General Maintenance Practices effectively address visible components of Electrical System including Earth Electrodes (Pits).
Since a majority of Earthing System is buried underground / inaccessible, it suffers from "OUT OF SIGHT IS OUT OF MIND" syndrome.
Unseen Components like Equipment Earth Conductor (Strip), Earth Grids / Mats which are buried or inaccessible, tend to be taken for granted or ignored, though they are crucial.
A Complete Earthing System with all its components- Equipment Earth Conductor, Earth Electrode, and Earth Grid / Mat, should offer low resistance path for fault current to flow so that breakers trip in time to prevent accidents such as - Fire, Short Circuit, Electric Shock and so on.
The current methodology of inspection of earthing systems limits itself to measuring the resistance of the earth pits. The earthing system is considered safe if the measured values are one ohm or lower.
However, it is extremely important to note that the resistance of 1 ohm in itself NOT AT ALL an indication of safety/healthiness of the earthing system. In fact, irrespective of the measured value, the resistance of the earth pits can never indicate healthiness of the earthing system or otherwise.
The main function of the earthing system is to allow enough current to flow through the earth fault loop so that the protective device can sense a possible fault and isolate the faulty circuit in acceptable time
The magnitude of a current that will flow through the earth loop in case of a fault depends on the impedance of the earth fault loop. (The earth fault loop comprises of the path from the connected phase conductor of the equipment to the grounding system and the source neutral)
Hence, it is imperative to measure the earth fault loop impedance and maintain it low enough to allow sufficient current in case of a fault so that the protective devices can operate and isolate the fault.
In order to achieve this, EARTH LOOP IMPEDANCE TEST for star-connected low and medium voltage systems has to be carried out. Additionally, a prospective fault current test also helps to identify if the rating of the protective device is appropriate. These steps automatically check the integrity of the buried section of the earth conductor loop for the above-cited systems.
For substations and other MV, HV and EHV applications, where there is a large grid/earth mat provided for the facility, a specific riser, and grid integrity test needs to be carried out to check the integrity of the earth connections buried in soil. These tests typically are carried out by injecting a small current through the earthing system and checking the current division and voltage drop. This method is extremely effective as it can pinpoint the connections/risers which are faulty in the buried earthing system.
Both the above tests can be done on live electrical systems without calling for the shutdown.