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Electrical ground lost: what happens. This document describes the loss of both neutral (utility company) and local building ground connections at a building leading to loss of electrical power and dangerous risk of electrocution.
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Case History of an Lost Electric Company Neutral AND a Local Electrical Ground Failure: a Double Fault Leads to Loss of Electrical Power in San Miguel de Allende, Mexico
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.
“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.” The example discussed in this article helps explain what happens when the neutral connection to the utility company, or the ground connection at a building or both are lost.
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 possible return-current connections at a building:
One is the incoming neutral wire from the electrical service - grounded somewhere by the utility company - don't' assume that it's connected.
The second is the local electrical ground (connection to earth, typically through a ground rod or grounding electrode (sometimes water piping) at a building. This 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. Also see Why Grounding is Needed.
Dim lights and no refrigerator after a storm: In a home in San Miguel de Allende, Guanajuato, 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.
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.
Frequently Asked Questions (FAQs) about the effects of lost elecrical ground at a building: detection, tests, risks, repairs
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