ELCBs Sensitivity Requirement

I don't know. How often do you mind having to reset a breaker.

One RS 422 transmit line requires almost 30 mA of current to drive just the terminating resistance. It seems to me that you are need to find somebody that knows more electronics than you. Your question is so incomplete, I do not know where to start.

I'm not sure electronics is the right depth. , I was referring to MCCs that run industrial pumps and motors. Would harmonics from 3x7W pumps be present in the linked MCC for instance and cause nuisance tripping to 30 mA ELCBS?

I said this in another thread and it needs repeating here.

Circuit breakers and fuses protect wires from igniting the insulation when too much current gets drawn by an intended or unintended load. They do not protect loads.

As such there are a lot of "rule of thumb" cable ratings that change from manufacturer to manufacturer, application, insulation used and cooling capability. Thus one cable company recommends no more than 0.15 A for 30 AWG wire while another advertises a 30 AWG Kapton insulated wire can handle 4 A.

As such it depends on a lot of different factors. Apparently you do not know the nature of these critical factors or you do not know which factors are critical. Either reason, you have not informed us enough to choose anything for you.

As 30mA ELCBs are installed for personal protection, what do you think the chances would be in finding them in a motor circuit, or the control circuit for one? You need to read your local electrical standards before dabbling.