1-It is clear, when a system is designed as a TNC grounding system, the separation of neutral and ground bars may increase the impedance of system ground loop; because in TNC system the neutral path is a part of grounding system.
2- Good grounding path of sufficiently low impedance ensures fast clearing of faults. A fault remaining in the system for long may cause several problems including those of power system stability. Faster clearing thus improves overall reliability. It also ensures safety. A ground fault in equipment causes the metallic enclosure potential to rise above the ‘true’ ground potential. An improper grounding results in a higher potential and also results in delayed clearing of the fault (due to insufficient current flow). This combination is essentially unsafe because any person coming into contact with the enclosure is exposed to higher potentials for a longer duration. Therefore, substation reliability and safety must be as ‘built-in’ as possible by good grounding scheme, which in turn will ensure faster fault clearing and low enclosure potential rise.
3- Conductors must be large enough to handle any anticipated faults without fusing (melting).
Failure to use proper fault time in design calculations creates a high risk of melted conductors. Two aspects govern the choice of conductor size: the first is the fault current that will flow through the conductor and the second is the time for which it can flow. The fault current depends on the impedance of the ground fault loop. The time of current flow is decided by the setting of the protective relays/circuit-breaking devices, which will operate to clear the fault. The IEEE 80 suggests using a time of 3.0 s for the design of small substations. This time is also equal to the short-time rating of most switchgear.
4- It is very evident that the connections between conductors and the main grid and between the grid and ground rods are as important as the conductors themselves in maintaining a permanent low-resistance path to ground. The basic issues here are:
-The type of bond used for the connection of the conductor in its run, with the ground grid and with the ground rod
-The temperature limits, which a joint can withstand
The most frequently used grounding connections are mechanical pressure type (which will include bolted, compression and wedge-type construction) and exothermically welded type. Pressure-type connections produce a mechanical bond between conductor and connector by means of a tightened bolt-nut or by crimping using hydraulic or mechanical pressure. This connection either holds the conductors in place or squeezes them together, providing surface-to-surface contact with the exposed conductor strands.
On the other hand, the exothermic process fuses the conductor ends together to form a molecular bond between all strands of the conductor.
Temperature limits are stated in standards such as IEEE 80 and IEEE 837 for different types of joints based on the joint resistance normally obtainable with each type.
Exceeding these temperatures during flow of fault currents may result in damage to the joint and cause the joint resistance to increase, which will result in further overheating.
The joint will ultimately fail and result in grounding system degradation or total loss of ground reference with disastrous results.