Clean Earthing vs. Dirty Earthing

Earth is earth is earth.

However, the principle between power and instrument earths is to gather all the power earth conductors onto a busbar and bond that to earth, then gather all the instrument earths onto a separate busbar, and bond that to earth via a separate conductor. The earthing point can be the same.

Grouping the earths onto separate busbars minimises the risk of noise on the power earth conductors affecting the instrument earth conductors and inducing unwanted signals onto those circuits.

Electronic equipment are sensitive to raise in Earth potential, which in turn is due to increase in Earth resistivity.

Hence, a separate "Treated earth pit" is assigned for terminating all Clean Earthing Signals. Resistivity is to be monitored and periodic maintenance is to be made (adding water to the pit). In a big plant, it is not possible to consider "Treated earth pits" for all points.

Noise is another concern.

As per IEEE 1100, there is no such thing as Clean Earth, Dirty Earth, Signal Earth, etc. To quote verbatim:

Vide Cl. 9.10.12.2 of IEEE 1100, "the use of any separate, isolated, insulated, dedicated, clean, quite, signal, computer, electronic or other such improper form of earth grounding electrodes for use as a point of connection of the EGC (Equipment Grounding Conductor) is not recommended".

Even IEC does not permit this.

Vide IEC 60364-7-707: The use of independent, "isolated" earth electrodes for computer or

electronic systems is not recommended/

permitted.