Generally speaking, the purpose of grounding is to electrically interconnect conductive objects, such as equipment, in order to minimize voltage differences between them. National Electric Code (NEC) requires that 120-volt AC power distribution in homes and other buildings must be a three-wire system
One of these incoming utility wires, which is often un-insulated, is the grounded or "neutral" conductor. Note that both neutral (white) and line (black) wires are part of the normal load current circuit shown by the arrows. Code requires that the neutral (white) and safety ground (green) wires of each branch circuit be tied or "bonded" to each other and to an earth ground rod at the service entrance.
Any AC line powered device with exposed conductive parts (that includes signal connectors) can become a shock or electrocution hazard if it develops certain internal defects. Insulation is used in power transformers, switches, motors and other internal parts to keep electricity where it belongs. (figure :A look at how AC power is typically delivered.)
However, for various reasons, the insulation can fail - effectively connecting "live" power to exposed metal as shown in above figure. such a defect is called a fault.For example, if the motor in a washing machine overheated and its insulation failed, the full line voltage could energize the housing of the machine! Anyone who accidentally touched the machine and anything grounded, such as a water faucet, at the same time could be seriously shocked or electrocuted.
Remember: current will always return to its source, whether the path is intentional or accidental. Electrons don't care - they can't read schematic. (figure : Watch out for faults… They can be mighty unpleasant)
TRIP THE BREAKERTo return this fault current directly to its source, many devices have a third wire connecting exposed metal to the safety ground pin of their plugs. The outlet safety ground is routed, either via the green wire or metallic conduit, to the neutral conductor at the main breaker panel.
This low-impedance connection to neutral causes a high fault current to flow, quickly tripping the circuit breaker that removes power from the circuit. To function properly, the safety ground must return to neutral. (Note that the EARTH connection had NOTHING to do with this process!)
LIGHTNING & DIRT
The earth itself is the return path for the current in a stroke of lightning. To protect people and equipment from lightning, we must make a connection to actual soil.
Overhead power lines are frequent targets of lightning. As a result, virtually all electric power distribution lines have one conductor connected to earth ground periodically along its length. Before this was done, power lines effectively guided lightning inside buildings, starting fires and killing people.
The (NEC) code-required earth ground at the service entry panel serves to direct lightning to earth ground before it enters the building. For the same reason, the code requires telephone, CATV, and satellite TV cables to "arrest" lightning before it enters a building.
Because soil has resistance just like any other conductor, earth ground connections are not at zero volts with respect to each other or any other mystical or "absolute" reference point. Code allows the resistance of this earth connection to be as high as 25 O.
Since this is far too high to trip the circuit breaker under fault conditions, an earth ground should never be confused with a safety ground. Safety ground must be connected to neutral at the main service entry panel. If more than one ground rod is used, Code requires that all must be bonded to the main utility power-grounding electrode.
