Protection Relay Earthing in Power System

Am confused several times while working with different consultants & clients. Instrumention (PLC, Meters etc) & Protective relays Installed in Power system needs to be earth seperatly to Instrumentation earth pit or to Electrical earth or No earth?

Finally in 1998 I had experiment running Instrumentation+ protection Relays earth bus & electrical bus seperatly upto Pit level & combine at pit level.

My Explaination as

My recommendation here is to bond together the clean earth & the dirty earth systems at lowest point. i.e. as close as possible to ground & preferably incorporating the point where the transformer starts point is earthed. The reason for this is as follows.

If there is a lighting surge or large earth fault, the Potential of all metallic parts connected to the dirty earth system will rise above the Potential of the general mass of earth. This is because of the volt drop cause by the surge/fault current flowing to earth through the resistance in the earth electrode system. Now if the clean earth system is not connected to dirty earth system, i.e. It has its own earth electrode, and then all parts connected to the clean earth system well remain at the true earth potential.

Therefore a high potential difference could appear between the "clean earths". Parts & their metallic enclosure (because the later is sure to be connected to dirty earth) & this could in turn lead to voltage break down. [High voltages can similarly occur between the system neutral & the polls of any isolators (i.e. unearthed) Power supply]. If, how ever the clear earth system were to be connected to the dirty earth system than the potential of both system would rise & fall together & there would not be poteinal difference between the paths in question & there enclosure Voltage breakdown is thus avoided.

During steady state condition, no currents are flowing to earth & hence the paths connected to both Clean & dirty earth will be at the same potential. This is why the DC potential the u refers to are the same when u measure between dirty & clean earth.

Incidentally, the reason for connecting together the two earths at the lowest point is to minimize the voltage arising from any small "Noise" currents flowing in earth system.

Please suggest.