How Loss of the Neutral Connection in a Sub Panel Led to a Dangerous Electrical Shock InspectAPedia® -
Case history of a neutral wire failure, improper joining of neutral and ground wiring in a sub panel, burning-up electrical ground wires, and a homeowner who received a nasty electrical shock
Photographs of electrical sub panel mis-wiring, overheating, and shock hazards
How to inspect the electrical ground system: wires, grounding conductors, connectors, ground rods
Why Do Electrical Power Surges or Lightning Strike Current Go to Ground While Short Circuits Follow a Path to the Utility Ground?
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This document describes a case history of loss of the neutral connection in an electrical panel combined with failure to isolate the neutral and ground buses leading to an electrical shock. 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. Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for Inspectors examining Residential Electrical Systems.
“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 at a remote sub panel in a garage is lost and all current flows through the grounding conductor to earth.
Case History of an Electrical Ground Failure - Loss of a Neutral Connection in an Electrical Sub Panel Badly Shocks a Homeowner
Building owner gets a nasty electrical shock: In an electrical failure case which we investigated, a building owner asked us to determine why he had received a severe electrical shock when he touched his metal work bench located in a detached garage.
Here is what we found in the electrical sub panel in the detached garage:
[Click any image in these articles to see an enlarged, detailed version.]
Remote sub panel: The detached garage received its electricity through a sub panel mounted in the garage and connected back to the main electrical panel in the home.
Local grounding: A ground wire ran from the panel ground bus to a local grounding electrode at the garage.
Evidence of overheating ground wires: The panel's copper ground wire was blackened - it was obviously dark from overheating (none of the other copper wires in the panel were discolored).
Other circuit ground wires that had been twisted together in the sub panel were also black from overheating.
Improper bonding of neutral bus and ground bus in a sub panel led to electric shock: The owner, intending to make his electrical panel "safer", had violated electrical code and good practice by bonding together the neutral bus and the grounding bus in his electrical panel.
There was no ground wire connection back to the main electrical panel, just incoming hot wires and incoming neutral wire.
The panel's incoming neutral wire from the home was not connected. It looked connected but when we wiggled it (power had been shut off at the main building) the neutral wire lifted right out of the connector.
Our photo at left has a black arrow pointing to the incoming neutral wire in the sub panel. The neutral wire looked connected, but it was not.
The owner had a high horsepower 3-phase ("converted magically to 2-phase") electric motor driven air compressor in use in the garage. He had modified the motor's electrical wiring as well.
The owner had, for safety, grounded his compressor motor and his metal work bench back to the electrical panel's grounding bus.
Question: What did all of these sub panel neutral and ground wire problems & electrical wiring errors mean?
When the air compressor motor was running it was producing a significant current on the compressor's neutral circuit.
The garage neutral circuit had no connection back to the main building (where it would have been connected to earth in the main panel, but because the owner had (improperly) bonded together the ground and neutral bus in his garage sub panel, the garage neutral circuit was indeed finding a path to earth through a small diameter (and thus overheated and dark) copper ground wire and a local grounding electrode.
Bud, a master electrician from Minnesota points out that in the 2005 NEC there are 2 ways to connect a subpanel in a detached garage. In both methods grounding electrode(s) are required at the garage.
Hot-hot-neutral-ground are run to the garage. The neutral bar is isolated. Separate ground bar. The grounding electrode(s) connect to the ground bar.
Hot-hot-neutral are run to the garage (NO ground). This is connected like a service. A “main bonding jumper” is installed to connect the neutral bar to the enclosure. The grounding electrodes are connected to the neutral (or ground) bar. (This method is no longer allowed in the
2008 NEC.)
The description of the garage event above is a case of example #2 above - there needs to be a MBJ [jumper wire] to connect the neutral and “ground”. The connection was not an error. Without the Neutral-to-Ground bond there is no metal return path for ground fault currents back to the utility transformer. The path would depend on an earth connection, which is not effective and is not allowed. (This is the same error as in my original complaint.)
Normally the neutral-to-ground bond is made in the main electrical panel and not in sub panels, lest grounding conductors end up carrying current during normal operations - a shock hazard. As Bud describes above, in the now obsolete, not allowed case, it was possible to wire a remote outbuilding sub panel as if it were a "main", with no ground returning to the actual remote main panel, and with a neutral-ground bond in the sub panel plus an effective local grounding electrode. We do not recommend this obsolete wiring approach. - Ed.
The problem in this case is the loss of the feeder neutral.
Summarizing some key observations in this lost neutral sub panel case:
Neutral wire between sub panel & main panel was not connected; there was a loose, bad connection in sub panel
The only grounding in this outbuilding was a local ground rod (maybe ineffective?)
A 3-phase compressor was "DIY" wired to run 240V - something fishy there
The compressor circuit included two "hot" wires (black and neutral - the upper right two-pole breaker in the page top photo), and a bare ground wire that was overheated and burned.
A metal workbench was connected to subpanel ground and was "live" when the compressor was running - owner got shocked
Wires to circuit breakers in the panel were overheated (feeding the compressor)
The internal bus in the panel was overheated (behind the CBs)
Ground wires were overheated and burned (and were new at the outset) at the sub panel
No mis-wiring nor overheating related to this case were observed in the property's main electrical panel
In the sub panel, the neutral bus and ground bus were connected, so any neutral circuit current that should have traveled back on the main neutral to the main panel was forced to travel on the local ground wiring and grounding conductors.
Summarizing our conclusions (we will amend if with readers we develop a different analysis of this case):
A combination of missing main neutral in sub panel back to main panel, connecting neutral and ground buses in the sub panel, local ground at the sub panel, connection of metal work bench to panel ground bus, and modified 3-phase compressor in operation produced an electrically live workbench (owner was shocked), and conspired to visibly overheat grounding conductors, hot wires to the panel, panel bus behind the breakers, and grounding wire to the local electrode.
Additional photographs from this case are shown just below. Thanks to reader Randy Gardner for discussing this lost neutral case and opening an argument for clarification of just what was going on.
Below left: metal workbench that was connected to ground wire bus in the sub panel. Below right: overheated corroded main bus connections in the sub panel.
Below left: 3-phase compressor "re-wired" to run on 240V (we suspect that a 3rd leg was connected to a neutral or ground wire at this hookup. Below right: overheated wires in the sub panel at circuit breakers (we suspect these were powering the compressor motor).
Why Do Electrical Power Surges or Lightning Strike Current Go to Ground While Short Circuits Follow a Path to the Utility Ground?
Question: We were a little unclear as to why surges find their way to the local earth connection (higher resistance path) while in an emergency such as a short circuit hot to neutral or hot to ground wire, the current does not.
Answer: Bud continued this helpful explanation:
It is a question of what is path is necessary to complete the circuit.
If you want to trip a circuit breaker the fault is hot to “ground” you must complete a path back from “ground” to the transformer neutral. If you wan t to trip a breaker, the path must be metal. That requires a N-G bond and return on the service neutral.
For lightning, you accumulate a different charge between the cloud and the earth. The path is then to the earth to neutralize the charges. (A service panel surge suppressor would have current hot-to-ground/earth.) (Any surge current on the neutral connects to earth through the N-G bond.)
The next most major cause of surges is probably utility switching (normal and abnormal). Switching power factor correction capacitors can be a major source. There the path is probably hot to neutral. (A service panel suppressor would have current hot-to-neutral.)
If there is a utility short that causes a fuse to open, there is high current through the inductance of the power wires that stores energy in a magnetic field. When the fuse opens, the current falls causing the magnetic field to collapse which produces a high voltage. Surge current is probably hot to neutral.
If a high voltage distribution wire falls on the wires to a house, the path needs to be back to the high voltage transformer. The path is likely through the earth, but could be through the service neutral if there is a path (there is probably a continuous neutral connection between transformers that feed houses).
Question: Isn't it the case that current always prefers the path of least resistance and that the current can actually be calculated to flow on two (for example) different paths as a direct function of the relative resistance of each path?
Answer: Nicely stated, particularly the end.
And in the case of the ground fault tripping a breaker there will be some current through the earth, just like there is some normal neutral current through the earth. But the resistance of the service neutral is far lower than the earth.
The root causes of this electrical shock were:
Improper electrical wiring - bonding the ground and neutral wires together in the sub panel
A binding steel screw in an aluminum wiring lug in the sub panel's incoming neutral wire connector - tightening the screw made the installer think he was securing the neutral wire when he was not.
Because the garage electrical system was carrying current that should have been flowing only on the insulated neutral wires in the building, when the owner touched his grounded metal work bench and was also touching a damp floor, current flowed through the ground wire, through the metal work bench, through the owner, to earth.
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Special thanks to Bud -
a master electrician in Minnesota who contributed text and suggestions for explaining why we need electrical grounding, electrical neutrals, and for discussing the shortcomings of neon testers and plug-in receptacle testers - 1/22/2009
Thanks to Randy Gardner for discussing this lost neutral case and opening an argument for clarification of just what was going on - 12/27/2010
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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.
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.
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