Property of Electrical Grounding
Electricity is hard to visualize. It can’t be seen or smelt, but to touch it can be painful if not deadly. In an attempt over the years to make easier to understand the way it works has been often compared to properties of water. Maybe a little absurd since water reacts to gravity which has no measurable effect on electricity.
Electrical current is the movement of electron energy from one molecule to the adjacent. Like billiard balls, energy impacted on the first ball hit transfers to the next and so on. The speed of the cue ball transfers from one ball to the others and if they’re in a straight row only the last one will move much. It happens quickly in a good conductor. Static electricity generated by dragging feet across a shag rug carpet is an exception. Lightning is the result of water molecules in clouds rubbing against each other with excess energy going to earth where it came from and will eventually return. Maybe or maybe not the result of gravity but then again earth generates this attraction.
The concept that generated electricity tends to go to earth, probably created by Benjamin Franklin’s kite experiment with static electricity, has been replaced. It is now known that electricity returns to its source being a generator, transformer or battery. Dirt makes a poor conductor although better when wet.
What does this have to do with electrical grounding, much less water since water ultimately responds to gravity? For the purposes of this comparison, earth must now be thought of as a generator, transformer or battery when comparing properties of water.
Given multiple paths, more water will go down the steepest slope although lesser amounts will still run down others. The same with electricity proportionately returning to its source through paths of least resistance. More will return through a copper wire than a human body. With that said, who wants any electricity going through their body? Larger voltages create more current to be distributed. If the steeper slope is dammed by a switch placed on the return path of the devise(s) using energy being turned off, or a loose wire, all the water will go down lesser slopes. Likewise, if the conductor supplying the resistive or reactive devise contacts a person before reaching this device intended to be operated, current will divide proportionately between its resistance and ours because the easy copper return path of the devise has now been partially blocked by the light, motor, heater and so forth.
The National Electric Code requires all exposed metal parts of an electrical circuit be grounded with a third conductor or metal raceway such as conduit going directly to its source. Not infallible since splices in it or connecting other metal enclosures might come loose but suppose it’s better than jumping out of an airplane without a reserve parachute. The intent is virtually all fault current will take this path with almost no resistance and trip the over current protective devise immediately like a waterfall.
The NEC can be somewhat confusing as the intended return path, often called the neutral, is referred to by it as the grounded conductor, and wires in the safety path required so all exposed metal parts of a circuit be grounded are referred to as grounding conductors.
That’s not to say earth can be taken out of consideration when designing electrical systems. Produced electricity tends to fluctuate and this variance in voltage is eliminated when tied to ground considered to have no voltage. Because dirt is a poor conductor, resistances in it may cause slight voltage differentials back to the delivering service equipment. Differences in electrical potential cause currents to flow through conductive paths. Fluctuations in voltage are even more critical in modern times. Computers don’t behave well when supplied with erratic voltages.
Grounding circuits must be isolated from grounded circuits at devices back to the source or some of the returning current might take the path through exposed metal parts, putting us right back where we started regarding safety. Once reaching the device creating the desired voltage, or its main disconnect switch, it is highly recommended they be bonded together there so the neutral branch conductors will have a constant zero potential at that location required to be grounded to earth.
It is clear in the NEC what grounding electrodes are to be used and bonded together. Buried building steel and metal water pipes of structures require larger bonding conductors since they are more likely to be energized by short circuits downstream and take that path back to the neutral bus bar in the main service panel. Rebar in concrete foundations only need a smaller bonding wire and a ground rod smaller yet since their only purpose is to eliminate voltage difference between ground and electrical service. Back in the day only one ground rod was used. Possibly due to modern electronic developments, when resistance between one rod and earth exceeds twenty-five ohms a second rod is required not less than six feet apart from the first bonded by a small number six bonding conductor. When in doubt, use a larger number two conductor good for ground rings.
Article 250 of the 2020 National Electric Code contains twenty-nine pages and addresses Grounding and Bonding requirements in much more detail.
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