Reply from: Mike Holt
I'm sorry if I in any way communicated that the equipment grounding conductor from the source to the metal parts was not important. I thought I explained that this is VERY important to remove dangerous touch voltage, by clearing the ground fault. And what is not important, in relationship of electric shock from a ground fault, is the grounding of metal parts to the earth.
Touch Voltage - Where did you get the 1ft distance. Did you mean 1 meter?
Earth resistance - I'm talking about a single ground rod, not a substation where you are standing on top of a grounding grid. Maybe the following from IEEE 142 - Green Book will clarify this topic.
2.2.8 Connection to Earth
(See Chapter 4 for more details.) The well-established usage of the terms ground and earth in our technical literature leads to many misconceptions, since they seemingly are almost alike, yet in fact are not. The electrical system of an aircraft in flight will have a ground bus, grounding conductors, etc. To suggest that ground and earth can be used interchangeably is obviously in error here. To an electrician working on the tenth floor of a modern steel-structured building, the referenced ground is the building frame, attached metal equipment, and the family of electrical system grounding conductors present at the working area. What might be the potential of earth is of negligible importance to this worker on the tenth floor.
If the worker is transported to the building basement in which the concrete floor slab rests on soil, or to the yard area of an outdoor open-frame substation, earth does become the proper reference ground to which electric shock voltage exposure should be referenced.
Thus, the proper reference ground to be used in expressing voltage exposure magnitudes may sometimes be earth, but (outside of the outdoor substation area) most likely will be the electric circuit metallic grounding conductor. The following paragraphs will show that the potential of earth may be greatly different from that of the grounding conductor. It therefore becomes very important that shock-exposure voltages be expressed relative to the proper reference ground.
All electrical systems, even those installed in airborne vehicles (as at least one Apollo crew can testify), may be faced with circumstances in which sources of electric current are seeking a path to ground. These conditions can do serious damage to electrical equipment or develop dangerous electric-shock-hazard exposure to persons in the area, unless this stray current is diverted to a preplanned path to a ground of adequate capability.
A comprehensive treatment of the behavior of earthing terminals appears in Chapter 4 and in References [2], [4], [7], and [22]. The prime purpose of this discussion is to develop a concept of the potential gradients created in discharging current into earth and the manner in which the equipment grounding problem is influenced thereby.
Earth is inherently a rather poor conductor whose resistivity is around one billion times that of copper. A 10 ft (3 m) long by 5/8 in (16 mm) diameter ground rod driven into earth might very likely represent a 25 connection to earth. This resistance may be imagined to be made up of the collective resistance of a series of equal thickness concentric cylindrical shells of earth. The inner shell will of course represent the largest incremental value of resistance, since the resistance is inversely proportion to the shell diameter. Thus the central small diameter shells of earth constitute the bulk of the earthing terminal resistance. Half of the 25 resistance value would likely be contained within a 1 ft (0.15 m) diameter cylinder (see 4.1.1).
For the same reason, half of the voltage drop resulting from current injection into this grounding electrode would appear across the first 0.5 ft (0.15 m) of earth surface radially away from the ground rod. If a current of 1000 A were forced into this grounding electrode, the rod would be forced to rise above mean earth potential by 25 000 V (1000 • 25). Half of this voltage (12 500 V) would appear as a voltage drop between the rod and the earth spaced only 0.5 ft (0.15 m) away from the rod. While this current is flowing, a person standing on earth 0.5 ft (0.15 m) away from the ground rod and touching the connecting lead to the electrode would be spanning a potential difference of 12 500 V. A three-dimensional plot of earth surface potential versus distance from the ground rod would create the anthill-shape displayed in Fig 36. The central peak value would be the rod potential (referred to remote earth potential), namely, 25 000 V. Moving away from the rod in any horizontal direction would rapidly reduce the voltage value. The half-voltage contour would be a horizontal circle 1 ft (0.3 m) in diameter encircling the rod.
Imagine a 50 by 50 ft (15.2 by 15.2 m) substation area within which 25 driven rods, each of the type previously described, had been uniformly distributed. Because of the overlapping potential gradient patterns, the composite resistance will not be as low as 25/25 W. For the case at hand, a 2 value would be typical (see Chapter 4). Should a line-to-ground fault at this station produce a 10 000 A discharge into the earthing terminal, the resulting voltage contour map would display 25 sharp-pointed potential mounds peaking at 20 000 V. In between would be dish-shaped voltage contours with minimum values ranging from perhaps 2000 to 5000 V, depending on location.
Such a highly variable voltage contour pattern within the walking area of the substation would not be acceptable. Additional shallow buried grounding wires can be employed to elevate the earth surface potential between main electrodes (see Reference [2]). Note particularly the concepts of step, touch, and transferred potentials. Additional shallow buried grounding wires can be employed to tailor the voltage contour adjacent to but external to the enclosing fence. Beds of coarse cracked rock, well drained to prevent standing water, can contribute to improved electric shock security. Metal grill mats bonded to the steel-framework supporting switch operating handles and located at the “standing” location of switch operators can ensure that the operator's hands and feet are referenced to the same potential.
The only reason I'm continuing this dialog is because the issues you bring up are what many engineers and other think. My concern is that you and others think that somehow we can ground metal parts to the earth to make it safe from touch voltage resulting from a ground fault, when nothing can further from the truth.
What must be done is that all metal parts containing electrical conductors be 'bonded' to the source so that it provides a low impedance path to clear the ground fault, thereby removing dangerous touch voltage.
I'm hoping that we you and I both get on the same page you'll see my point and agree.
Note: I used to think, teach, and write that grounding reduced touch voltage to a safe level at one time. I feel like a fool now that I know better, but this is the process. We are always learning.
Reply to Mike Holt
Reply from: Mike Holt
It would be wonderful if you could take a TN-C power system, install the circuit conductors to metal equipment improperly, then call it a TT power system.
But, one small problem, the power system is still a TN-C system! So if NASA wants to change the power system at the transformer to a TT power system and place RCD detectors in accordance with that system (set at about 500 mA of fault current). Then the system would be safer.
Again the earth is not what's going to make this installation safe, it's the power system with RCD.
Mr. Haja Sahib why don't you just admit that earthing of metal parts that are energized and not properly bonded is not going to reduce the touch voltage to a safe level.
You keep trying to change the issue. So with this all said, I'll bid you farwell. If you still think that driving a ground rod and attaching it to metal parts that are energized from a ground will make an installation safe, then I'm sorry that you don't understand the issues.
Reply to Mike Holt
