Triangular Instrument Grounding

For Instrument grounding often separate bars are provided. Those ground bars are then tied to the grounding grid. This is often done with a triangular arrangement with three earth rods that are tied together via copper conductor. One rod is connected to the Instrument ground, one to safety ground etc. The third rod ties to the ground grid and this rod is more deeper that the other two. This system is also called Master Safety ground (MSD).

I want to know the bases of this system meaning which standard is this system following? how do we calculate the grounding electrode size, its depth and distance between electrode etc. Often it is said that it is a vendor recommended system if so that what is the proper grounding method (clean earth) as per standards for instrument and electronics equipment on the plant.