Open Service Neutral Causes Dangerous Touch Voltage on Metal Parts
Question: Mike, I was reviewing one of your previous articles in the Electrical Contractor magazine and have a question regarding water line bonding. Our department conducts all of the electrical inspections on one, two, and three family dwellings within our city and part of that inspection does require the water system to be bonded with the electrical system.
Recently the water department superintendent called me to investigate a problem his crew encountered on a water line repair. They uncovered the bad section and cut the copper service line. Once cut, the repairman grabbed hold of the water line and apparently received a substantial shock.
I realize this was probably due to an open neutral somewhere in the system, and that the electrical service might possibly be using the water line as a return path. However there was approximately twenty-five feet of copper water line between the residence and the work crew. I might also mention that in the last incident, there was probably sixty feet of pipe to the work crew. Their question - why are we getting nailed?
My response was that the water system offered less resistance than was offered by the driven ground rod, therefore that is the path that the service followed. Obviously when they cut the line and got between the line and earth, they will get shocked due to the fact that the water line was no longer in the system. I have told them repeatedly to place mechanical jumpers across the line prior to cutting; however it seems to fall on deaf ears.
I would like to ask of you if I may, if I am correct with my reasoning or if there may be another explanation for this situation. Your opinion would sincerely be appreciated.
Gary L. Smith, City of Lancaster
Mike's Answer: The following is an extract from my Grounding and Bonding book: "The bonding of the grounded (neutral) conductor to the service disconnect creates a condition where ground-faults can be cleared and the elevated voltage on the metal parts will not be much more than a few volts (voltage drop of the service conductors). Figure 250-64
However, if the grounded (neutral) service conductor which serves as the effective ground-fault current path is opened, a ground fault cannot be cleared and the metal parts of electrical equipment, as well as metal piping and structure steel will become and remain energized providing the potential for electric shock. Figure 250-65
When the service grounded (neutral) conductor is open, objectionable neutral current flows onto the metal parts of the electrical system because a neutral-to-case connection (main bonding jumper) is made at service equipment. Under this condition dangerous voltage will be present on the metal parts providing the potential for electric shock as well as fires. This dangerous electrical shock condition is of particular concern in buildings with pools, spas and hot tubs, Figure 250-66
Fire Hazard. If the service grounded (neutral) conductor is open, neutral current flows onto the metal parts of the electrical system. When this occurs in a wood frame construction building or structure, neutral current seeking a return path to the power supply travels into the moist wood members. After many years of this current flow, the wood can be converted into charcoal (wood with no moisture) because of the neutral current flow which can result in a fire. This condition is called pyroforic-carbonization, Figure 250-67
Author's Comment: We can't create an acceptable graphic to demonstrate how pyroforic-carbonization causes a fire by an open service neutral. However, if you would like to order a video showing actual fires caused by pyroforic-carbonization, call 1-888 NEC CODE.
If the grounded (neutral) service conductor is not run between the electric utility and the service, there would be no low impedance effective ground-fault current path. In the event of a ground fault, the circuit protection device will not open and metal parts will remain energized because sufficient ground-fault current cannot flow through the earth to the power supply. Figure 250-60
Author's Comment: To determine the actual voltage on the metal parts from an open service grounded (neutral) conductor, you need do some fancy math calculations with a spreadsheet to accommodate the variable conditions. Visit http://www.mikeholt.com and go to the "Free Stuff" link and download my touch voltage spreadsheet.
Copyright © 2003 Mike Holt Enterprises,Inc.