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Electrical power arriving at a home- schematic (C) Carson Dunlop Associates Electrical ground inspection
How to Inspect Residential Electrical System Grounds, Ground Wiring, Grounding Conductors, Grounding Electrodes

  • ELECTRICAL GROUND SYSTEM INSPECTION - CONTENTS: Class on how to inspect electrical panels, home inspection procedures and safety, definition of ground, grounding, grounded, grounding conductor, grounded conductor, bonding, earth, earthing
  • POST a QUESTION or READ FAQs about how to inspect residential electrical wiring ground system, & about defects in electrical ground system wiring
  • REFERENCES
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Electrical ground system inspection procedures & checklists.

This document discusses procedures the inspection of the grounding system components of a building electrical system when performed by trained building inspection professionals, home inspectors, electrical inspectors, and electricians.



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Guide to Inspecting Electrical Service Grounding Equipment for Defects

Why we need electrical grounding (C) Carson Dunlop Associates

\Here we define electrical ground, grounding, bonding, and earthing terms and explain why there are important differences among these words.

[Click to enlarge any image]

“Grounding”, article 250 in the NEC, is probably one of the most difficult of the often used articles. In 2005 article 250 became “Grounding and bonding”. In the 2008 NEC there has been a major revision in language, and phrases like “shall be grounded” have changed to “shall be connected to an equipment grounding conductor.”

Sketches above and at page top courtesy of Carson Dunlop Associates.

Article Contents

Also see DEFINITIONS of ELECTRICAL GROUND, Grounding Electrode, Grounding Conductor, Grounded Conductor, Ground Wire, Neutral Wire, Ground Rod, for definitions of these confusing electrical terms.

Why we need electrical grounding

The grounding system at a building provides an easy path for electricity to flow to earth should a problem, such as a short circuit, occur.

Allowing current to flow to earth through the ground system helps assure that a circuit breaker will trip or fuse will blow should a problem occur. Properly operating these overcurrent devices help prevent fire and shock.

Should an electrical fault occur where no ground path is present, the electrical potential is just sitting there waiting for a person to come along, touch some component of the system, and by accidentally providing a path to earth through their body, receive a burn or potentially fatal shock.

Sketch courtesy of Carson Dunlop Associates.

Details of why we need grounding, and definitions of electrical grounding and electrical bonding (what's the difference between these two terms) can be read at Why Grounding is Needed.

Bud, a master electrician from Minnesota has offered these important clarifications:

"Grounding" has 2 main functions.

One is to provide a path to trip a breaker in the event of a 'short' as in the text above. That function relies on a "ground"-to-neutral connection required at services in the US (the "main bonding jumper"). The path is (branch circuit ground wire) to (N-G bond at the service) to (service neutral) to (utility power transformer).

This path *must* be metallic back to the power transformer to provide low resistance to trip a circuit breaker. This function will work even if the service is not connected to earth. And the NEC *does not allow* earth to be used as part of this path.

One reason is the resistance of an earth path is too high. Assume the earthing is only through a ground rod and the rod has a quite good 10 ohms resistance to earth. Further assume there is a 'short' connecting hot to "ground". The current to earth will be 12A. There is a good chance this won't even trip a 15A circuit breaker. If the circuit is loaded the breaker will trip, but after a significant time delay. In the mean time, the "ground" potential with respect to the earth away from the ground rod will be 120V.

Note that if you are using the earth as in the quote above, the path is not just into the earth. It is back to the power source, and also depends on the earth connection at the power transformer.

This would be better termed a *bonding* function.

Showing the elecrical path to earth (C) Carson Dunlop Associates

Carson Dunlop's sketch shows how the electrical current in a building can find its way to earth by way of the electrical grounding system. But as you may want to read in our case study of loss of all ground connections at a building, don't assume that the current will always find its way to earth.

Loss of electrical ground at a building is extremely dangerous and risks electrocution.

Some discussion points about electrical grounding are listed just below.

What is Meant by the "Grounding Equipment " in a Building?

As Carson Dunlop's sketch shows, the grounding equipment includes wires which bond the ground and neutral bus in the main electrical panel with an outdoor component that conducts electricity to the earth (ground).

Sketch of basic grounding equipment (C) Carson Dunlop Associates

The outdoor component may be grounding electrodes (ground rods), or in some jurisdictions a metal water pipe or possibly other metal components.

Where do Ground Wires Go?

Where ground wires should go (C) Carson Dunlop Associates

As Carson Dunlop's sketch shows here, from the main electrical panel a grounding conductor connects to:

 

 

 

 

 

Why is Building Water Supply Piping Connected to the Ground System?

Loose ground wire (C) Daniel Friedman

The NEC (section 250-81 through 250-83) requires that the electrical system connected to all of the following, if available for grounding purposes:

The reason we ground in-building plumbing is not to provide an additional grounding conductor in a building but to ground the plumbing.

Picture someone knocking a toaster into a stainless steel sink or into any sink with a metal drain and drain piping.

If the sink and piping are grounded the fuse or breaker will blow. If not, the system is waiting to electrocute the building occupant when s/he touches the live water/toaster in the sink and perhaps a nearby metal faucet, radiator, or other component that is ultimately connected to earth. Similar hazards exist at other building locations such as basement laundry equipment & sinks, at building tubs and showers, etc.

In a properly-wired building, the grounding conductor and bonding system do not normally carry current, and would not be blamed for copper pipe pinholing etc. The grounding system is intended to conduct electrical current only in the event of a fault or emergency [such as a lightning strike or a hair dryer dropped into the bath tub or sink].

Details about the causes of copper pipe pinhole leak complaints are at COPPER PIPE PINHOLE LEAKS, Pinhole Leaks: cause, cure, prevention

NEC Citations on grounding water piping

Gas Piping May Need to Be Bonded to the Electrical Ground System

Bonding gas piping to the bulilding electrical ground system (C) Carson Dunlop Associates

In some communities, as Carson Dunlop's sketch shows, the metal gas piping in a building must be bonded to the electrical ground system.

Bonding anything to the ground system, including metal gas piping, helps prevent an electrical spark that might otherwise result in an explosion in the case of a gas piping system.

The bonding of the gas piping to the building ground system is not the same thing as attempting to use the metal gas piping as the primary or only connection to earth in a building.

Table of Electrical Ground Wire Sizes

Ground Wire Gauge CU (Copper) - wire size  Electrical Service Size
#8 to 100A
#6 to 125A
#4 to 165A
#3 to 200A

See Definitions of Electrical Ground, Grounding Electrode, Grounding Conductor, Grounded Conductor, Ground Wire, Neutral Wire, Ground Rod, for definitions of these confusing electrical terms.

More details about electrical grounding can be read at ELECTRIC SERVICE GROUNDING SYSTEM INSPECTION and

and at OLD HOUSE ELECTRICAL WIRING.

Also, see details about electrical grounding at ELECTRICAL CIRCUITS, SHORTS,

and at ELECTRICITY BASICS, HOW IT WORKS.

At ALUMINUM GROUND WIRES we discuss proper repair of aluminum ground wires found in solid conductor branch circuit wiring.

Guide to Inspecting Electrical Service Grounding Equipment

Grounding Equipment (C) Carson Dunlop Associates

Readers should see our complete electrical ground inspection information at ELECTRICAL GROUND SYSTEM INSPECTION

This article series describes procedures for safe and effective visual inspection of residential electrical systems including electrical panels and other components, when the inspection is conducted by trained building inspection professionals, home inspectors, electrical inspectors, and electricians.

This information was presented by Daniel Friedman - InspectApedia.com, at & discussed by the Hudson Valley chapter of the American Society of Home Inspectors - HVASHI Seminar 12 Sept 2002, Updated April 2006, April 2009.

 

Carson Dunlop's sketch at page top shows where the electrical inspection starts at a residential property.

 


Table of Electrical Ground Wire Sizes Required by Electrical Service Size

Ground Gauge CU (Copper) Service Size
#8 to 100A
#6 to 125A
#4 to 165A
#3 to 200A

Electrical Service Grounding Defects & Conditions to Check For During an Electrical Inspection

Here are the Most-Common Electrical Service Grounding Defects & Conditions

Ground locations (C) Carson Dunlop Associates


Bad grounding electrode conductor connection (C) Daniel Friedman


Notes on types of defects courtesy of Arlene Puentes. Sketch courtesy of Carson Dunlop Associates.

(a) Equipment grounding conductor defect

(b) Grounding electrode conductor defect

More about the galvanic scale and corrosion between dissimilar metals is at GALVANIC SCALE & METAL CORROSION.

 

 

What Other Defects Should We Check for in an Electrical Grounding System?

Jumper wires needed at non-conductive pipe fittings (C) Carson Dunlop Associates

The Dialectic Philosophy of Dielectric Electrical Separation of Materials

Thanks to reader Bill O'Reilly (not that one) for the following excellent comment.

7/21/2014 Bill O'Reilly (not that one) said:

There's a spoonerism in the "Electrical ground inspection" article in the phrase ..."if the building plumbing includes dialectic fittings"... Dialectic fittings could be decorative, but wouldn't serve the intended function.

The word dialectic should be dielectric.

A dialectic is a form of formal argument first popular in the golden age of greece. [Dialectic is defined as The art of investigating or discussing the truth of opinions, inquiry into metaphysical contradictions and their solutions - Ed.]

A dielectric separates two electrically conducting materials so that they can be at two different potentials, more formally an electric di-pole. In the formal sense, a dielectric can be a solid (e.g. glass, porcelain, nylon, polyvinylchloride), a liquid (e.g. oil), a gas (e.g. dry air, sulfur hexafluoride), or a vacuum, which is nothing at all. In the particular sense of the phrase's context, the dielectric blocks the flow of electrons between the dissimilar metals, thus preventing the flow of dissolved ions from the water to the cathode and (metallic) ions from the anode into the water.

Suggested citation for this web page

XXX at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Electric meter ground bypass has fallen apart (C) Daniel Friedman

Corroded copper ground wire (C) Daniel Friedman

More about the galvanic scale and corrosion between dissimilar metals is at

GALVANIC SCALE & METAL CORROSION.

GALVANIZED STEEL WATER PIPING

Electrical Code Requirements for Grounding Water Pipes: NEC Citations on grounding water piping

List of Still More Electrical Grounding System Checkpoints:

Ground rod and ground wire (C) Daniel Friedman T Hemm

Corroded and disconnected aluminum electrical ground wire (C) Daniel Friedman

At ALUMINUM GROUND WIRES we discuss proper repair of aluminum ground wires found in solid conductor branch circuit wiring.

Loose ground wire (C) Daniel Friedman


No one may notice this problem because even if this ground connection is totally ineffective, the building may be still grounded through the service entry ground wire. As we demonstrated at DOUBLE FAULT, LOSS OF ELECTRICITY, it's not safe to rely on just the utility company's ground connection.

Electrical ground path used for neutral path (C) Daniel Friedman

The ground system wiring is for emergency-use only - it should never be wired so as to carry current during normal operation. (E.g. This occurs if a sub panel bonds the neutral to ground wires).

We've found cases in which someone used the ground path to complete an electrical circuit because the neutral wire was broken somewhere that could not be found.

As a result, the ground path was electrically live when it should not have been, leading to an electric shock.

In our photo at left, someone used telephone wire to connect the neutral side of this electrical receptacle to the receptacle's steel mounting strap, knowing that that would in turn connect the neutral side of the receptacle to the steel junction box and through it, to the armored BX electrical cable, forming an electrical path back to the main electric panel. We discuss this crazy wiring in more detail at False Neutral Connections.

Indeed this got the receptacle "working" by using the ground path in the system after the original neutral path had been lost.

We were working on renovating the home where we found this condition. How did we find it? We were replacing two-prong un-grounded receptacles with grounded devices. We turned off electrical power to this circuit and began working on it. When our assistant plugged in and began using a vacuum cleaner in the same room we got an electrical surprise - a shock while touching the BX cable!

Fake ground electrode disclosed (C) Daniel Friedman Fake electrical ground rod (C) Daniel Friedman

Ground rod cut off or short - don't assume that because you see a grounding electrode that it has been properly installed. If the installer hit rock and couldn't drive the rod fully into the soil s/he may have cheated and simply cut off the top of the rod.

Grounding electrodes in some locales have an embossed code on their upper end - if the rod was cut off the embossed letters will be missing. If a grounding electrode cannot be fully driven into the soil the electrical code provides procedures for driving the electrode in cut-sections to achieve sufficient total earth contact.

As we and our inspection client discovered (photos above), the bent-over grounding electrode made us wonder what was happening. When the grounding electrode was just nudged with a toe, it fell over. Our client was kind enough to demonstrate just how ineffective this electrical ground system was, thanks to someone's shortcut.

Readers should also see Definitions of Electrical Ground, Grounding Electrode, Grounding Conductor, Grounded Conductor, Ground Wire, Neutral Wire, Ground Rod, for definitions of these confusing electrical terms.

SYSTEM GROUNDING - A Summary of Inspecting Residential Electrical System Grounding for Defects

While we have frequently updated and added to the material, in its original form this information was presented by Daniel Friedman - InspectAPedia.com, at the Hudson Valley chapter of the American Society of Home Inspectors - HVASHI Seminar 12 Sept 2002, Updated April 2006, February 2013, March 2014, July 2014, December 2014

Watch out: for safety, also review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for Inspectors examining Residential Electrical Systems

Testing the Local Electrical Ground

Reader Question: 4 June 2015 Bill Said:

I am replacing my service panel,in my condo, and the conduit running from the sub panel is being used as the earth ground. How can I test this ground?

Reply:

Bill,

When the conduit is being used as the earth ground it must meet certain UFER specifications and impedence requirements. Below is a general answer discussing local electrical ground electrode testing.
See ELECTRICAL GROUND REQUIREMENTS

If a local ground is isolated from the electrical company's ground (neutral wire in the panel) and does not conduct electricity or shows high resistance (or more generally, high impedence) it is not safe. How much is "high impedence" when testing an electrical ground system at a building?

The NEC specifies 25 ohms as an acceptable limit for electrode impedance. The IEEE Standard 142 Recommended Practice for Grounding of Industrial and Commercial Power Systems (“Green Book”) suggests a resistance between the main grounding electrode and earth of 1 to 5 ohms for large commercial or industrial systems.  - Fluke Corporation, "Checking ground electrode impedence for commecial, industrial, and residential buildings", [PDF] Fluke COrporatio, PO BOx 9090, Evrett WA, USA 98206, Tel U.S.A. (800) 443-5853, Fluke Europe, B.V., PO Box 1186, 5602 BD Eindhoven, The Netherlands, In Europe/M-East/Africa +31 (0) 40 2675 200 , In Canada (800)-36-FLUKE From other countries +1 (425) 446-5500 Website: http://www.fluke.com

Watch out: this test needs to be performed by a trained electrician as there are shock and electrocution hazard risks. As Fluke and other experts point out, to perform testing of the local grounding electrode it must be disconnected from the building. The ground testing instructions that we cite below include additional important safety warnings and procedureal details from which we excerpt. From reading the literature our opinion is that this test is technically difficult, requires expertise, and should not be attempted by a homeowner nor by anyone else who lacks the necessary expertise.

Here is an excerpt from Fluke Corporation, a producer of a wide range of electrical test equipment:

There are two types of ground impedance testers. Three and four point ground testers and clamp-on ground testers. Both types apply a voltage on the electrode and measure the resulting current.

A three or four-pole ground tester combines a current source and voltage measurement in a “lunch box” or multimeter-style package. They use multiple stakes and/or clamps.

Ground testers have the follwing characteristics:

  • AC test current. Earth does not conduct dc very well.
  • Test frequency that is close to, but distinguishable from the power frequency and its harmonics. This prevents stray currents from interferring with ground impedance measurements.
  • Separate source and measure leads to compensate for the long leads used in this measurement.
  • Input filtering designed to pick up its own signal and screen out all others.

Clamp-on ground testers resemble a large clamp meter. But they are very different because clampon ground testers have both a source transformer and a measurement transformer. The source transformer imposes a voltage on the loop under test and the measurement transformer measures the resulting current. The clamp-on ground tester uses advanced filtering to recognize its own signal and screen out all others. - Op. Cit.

Grounding Electrode Testing Shortcut using the 62% Rule

Fluke Corporation describes a simplified grounding electrode test in which the technician drives additional spikes into the ground to permit several measurements fof impedence that are compared at different distances from the grounding electrode under test. Measurements are made by connecting test leads to the grounding electrode, to a "current spike" driven at a specified distance "d" from the test electrode, and one to three "potential spikes" driven at a distance equal to 62% of distance "d". Details are in the document that we cite and link-to above.

You may be able to use a shortcut if your test meets the following criteria:

  • You are testing a simple electrode (not a large grid or plate)
  • You can place the current stake 100 feet or more from the electrode under test
  • The soil is uniform

Under these conditions you can place the current stake 100 feet or more from the electrode under test. Place the potential stake at 62 % of the distance between the current stake and the electrode under test and take a measurement. As a check, take two more measurements: one with the potential probe 3 feet closer to the electrode under test, and one 3 feet farther away (see Figure 5 in Fluke's document). If you are on the flat portion of the fall-of-potential curve then the readings should be roughly the same and you can record the first reading as your resistance. - Op. Cit.

Electrical Grounding System Articles

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Continue reading at ALUMINUM GROUND WIRES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see DOUBLE FAULT, LOSS OF ELECTRICITY

Or see ELECTRICAL GROUND ERROR-CAUSED LEAKS - leaks in metal water pipes traced to improper grounding

Or see ELECTRICAL SERVICE ENTRY DAMAGE - service entry wire melts & shorts to ground

Or see GAS PIPING, FLEXIBLE CSST for a discussion of lightning protection needed for flexible stainless steel tubing used as gas piping

Suggested citation for this web page

ELECTRICAL GROUNDING BASICS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

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