The AC Grounded system is a system of electrical conductors in which at least one conductor or point (usually the middle wire or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through a current-limiting device.
The practice of grounding of electrical systems is almost as old as the development and widespread use of electric power itself.
Grounding serves the following principal purposes:
-It provides an electrical supply system with an electrical reference to the groundmass. By connecting a particular point of the supply source to the ground (such as the neutral of a three-phase source), it is ensured that any other point of the system stays at a certain potential with reference to the ground.
-A metallic surface of the enclosure of an electrical system is grounded to ensure that it stays at ground potential always and thus remains safe to persons who may come into contact with it.
-It provides a low-impedance path for accumulated static charges and surges caused by atmospheric or electrical phenomenon to the ground thus ensuring that no damage is caused to sensitive equipment and personnel.
We all know that the insulating layer around the current carrying conductors in electrical systems is prone to deterioration. When a failure of insulation takes place due to aging, external factors or due to electrical or thermal stress, it is necessary to detect the point of failure so that repairs can be undertaken. In a system that has no ground reference at all, it is not easy to correctly pinpoint the faulted location. If, however, a second fault occurs in the unaffected line at some other point in the system, it can cause a shorting path and results in the flow of high magnitude fault currents that can be detected by protective devices.
Thus, one of the primary purposes of grounding is to permit easy detection of faults in electrical systems by providing a path for the flow of currents from the fault point through the ground (and sometimes the earth mass) back to the neutral point of the source.
Some standard references:
-ANSI/IEEE Std 80-2000: IEEE Guide for Safety in AC Substation Grounding
-IEEE Std 81-1983: IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System
-IEEE Std 81.2-1991: IEEE Guide for Measurement of Impedance and Safety
Characteristics of Large, Extended, or Interconnected Grounding Systems
-IEEE Std 367-1996: IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault
-IEEE Std 524a-1993: IEEE Guide to Grounding During the Installation of
Overhead Transmission Line Conductors—Supplement to IEEE Guide to the Installation of Overhead Transmission Line Conductors
-IEEE Std 789-1988 (R1994): IEEE Standard Performance Requirements for Communications and Control Cables for Application in High-Voltage Environments
-IEEE Std 837-1989 (R1996): IEEE Standard for Qualifying Permanent Connections Used in Substation Grounding
-IEEE Std 1048-1990: IEEE Guide for Protective Grounding of Power Lines
-IEEE Std 1050-1996: IEEE Guide for Instrumentation and Control Equipment Grounding in Generating Stations
-IEEE Std 1243-1997: IEEE Guide for Improving the Lightning Performance of Transmission Lines
-IEEE Std 1313.1-1996: IEEE Standard for Insulation Coordination-Definitions, Principles, and Rules
-IEEE Std 1410-2004: IEEE Guide for Improving the Lightning Performance of Distribution Lines