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This document describes the loss of ground connections at a building leading to loss of electrical power and dangerous risk of electrocution. Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for Inspectors examining Residential Electrical Systems and Local Electrical Grounding for safety procedures during inspection of the grounding system.
Also, see details about electrical grounding at Electrical Circuits, shorts, and at Electrical Wiring in Old Houses and at Electricity Basics - how it works.
© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Case History of an Electrical Ground Failure - a Double Fault Leads to Loss of Electrical Power in San Miguel de Allende, Mexico
Never assume anything during a building inspection. For example, never assume that because you see that ground wires are present, that the building and its electrical system are safely grounded.
First of all, there are usually at least two ground connections at a building:
One ground source is an incoming ground wire from the electrical service is grounded somewhere by the utility company - don't' assume that it's connected.
The second and critical ground source at a building is the building's own local connection to earth through one or more grounding electrodes, or in older buildings, possibly by a connection between the electrical panel's ground bus and a metal water pipe entering the building from outdoors.
Dim lights and no refrigerator after a storm: In a home in San Miguel de Allende, Mexico after a severe rain storm the building's lights were severely dimmed. If the occupants turned on any high-amperage device such as a toaster or refrigerator, the lights began to flicker. The home's refrigerator motor would not even start. We assumed (that was the first mistake) that the building's connection to the electrical utility company's overhead service wire had been lost because we saw that wet tree branches had leaned over, pulling on the service drop wire.
Measuring electrical voltage showed that 120V was available at the wiring lugs in the electric meter base (photo below), but when any electrical load was placed on the building electrical system voltage plummeted.
After more than a week of haggling with CFE, the electric utility company, and after watching their service tech climb poles, cut and re-make electrical connections to the building's service entry cables, nothing was changed,and the building still did not have functional electrical power.
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Each electric meter feeds one side of the electrical panel indoors. Circuits in the building are thus fed from one or the other electric meter. Why people do this bears explaining: all that this dual meter setup is doing is splitting the electric bill into two bills. People do this when they want to create an apartment whose tenant pays their own utilities, or in some cases because the size of a utility bill can affect the electricity rates. In this home there is no apartment. The second meter was installed by prior owners who thought they were obtaining doubling the electrical power at their home. They were completely mistaken. Above the incoming blue "hot" wire (the service drop wire) you can see the aluminum ground wire coming from the utility pole and being fed into the meter enclosure from the overhead electrical service drop. Everyone was focused on testing the "hot" blue wire back to the pole and we forgot to also test the ground wire back to the same pole. |
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Before the replacement service entry cable could be installed, we asked for some additional diagnostic help from a local electrician familiar with electrical wiring problems in Mexico. Figuring out what was wrong - a bad service drop ground wire: The electrician too thought at first that the problem was a shorting service drop cable. But we also discussed the alternative possibility that the problem could be not the loss of the building's incoming or "hot" service entry wire, but rather the loss of the building's ground connections. No ground-connection means no current flows in a building's electrical circuit. A very weak connection between a building's electrical system and earth could result in power loss when the electrical system was under load - not enough electrons could flow to earth. The loss of the building ground connection seemed unlikely. After all, we could see a grounding electrode driven into the soil (for some unknown depth) and surely, all that rain had wet the soil, improving the ground connection. The left-hand photo below shows the original building local ground wire (red) connected to the original building grounding electrode. (The green wire was added as we explain below).
Our photo above of the electric meter enclosure shows the temporary green ground wire that the electrician connected between the meter-ground (which also connected to our electrical panel ground and neutral buses). Our photo at left shows the ground wire wrapped around the building's incoming water pipe. (A clamp would be added later.) In the photos we show here we had not yet found and installed a proper grounding clamp to secure the ground wire to the water pipe. A ground wire that is just wrapped around a water pipe is certainly not a reliable connection, but it was sufficient to demonstrate that providing a good local ground at the building turned the lights back on. We determined that the building had experienced a "double fault" in its grounding system: the local building ground had always been inadequate. But when the storm damaged and pulled loose our overhead connection to the utility company's ground system, we had lost our second connection to earth. |
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While waiting for the electrical utility company, CFE (Comision Federal de Electricidad) in San Miguel to get around to fixing their own ground ("cable de tierra") back at their pole, the electrician installed a new and longer grounding electrode at the building's electrical service.
Loss of all building ground connections was extremely dangerous: Assuring that the building had at least one working ground was very smart. The near total loss of electrical grounding at this building could certainly have killed someone.
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Keeping the building well grounded: At this building the electrician kept the additional bond between the local ground system and the local municipal water pipe as well as installing a new grounding electrode. Having a local electrical ground at the individual building is very important, to give the building electrical system a ground reference point, and for surge dissipation, but the local electrical ground at a building should never be the only path for return current. -- J.P. Simmons Other risks remain when the utility company ground is not effective: After this temporary fix we were still very nervous about the missing utility company ground. Why? Because we don't know how many other homes may be affected by the failure in the utility company's ground system. The fact that only our building lost power does not mean that we were the only one connected to this "bad ground" back at the pole - it could simply be the case that the other houses on the street each had their own good local ground connection. But should someone else's ground connection fail, and should the utility company's ground connection remain un-repaired, then our neighbors could be depending on our local ground wire (which is pretty small) for their own safety - a dangerous dependence. The utility company ground was finally restored when CFE, the electric utility company, routed a new neutral wire between the building's electrical meters and their overhead neutral wire. Our case study of a shock and fire hazard in a garage shows what happens when a local ground is the only connection to earth at a property and when that condition combines with other wiring defects. |
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ELECTRICAL INSPECTION, DIAGNOSIS, REPAIR
ELECTRIC PANEL AMPACITY
ELECTRIC PANEL INSPECTION
ELECTRICAL BASICS
Electricity Basics - how it works
Electrical Circuit ID, Map & Label
Electrical Circuits, shorts
Electrical Code Basics
Electrical Conduit Tips
Electrical Definitions
Electrical Grounding Basics
Electrical Outlet-how to add
Electrical Splices, how to make
Electrical Tools & Tests
Electrical Wire Stripping Tips
Electrical Wiring Books & Guides
Electrical Wiring in Old Houses
ENERGY SAVINGS in BUILDINGS
FEDERAL PACIFIC FPE HAZARDS
GROUND SYSTEM INSPECTION
Why Grounding is Needed
Gas Piping Ground Bond
Table of Ground Wire Sizes
False Ground at Receptacles
False Neutral Connections
Case History: Double Fault Leads to Loss of Power
Case History:Loss of Neutral Shocks Homeowner
Electrical Service Grounding Checklist
KNOB & TUBE WIRING
LIGHTNING PROTECTION
LOW VOLTAGE BUILDING WIRING
MAIN DISCONNECT AMPACITY
- Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
- Thanks to James P. Simmons, Mr. Electric, for technical review and advice on this material. 9/19/2008. Jim P. Simmons, Mr. Electric, Licensed Master Electrician, 1320 Dayton Street SE, Olympia, WA 98501, 360-705-4225, Fax 360-705-0130 or by email to mrelectricoly@msn.com
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More Information on Building Diagnostic Inspections and Repairs
- * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
- 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.
- Ampacity of an Electrical Service: How to determine the electrical service size or ampacity entering a building
- 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.
- Lightning Strike Risk Assessment, Protection Systems & Services
- Multi-wire branch circuit inspection and defects
- Zinsco Electrical Circuit Breaker Failures: overheating, failure to trip, burn-ups involving Zinsco and certain Sylvania electrical panel components - page under development March 2006.
- "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.
- "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.
- Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
- Directory of Home Inspectors accessible via internet
- Electrical System & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page
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