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Lost neutral or weak neutral wire electrical connections at a building: what can happen? 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.
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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.]
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.
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.
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:
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:
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:
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.
Sketch courtesy of Carson Dunlop Associates.
Our case study of a double fault involving loss of both the utility company ground and the local building ground shows how power can simply be lost in a building due to grounding system defects.
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