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#129546 - 04/16/05 07:53 AM Grounding Quiz, by Mike Holt
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
Grounding Quiz

By Mike Holt

When answering these questions it very important that you base your answers on the following terms as defined by the 2005 National Electrical Code.

• Bonding (Bonded). The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed [100].

• Bonding Jumper, System. The connection between the grounded circuit conductor and the equipment grounding conductor at a separately derived system [100].

• Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device [250.2].

• Ground. A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth [100].

• Ground Fault. An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally noncurrent-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth [250.2].

• Ground-Fault Current Path. An electrically conductive path from the point of a ground fault on a wiring system through normally noncurrent-carrying conductors, equipment, or the earth to the electrical supply source [250.2].

• Grounded. Connected to earth or to some conducting body that serves in place of the earth [100].

• Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode [100].

• Grounding Conductor, Equipment. The conductor used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system [100].

• Grounding Electrode. A device that establishes an electrical connection to the earth [100].

This quiz is not designed to upset you or to cause you conflict, but it might; it’s simply designed to demonstrate how easily it is to get confused if you are not careful.

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.

Current Flow
1. When electrical current is given multiple conductive paths on which to flow, current will always take the path of least resistance.
False. In parallel paths, current divides and flows through each individual parallel path in accordance with Kirchoff's current law. So, when given multiple conductive paths on which to flow, current will take all of the available paths. Yes, it’s true that more current will flow through the lower resistive path, as compared to a higher resistive path in a parallel circuit, but that’s not the question.

Current Flow
2. It is important to ground metal parts to a suitable grounding electrode, so that in the event of a ground fault, dangerous ground-fault current will be shunted into the earth, away from persons; thereby protecting them against electric shock.
False. A person touching an energized metal pole, which is grounded, will experience between 90 and 120 mA of current flow through the body, which is more than sufficient to cause electrocution.

Remember: In parallel circuits, current divides and flows through each individual parallel path in accordance with Kirchoff's current law.

Current Through Person
I = E/R
I = 90V*/1,000 ohms**
I = 0.090A or 90 mA

*IEEE 142, Grounding Industrial and Commercial Installations.
** IEEE 80, IEEE Guide for Safety in AC Substations.

Current Through Ground
I = E/R
I = 120V/25 ohms
I = 4.8A

Voltage on metal parts can never be reduced or removed by grounding the metal parts to the earth. The only way to make an installation safe from a ground fault is to bond the electrical equipment to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device and clear the ground fault [250.2 and 250.4(A)(3)].

Current Flow
3. The grounding conductor for a supplementary grounding electrode (for example, a ground rod for a machine tool) must have the capacity to conduct safely any fault current likely to be imposed on it. This is accomplished by sizing the conductor in accordance with Table 250.66 or Table 250.122, depending on the conditions.
False: During a ground fault, the amount of current flowing through the grounding conductor into the earth, to the power supply, is dependent on the circuit voltage and the earth’s resistance.

Assuming a circuit voltage of 120 and a ground rod resistance of 25 ohms, the current flowing through the grounding conductor into the earth, to the power supply, will be only 4.8A.
I = E/R
I = 120V/25 ohms
I = 4.8A

Because of the earth’s high resistance, it cannot be used as an effective ground-fault current path [250.4(A)(5)]; therefore, the grounding conductor for a supplementary electrode is not sized in accordance with the NEC [250.54].

Clear a Fault
4. Electrical equipment must be grounded so that sufficient fault current will flow through the circuit protection device to quickly open and clear the ground fault. For example, a 20A circuit breaker will trip and de-energize a 120V ground fault to a metal pole that is grounded to a 25 ohm ground rod.
False: A 120V ground fault that uses the earth as part of the fault return path is not capable of clearing the ground fault [250.4(A)(5)]. Result… dangerous voltage will remain on all metal parts.

If the metal pole were bonded to an effective ground-fault current path, the ground-fault current would be sufficient to quickly open the 20A circuit protection device [250.2 and 250.4(A)(3)]. Result… dangerous voltage on metal parts will be removed.
I = E/Z
I = 120V/0.405 ohms*
I = 296A

*Effective ground-fault current path:
• Service Feeder: 100 ft of 3/0 AWG Copper
Z = 0.0766 ohms per 1,000 ft x 0.20 (Chapter 9 Table 8)
Z = 0.015 ohms

• Branch Circuit: 100 ft of 12 AWG Copper
Z = 1.93 ohms per 1,000 ft x 0.20 (Chapter 9 Table 8)
Z = 0.39 ohms

Electrical Equipment
5. Electrical equipment must be grounded to ensure that dangerous voltage on metal parts resulting from a ground fault can be reduced to a safe value.
False. Grounding metal parts to the earth does not reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond the electrical equipment to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2 and 250.4(A)(3)].

Electrical Equipment
6. Metal traffic signal poles and handhole covers must be grounded to a suitable grounding electrode to ensure that dangerous voltage on metal parts resulting from a ground fault can be reduced to a safe value.
False. Grounding metal parts to the earth does not reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond the metal traffic signal poles and handhole covers to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2 and 250.4(A)(3)].

Electrical Equipment
7. Grounding of metal manhole covers to a suitable grounding electrode ensures that dangerous voltage on metal parts resulting from a ground fault can be reduced to a safe value.
False. Grounding metal parts to the earth does not reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to isolate the manhole cover from energized parts or to bond the metal parts to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2 and 250.4(A)(3)].

Service Equipment
8. Service equipment must be grounded to a grounding electrode to ensure that dangerous voltage on metal parts resulting from a ground fault can be removed or be reduced to a safe value.
False. Grounding metal parts to the earth does not removing or reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond service equipment to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2, 250.4(A)(3), and 250.24(C)].

Service Equipment
9. Grounding service equipment to a low resistive grounding electrode helps in protecting interior wiring and equipment from lightning damage.
False. Grounding metal parts to the earth does not protect interior wiring or equipment from lightning.

However, grounding service equipment to the earth does reduce the voltage on the metal parts from lightning, which can help prevent a fire caused by elevated lightning voltage seeking a path to the earth by arcing across combustible materials.

Interior wiring and equipment can be protected from lightning-induced voltage transients on the circuit conductors by the use of properly designed cascading TVSS protection devices: one at the service equipment, one at each downstream panelboard, and one at each point of use.

Service Equipment
10. Service equipment is grounded to a grounding electrode to ensure that metal parts, subject to a ground fault, remain at the same potential as the earth.
False. Grounding metal parts to the earth serves no part in reducing the difference of potential between metal parts and the earth from a ground fault.

The only way to make this installation safe is to bond service equipment to an effective ground-fault current path so that the ground fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2, 250.4(A)(3), and 250.24(C)].

Service Equipment
11. Grounding of service equipment to a grounding electrode is necessary to stabilize the system voltage.
False. The earth serves no part in stabilizing the system voltage.

To stabilize the system voltage, a system bonding jumper must be installed at the separately derived system in accordance with 250.30(A)(1).

Service Equipment
12. Service equipment is grounded to a grounding electrode to ensure that the voltage between the metal parts of the electrical installation and the earth remains at the same potential.
False. Grounding service equipment to the earth serves no purpose in establishing or maintaining a zero difference of potential between metal parts of electrical equipment and the earth.

Separately Derived System
13. The metal case of a separately derived system is grounded to a grounding electrode to stabilize the system voltage during normal operation.
False. The earth serves no part in stabilizing the system voltage.

To stabilize the system voltage, a system bonding jumper must be installed between the separately derived system and its metal case in accordance with 250.30(A)(1).

Separately Derived System
14. Separately derived systems are grounded to a grounding electrode to ensure that the voltage between metal parts and the earth remains at the same potential.
False. Grounding a separately derived system to the earth serves no purpose in establishing or maintaining a zero difference of potential between metal parts of electrical equipment and the earth.

Separately Derived Systems
15. Separately derived systems must be grounded to a grounding electrode to ensure that dangerous voltage on metal parts, caused by a ground fault, can be removed or be reduced to a safe value.
False. Grounding a separately derived system to the earth serves no purpose in removing or reducing voltage on metal parts caused by a ground fault.

The only way to make this installation safe from a ground fault is to bond the metal parts of the separately derived system by using a system bonding jumper so that the ground fault current will be sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2, 250.4(A)(3), and 250.4(A)(3)].

Separately Derived Systems
16. An ungrounded system gets its name from the fact that both the separately derived system and the metal case of the separately derived system are isolated from ground (earth).
False. The NEC requires the metal case of ungrounded separately derived systems to be grounded to a grounding electrode [250.30(B)(1)].

Transformers
17. Failure to ground the metal case of a transformer to a grounding electrode can result in a dangerous difference of potential between the metal parts of different separately derived systems.
False: Because all metal parts of electrical installations are required to be bonded to an effective ground-fault current path [250.4(A)(3)], there is no difference of potential between different separately derived systems.

The NEC requires the metal case of all separately derived systems to be grounded to a suitable grounding electrode [250.30(A)(4)], even though there is no technical reason for this.

Generators
18. The metal case of generators are grounded to a suitable grounding electrode to ensure that dangerous voltage on metal parts, caused by a ground fault, can be reduced to a safe value.
False. Grounding metal parts to the earth does not remove or reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond the metal case of the generator to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2, 250.4(A)(3), and 250.30(A)(1)].

Remote Building
19. Building disconnecting means at a remote building supplied by a feeder must be grounded to a grounding electrode to ensure that dangerous voltage on metal parts, caused by a ground fault, can be removed or be reduced to a safe value.
False. Grounding metal parts to the earth does not remove or reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond the building disconnecting means to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2, 250.4(A)(3), and 250.32(B)].

Remote Building
20. The metal disconnecting means at a remote building, supplied by a feeder with an equipment grounding conductor, is not required to be grounded to a grounding electrode.
False: Grounding of the remote building disconnecting means to the earth is necessary to reduce voltage on the metal parts from lightning; thereby reducing the likelihood of a fire caused by elevated voltage seeking a path to the earth by arcing across combustible materials.

The equipment grounding conductor provides the low-impedance path to the source necessary to clear a ground fault; its function is not to serve as a path for lightning to the earth.

Remote Building
21. The grounding of a building disconnecting means to a suitable grounding electrode helps in protecting interior wiring and equipment from a lightning strike.
False. Grounding metal parts to the earth does not assist in protecting interior wiring or equipment from lightning.

However, grounding the building disconnecting means to the earth does reduce the likelihood of a fire caused by elevated lightning voltage seeking a path to the earth by arcing across combustible materials.

Interior wiring and equipment can be protected from lightning-induced voltage transients on the circuit conductors by the use of properly designed cascading TVSS protection devices: one at the service equipment, one at each downstream panelboard, and one at each point of use.

Outdoor Metal Pole
22. Outdoor metal light poles must be grounded to a suitable grounding electrode to ensure that dangerous voltage on metal parts, caused by a ground fault, can be reduced to a safe value.
False. Grounding metal parts to the earth does not remove or reduce voltage on metal parts resulting from a ground fault because the earth cannot serve as an effective ground-fault current path [250.5(A)(5)].

The only way to make this installation safe from a ground fault is to bond the metal light pole to an effective ground-fault current path so that the fault current will be more than sufficient to quickly open the circuit protection device; thereby clearing the ground fault and removing dangerous touch voltage [250.2 and 250.4(A)(3)].

Outdoor Metal Pole
23. Grounding metal light poles to a grounding electrode helps in reducing lightning damage to the luminaires on the metal light pole from a direct lightning strike.
False: If lightning strikes the pole, the luminaire on the pole is toast. Nothing can be done about this.

Outdoor Metal Pole
24. Grounding metal light poles to a grounding electrode helps in preventing damage to building wiring and equipment from lightning striking one of the metal light poles.
False: Grounding a metal light pole to the earth does nothing to prevent damage to interior wiring and equipment of a building from lightning.

Interior wiring and equipment can be protected from lightning-induced voltage transients on the circuit conductors by the use of properly designed TVSS protection devices.

Outdoor Metal Pole
25. Grounding metal light poles to a grounding electrode is necessary to prevent lightning damage to the concrete pole base.
False: Ralph Lee, in a 1966 study, proved that lightning does not crack the concrete of a concrete encased grounding electrode.

Sensitive Electronic Equipment
26. Studies have shown that a low-resistive grounding system improves power quality for sensitive electronic equipment.
False: The earth serves no purpose in improving power quality.

Sensitive Electronic Equipment
27. Single-point grounding improves equipment performance by preventing ground-loop currents.
False: Grounding sensitive electrical equipment to the same electrode serves no purpose in preventing or reducing ground-loop currents. This is because ground-loop currents flow when improper neutral-to-ground connections are made on the load side of service equipment or separately derived systems in violation of 250.142. To remove ground-loop currents, simply ensure the installation is in compliance with the NEC.

Sensitive Electronic Equipment
28. Studies have shown that grounding sensitive electronic equipment to an isolated counter-poise ground improves equipment performance because of improved power quality.
False: Grounding sensitive electronic equipment to the earth serves no purpose in improving equipment performance or power quality.

Sensitive Electronic Equipment
29. If an electrical system is properly installed, the voltage between the neutral terminal and the ground terminal at a receptacle should be near zero.
False: The voltage between the neutral and ground terminals at a receptacle will never be near zero volts in a building that has power.

For example: the NEC recommends a maximum voltage drop of 3% for the feeder, which works out to be 3.6V for a 120V circuit. Under this condition, the voltage (feeder neutral voltage drop) between the receptacle’s neutral and ground terminals would be 1.8V if no current flows through the branch circuit supplying the receptacle.

A study by the Electrical Power Research Institute (EPRI) demonstrated that elevated neutral-to-ground voltage has no affect on equipment performance.

Stray Voltage or Neutral-to-Earth Voltage (NEV)
30. Grounding premises wiring to a low resistive grounding grid can help reduce stray voltage or neutral-to-earth voltage on metal parts.
False: Grounding metal parts to the earth serves no purpose in reducing stray or NEV voltage.

However, bonding metal parts together reduces the difference of potential between the metal parts, but the stray or NEV voltage, as measured between the metal parts and the earth, will not be reduced.

Stray voltage or neutral-to-earth voltage can come from the electric utility’s distribution system, the building’s electric system, or both of these sources.

Stray Voltage or NEV
31. Grounding metal parts of electrical equipment to an equipotential plane can help reduce stray or NEV voltage on the metal parts.
False: Bonding metal parts to an equipotential plane does reduce the difference of potential between the metal parts and the equipotential plane, but stray or NEV voltage, as measured between the metal parts and the earth, will not be reduced.

TVSS
32. A low resistive earth ground is necessary for the proper operation of transient voltage surge suppressors (TVSSs).
False. The earth serves no purpose in the operation of a TVSS device.

TVSS protection devices protect electrical equipment by shunting high-frequency impulse currents away from the load and back to the source via the circuit conductors, not via the earth.

General
33. Because salt water is more conductive than fresh water, a person is more likely to be electrocuted while swimming at a saltwater marina, than a freshwater marina.
False: Because the voltage gradient in salt water much lower than fresh water, the likelihood of death will be greater in a fresh water marina.
_________________________
Joe Tedesco, NEC Consultant

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#129547 - 04/16/05 08:27 PM Re: Grounding Quiz, by Mike Holt
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Joe,

Thanks for pasting this information here!

The information is - in my words "Covering The Subject, Using Very Basic Descriptions, &/Or A Non-Technical Approach", but this should not discourage anyone from reading it and learning from it too.

I feel some information may be ambiguous, lacking complete description, or possibly misquoted by accident (as mentioned above "In My Words"), due to the descriptions in Questions #10 through #15.
Those "Q"s may need to have some information added, which regards the effects on Conductor Insulation + related Equipment, on an Ungrounded System.

#16 relates to an Ungrounded System, but not the way I expected it to.

#26 relates to Power Quality - which I was hoping to see something discussed, covering more "angles" about things like Ground Loops, Supplemental Ground Rods, Filters w/ X-G connections, and something about how an Ungrounded System may effect these items.
(not necessary to include within the same "Q", but maybe referenced into additional "Q" numbers).

Please do not think I am being difficult here, just curious about the approach, and wondering if any one else feels similar regarding the items I have listed.

Once again, the document is good. It will make sense to all Persons reading it, and that is the point.
Along with that, it does not steer someone into incorrect &/or hazardous Grounding Techniques.
Just wished for a little more information regarding Ungrounded Systems - and the differences between the two (Grounded vs. Ungrounded).

If anyone feels I am totally off base, please let me know.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#129548 - 05/01/05 12:25 AM Re: Grounding Quiz, by Mike Holt
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
***Bump***

Bumping this thread to life again!

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

Top
#129549 - 05/01/05 11:05 AM Re: Grounding Quiz, by Mike Holt
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
 Quote:
Joe, I'm not a member, could you post the following for me? Thanks.

Scott35,
I hope the following helps.

Re: Questions 10-15.
None of these questions related to ungrounded systems. This was explained in the header of the quiz 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."

Re: Question 16
What did you expect?

Re: Question 26
Quiz was not intended to be an article on Power Quality. Sorry.


God Bless, Mike Holt, 1-352-429-5577


[This message has been edited by Joe Tedesco (edited 05-01-2005).]
_________________________
Joe Tedesco, NEC Consultant

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#129550 - 05/01/05 05:03 PM Re: Grounding Quiz, by Mike Holt
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
Joe, if I may make a suggestion:

For the next quiz, how about grouping the answers together after the questions? The way you did thos one, we're already reading the snawer by the time we realize we're finished reading the question.

Thanx,
Larry
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com

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#129551 - 05/06/05 09:28 AM Re: Grounding Quiz, by Mike Holt
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
***BUMP***

Bumping this thread to life

btw, I may have accidentially offended Mike Holt with my response to the Article!

Sorry...

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

Top
#129552 - 05/06/05 02:43 PM Re: Grounding Quiz, by Mike Holt
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
I've got to take issue with a number of answers here, although my quibbles are more language than physics.

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.

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.

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.

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.

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.)

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.

I have a similar response to question 14.

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.

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

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.

-Jon

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#129553 - 05/06/05 07:09 PM Re: Grounding Quiz, by Mike Holt
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
From Mike Holt

 Quote:
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.
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Joe Tedesco, NEC Consultant

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#129554 - 05/07/05 05:28 AM Re: Grounding Quiz, by Mike Holt
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
winnie: Mike asked me to ask you to call him:

Mike Holt, 1-352-429-5577


_________________________
Joe Tedesco, NEC Consultant

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