Ground Rod Diameters
NEC grounding is to help prevent accidents to persons and damage by fire to property in case of lightning, breakdown between primary and secondary windings of the transformers, or accidental contact between high-voltage wire and low-voltage wires. If some point on low-voltage circuit if grounded:
1. Lightning striking the wires will be conducted into the ground
2. Breakdown of the transformer insulation between primary and secondary coils will reveal itself through blowing of the primary transformer fuses if one wire of the primary circuit comes in contact with one of the secondary wires.
You need to keep in mined that the utility does ground their system and the ground (earth) does become a parallel path back to its sources. If the secondary side of the transformer is not grounded, and a crossover does occur the primary 7621 volts, (on a four wire 13.2KV destitution system) would be impressed on the secondary system. The insulation of the secondary system only max 600 volts, and numerous insulation breakdowns, which would cause considerable damage to equipment, and there would be extreme damager to human life when coming in contact with any part the secondary system.
An interior wiring system if considered as being ?exposed? to voltage higher than that for which the system is designed if it is supplied through a step-drown transformer, because there is a possibility that the insulation between the primary and secondary windings of the transformer may break down.
The purpose of NESC grounding is to provides practical methods of grounding for use where is required as a means of safeguarding employees and the public from injury that may be caused by electrical potential on electric supply or communications facilities.
The object of protective grounds on electric circuits or equipment, as required by the rules of the NESC, is to keep some point in the electric circuit or equipment at, or as near as practical to, the potential of the earth in the vicinity. Grounding helps to prevent harm to persons or damage to property in the event of accidental contact of high-voltage conductors with low-voltage conductors; breakdown between primary and secondary windings of transformer; etc.
In the order of descending effectiveness, grounding systems serve to
1. enhance prompt operation of system fault-protective devices and
2. minimize the exposure of personnel to electrical potential.
Under high-capacity grounding fault or lightning conditions, substantial voltages may develop between locations on the earth?s surface only a few feet apart, due principally to the very appreciable resistance of the earth itself. Good grounding alone will not remove this hazard; additional means are required. Where there is a high probability that personnel may be exposed to large step potentials resulting from the operation of fault-current or other protective devices, such as a supply substation, the effective potential may be minimized by the use of properly spaced buried grid conductors and by covering the earth with coarse crushed rock in the critical areas.
Utility prefer grounding connection at all building entrances served by any particular secondary circuit are desirable, since they (1) permit ready means for inspection and testing and (2) because of their number, (of grounding point), they provide good insurance against the entire loss of the ground connection. Since the resistance multiple grounds varies very nearly inversely as their number, a larger number will more readily open automatic protective devices in case of accident and provide a greater degree of safety.
