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Electric Signs and Lighting, Part 6

Electric Signs and Lighting, Part 6As noted in Part 5, 600.7(A) provides the "grounding" requirements but these are actually bonding requirements. This becomes clear when you read the requirements and remember that Article 100 defines "ground" as "the earth."

The first requirement we come across is that the metal equipment must be "grounded" by connection of the equipment grounding conductor (EGC) of the supply conductor circuit [600.7(A)(1)]. Now, the EGC is really a bonding conductor. Only conductors that provide connections to the earth (dirt) via ground rods, ground grids, counterpoise, etc., are technically "grounding" conductors.

It's important to avoid confusion between grounding and bonding, regardless of the language used in any standards, drawings, or specifications, because they serve entirely different purposes. Grounding where you should bond fails to remove potentially lethal differences of potential.

People often confuse these two, thinking that “ground” is the equipotential plane. It’s not. You can easily have a serious difference of potential between two ground rods that are only 30 feet apart. Or even just three feet apart.

You can confirm this by drawing out the circuit with the correct load impedances. We all know dirt isn’t nearly as conductive as copper wire. If you look up the soil resistance for your location, you may find (for example) it’s 1,200 ohms per foot. By applying Ohm’s Law, you can easily see why using earth as a bonding jumper just doesn’t work.

« Part 5 |  Part 7 » | Source: Mark Lamendola | Mindconnection