From Mike Holt

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My comments are in reference to Winnie post:

Re: For questions 1 and 2, mention is made of Kirchoff's Current Laws. While KCL is relevant, it simply says that the total current flowing into a node is zero, or that the amount of current flowing out is equal to the amount of current flowing in. KCL does not say anything about _how_ the current will divide up. For this we need Ohm's law. The equations shown are the Ohm's law calculations.

Mike’s Comment: You know I’ll have do some research on this, but I’m sure your right.

Re: For question 2, if the person is touching both an energized, grounded metal pole _and_ is themselves well grounded (say by also touching a non-energized grounded metal item), then lethal current flow is possible. However contact at two points to complete a circuit is a must. An perhaps relevant example is the concept of 'step potential', where high enough current is flowing through the ground that the voltage difference between the two feet of someone standing on the ground is sufficient to cause an electric shock.

Mike’s Comment: There is a difference of potential of about 90V between the energized metal pole and three feet away from the ground rod. This is called surface voltage gradient, which is in relationship to the resistance of the ground rod and the earth.
Further, the statement "Voltage on metal parts can never be reduced or removed by grounding the metal parts to the earth." is _not_ generally true. It is true in the specific case of metal parts energized by low impedance contact with a low impedance source such as a transformer. On the other hand, if a metal part is energized by a high voltage, high impedance source, then grounding can greatly diminish the potential present at the metal part. For example, a metal part energized by leakage from a high voltage primary, or one energized by wind induced charge separation.

Mike’s Comment: The scope of the quiz was identified as follows:

“Note: The questions are based on premises wiring systems having a nominal voltage of 120, 120/208, 120/240, 240, 277, 277/480, 480, 347, 347/600, or 600, and assume that all separately derived systems are customer owned inside a building.” So the statement is true as written.

Re: The answer to question 3 should be true, but it is misleading, and the _reason_ given in the question is false. Clearly a grounding conductor _must_ be sized to carry any expected fault current for the expected fault duration. However, as the calculation shows, this fault current is quite low. The sizing of these supplemental electrode grounding conductors is much larger than would be required in order to simply carry the expected fault current.

Mike’s Comment: I think you might have misread the definition of a grounding conductor. A grounding conductor is not an equipment grounding conductor (two different definitions). The NEC does not specify the size of the grounding conductor. See 250.54. However, if the question was relating to an equipment grounding (bonding) conductor, then your comment is 100% correct.

Re: I believe that the answer to question 9 should be true. If the service equipment were not grounded, then a lightning induced voltage on the service conductors would raise the potential of the entire electrical system. Eventually the high voltage would cause insulation breakdown and damage. Grounding the service helps mitigate voltage excursions caused by external sources (lightning, transformer failure, etc.)

Mike’s Comment: You know I think your right… let me think more about this.

Re: I have a similar disagreement with the answer to question 12. Grounding of service equipment does service to help stabilize the voltage of components relative to the earth. Low impedance ground faults (eg. phase to ground) can significantly change the potential of even grounded items, however high impedance (eg. lightning induced, or leakage across transformer insulation) sources _are_ significantly shunted by grounding.

Mike’s Comment: You are right, if we are relating this to lightning. I’ll reword the question to relate to a ground fault condition.

Re: I have a similar response to question 14.

Mike’s Comment: I’ll reword the question to relate to a ground fault condition.

Re: I agree with the answer to question 15 because it includes the limiting factor of 'dangerous voltages _caused_ by a ground fault'. Grounding would help protect from dangerous voltage caused by reasons other than ground fault.

Mike’s Comment: I got this one worded right… good.

Re: I don't see how the answers to questions 20 and 21 are consistent with each other. I agree with 20, disagree with 21.

Mike’s Comment: Since I was wrong on No. 12, I would be wrong on No. 21.

Re: On question 28 I would add the additional warning that grounding equipment to an isolated grounding electrode can _cause_ equipment damage if there is current flow through the earth and a potential difference between the separate grounding electrode systems.

Mike’s Comment: Good point.

Joe Tedesco, NEC Consultant