Case histories of ground system failures, loss of electrical power, electrical shocks
Class on how to inspect electrical panels
Home inspection procedures and safety
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This document discusses the details of why we need grounding, and definitions of electrical grounding and electrical bonding (what's the difference between these two terms. 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. Sketch at page top courtesy of Carson Dunlop.
“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.” Here we define electrical ground, grounding, bonding, and earthing terms and explain why there are important differences among these words.
Why Do We Need Electrical Grounding, Ground Rods, Earthing, Bonding in Electrical Wiring Systems?
Why we need electrical grounding
Speaking first a bit loosely (we'll get careful and detailed in a minute) the grounding system at a building provides an easy path for electricity to flow to earth should certain problems occur (a power surge, lightning strike).
Bonding or connection in the electrical panel between the neutral wires, ground wires, and the utility company's incoming neutral wire allowing current to flow to the utility company's wiring, assuring that a circuit breaker will trip or fuse will blow should a problem occur such as a short circuit or overloaded circuit.
Properly operating these overcurrent devices help prevent fire and shock.
Should an electrical fault occur where no neutral connection to the utility company or no local 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.
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.
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.
Required for safety, lets fuses blow
Example: toaster falls into metal sink
Grounding Electrode Conductor wire from service equipment to
House plumbing (grounds the plumbing)
Entering metal water pipe (grounds the system)
Grounding Electrode (two now recommended)
Continuous, no splices, meter bypass
Copper ground wires and grounding conductor (corrosion resistant)
Electrical “Grounding” has two jobs in a building: an “earthing” and a “bonding” function.
What is Electrical Grounding? "Earthing"
A major purpose of “grounding” is to provide a path so a “short” will trip a breaker. That requires a low resistance path back to the power source, which is the utility transformer. The path relies on a neutral-ground connection required at all [not entirely true] services.
The second "grounding" function is actually a collection of three safety functions:
minimize the voltage between exposed metal and the earth
minimize the voltage between the power wires and the earth
provide a sink for lightning, power line crosses, and similar hazards
The 1st grounding function described above is accomplished by connecting the grounding conductors at the service to an earthing electrode.
The 2nd grounding function described above is accomplished by connecting the power neutral (the neutral wire entering from the utility company's service) at the service to an earthing electrode (ground rod). Provided it's a real earthing electrode not a fake one as we found in our photo (left).
The 3rd grounding function described above is accomplished by both of the above.
Practically, all three grounding functions are accomplished by a required neutral-ground connection at the power service, with the combined neutral-ground connected to an earthing electrode or electrodes.
This function might best be called “earthing”.
What is Electrical Bonding?
The path is from a ground conductor [which is not necessarily a wire] to the service panel, through the neutral-ground connection, and back to the transformer via the service neutral. To provide high current to trip a breaker this must be a metal path.
The earth is far too high in electrical resistance and is not allowed to be the path. In fact this function will work if the service is not connected to earth.
This function might best be called “bonding”.
Connecting exposed metal together to minimize voltage between surfaces is also a “bonding” function.
There might be some virtues to showing the Neutral-to-Ground bond (called the main bonding jumper). As I wrote previously, it is barely visible in the Carson Dunlap diagram (above) with “Electrical path for ground and neutral wires”.
When a professional examines the interior of a main electrical panel, she looks to see if there has a strap for the bond between neutral and ground buses and to the grounding conductor leading to the earthing rod (ground rod). In some panels such as SquareD the bond is a screw (and very hard to identify if you don’t know what to look for - some home inspectors have trouble finding Neutral-Ground bonds.)
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Timothy Hemm, Yucala, CA, contributed the photographs of electrical equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
Special thanks to Bud - a master electrician in Minnesota who contributed text and suggestions for explaining why we need electrical grounding, and for discussing the shortcomings of neon testers and plug-in receptacle testers - 1/22/2009
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Recommended books on electrical inspection, electrical wiring, electrical problem diagnosis, and electrical repair can be found in the Electrical Books section of the InspectAPedia Bookstore. (courtesy of Amazon.com)
Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
Electrical Panels, How to Inspect in buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
"Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
"Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
"Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
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