Electric worker testing service drop connections (C) Daniel FriedmanHow Loss of Neutral & Electrical Ground Can Both Occur at a Building - Dangerous Results

  • DOUBLE FAULT, LOSS OF ELECTRICITY - Case histories of ground system failures, loss of electrical ground leads to loss of electrical power and risk of potentially fatal electrical shock - Case history of utility company electrical neutral wire connection lost leads to lost electrical power and dangerous shock hazard; Case history of double electrical fault: lost electrical company neutral AND lost local ground to earth; How to inspect the electrical ground system: wires, grounding conductors, connectors, ground rods
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Electrical ground becomes disconnected, corroded, weak, intermittent, or 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

“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.

Also, see details about electrical grounding at ELECTRICAL CIRCUITS, SHORTS, and at OLD HOUSE ELECTRICAL WIRING and at ELECTRICITY BASICS, HOW IT WORKS.

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.

Dual meter set up in San Miguel de Allende (C) Daniel FriedmanHere is a common dual electric meter setup at a single family home in Mexico. The blue wire entering at upper right is the 120V "hot" wire bringing in 120Volts to this pair of electric meter bases which are wired in parallel. We pulled the electric meters to give an easy view of the wiring itself. This setup simply splits the electric bill into two parts.

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.

Temporary ground wire test (C) Daniel Friedman

The wrong diagnosis of a bad service drop hot wire: Together with the utility company employee who came to the site, we concluded that our service entry cable had been damaged and was shorting to ground somewhere, and we ordered a replacement. The CFE employee left to obtain a replacement cable (and did not return).

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).

Ground wire temporarily wrapped onto the water pipe in San Miguel de Allende, Mexico (C) Daniel Friedman

A simple electrical system grounding experiment told the story of a double fault in both the utility company's ground and the building's local ground connection: the electrician made a temporary grounding connection between our electrical panel and an incoming metal municipal water supply pipe.

Suddenly all of our lights went on and everything worked.

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.

Bad grounding electrode connection (C) Daniel Friedman Good ground electrode connection in San Miguel de Allende (C) Daniel Friedman

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.

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

Additional comments from Bud, a master electrician in Minnesota:

Looking at the pictures makes me appreciate the NEC.

I read the newsgroup, which probably gets most of the US power wiring questions. A few times a year there are problems which are caused by a bad utility *neutral* connection. In the 240/120V US system, (where the 240V transformer is center tapped to provide 120V) if the service neutral is lost the balance of the 2 - 120V legs is lost.

With just a space heater on one side and just a light bulb on the other side the light bulb can get real bright (briefly). The path through the N-G bond and the earth back to the transformer helps a little but can’t prevent serious unbalance.

My understanding of this Mexico story is that what was lost was the utility neutral, not the utility ground (IMHO another error). A neutral is a current carrying conductor. A ground does not normally carry current.

With no utility neutral, all the power has to return to the utility transformer through the “ground” wire. With higher resistance to earth at the start, most of the voltage is lost in the earth path. When the ground was attached to a metal water pipe, the path was probably through the pipe to neighbor(s), through their service N-G bond, and back through their service neutral (which your article probably says). (This can also happen in the US, and was in a recent thread at

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. Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for Inspectors examining Residential Electrical Systems and Local Electrical Grounding for safety procedures during inspection of the grounding system.

Electrical Grounding System Articles


Continue reading at LOST NEUTRAL SHOCKS HOMEOWNER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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


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