Posted 23 November 2011 - 03:44 AM
Generally speaking in a properly wired house the 3rd round prong on your outlets is indeed an earth ground. Somewhere outside your house, generally below the meter and primary breaker box, is a large copper rod drove into the ground for which the main breaker box is earth ground to, providing the earth ground to all internal electrical outlets. However, some older house are not so well grounded, and/or age has corroded the grounding connection. When an inverter, or similar equipment, is sold the instructions often specify earth ground so that the installer understands to insure that in the event of a house or other circumstance of installation that lacks a proper good ground that one should be provided separately. Such equipment often contains op amps which are particularly sensitive to ground faults which will fry them even when normal motors and solenoids seem to work fine. As noted: if your house wiring meets modern codes the normal ground prong on any of your outlets is sufficient. If in doubt and you want to insure a proper ground you can buy a copper grounding rod from any Lowes, Home depot, etc. Drive it into the ground just outside your house next to the foundation with just a few inches left exposed above the ground. Then use a ground rod clamp to attach a single strand house wire to it and run it directly to the earth ground on the equipment.
Some (extra) technical details:
In an AC power system provided by the power company the alternating current means that there is no polarity difference between the hot and cold (black and white) wire. They both alternate between negative and positive. Hence what makes the difference between that black and white wire is not electrical polarity, but the fact that the white (cold) side is solidly grounded to the earth. Hence, under normal no fault circumstances, grabbing hold of the white wire with power on should not result in electrocution. The same cannot be said about the black (hot) wire because in that case your body becomes the grounding instrument resulting in electrocution. The power system could work in principle without this grounding at all, but would make nearly ANY circuit fault potentially deadly and/or destructive to the faulty equipment.
Now in older homes with 2-pronged outlets the fact that the white wire was is fact grounded, in the same manner as the 3rd ground wire, was deemed a sufficient safety measure against faulty equipment. Indeed it is sufficient, so long as the ground fault exist in the equipment, and not in the wiring itself. Hence it is not uncommon, in older house with 2-prong systems and a faulty ground, to feel a tingling sensation when touching the metal parts of an electrical cooktop for instance. Modern code adds a separate green (or bare) grounding wire to the system which, at the breaker box, is wired directly to the white wire to provide grounding to the white wire. This ground wire is then separately extended throughout the system to provide multiple redundant grounding points for each piece of equipment. Hence a ground fault a one piece of equipment will not result in the destruction of any other equipment on the system. Though it has an even bigger safety advantage. In the event of a systemic ground fault it becomes trivial to test at any point anywhere in the system simply by testing whether you can read any current between the ground wire and the white (hopefully) cold wire anywhere in the system.
You can even buy a cheap tester from Ace which makes testing for ground faults easy without any need to understand the system itself. If the little red light comes on you have a ground fault that needs attention. Likely due to corrosion at the ground rod clamp at the top of the ground rod, though other forms of faults can and do occur. This is what makes wiring a breaker box counter intuitive to anybody thinking in terms of electrical polarity. It is at the breaker box *only* in which crossing wires which is absolutely not to be crossed in any other circumstances is routine. It's not so obvious to many people why a wire that is connected directly to exposed metal surfaces on equipment in the house is connected directly to a current carrying wire in the breaker box.
Final note:
Should you decide to provide a separate grounding rod for a specific piece of equipment, such as your inverter, no inspector will ever take issue with it. In the event your grounding system is working properly without it, doing so only builds redundancy into the system in a manner that an inspector requires to be present with or without the redundancy, and might save your inverter from being fried by a trivial low current fault that would not otherwise pose any sort of safety or equipment issues.
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