC power. A connection to earth/ground on both sides of that power supply may cause a partial short circuit inside the power supply. Most of the newer power supplies now have complete isolation but any noise from the AC side can infiltrate the DC side and cause problems.

Mixing AC and DC successfully takes some specific electrical knowledge and then verification if you attempt it.