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Unjacketed Concentric Neutral Cable and Stray Voltage — Update

Mike, beyond stray voltage concerns, I believe that an underground cable with a deteriorated neutral can be very dangerous to anyone who should happen to dig into it.

With the concentric neutral in place, a backhoe or trencher blade that should cut into the cable will make contact with the neutral and then the primary. In this case, the fault current flows just through the small section of blade that is in close proximity to the cable, limiting the possibility that the operator of the equipment will be electrocuted.

If the concentric neutral is not in place, then much more fault current is forced to flow through the digging equipment and everywhere else trying to get back to the source. Not only will this more likely result in the digging equipment being energized at a lethal voltage, but also the high resistance fault current path could allow protective equipment on the utility system to take a lot longer to operate (it might not operate to clear the fault at all, depending upon the protective equipment settings).

If I were digging anywhere near energized electrical cables, I would prohibit anyone else from approaching the equipment while in use. Should a dig-in occur, the operator would be much better protected than someone on the ground touching the equipment. Furthermore, I would use a walk behind trencher only as a last resort. If someone was standing on the ground and touching a walk behind unit that digs into a cable with no concentric neutral, then that persons body will become a path for fault current to flow.

My opinion on this topic is based on my own understanding of the behavior of electrical distribution systems. I am not aware of specific experiments done with these circumstances, nor do I wish to experiment on my own. Please do not take my comments as gospel truth, but use them to think about your own work practices and stay safe.

Note: I am an engineer for an electric utility that serves 50,000 meters. During the year 2003, energized electric primary cables (7200 Volts Line to Ground) belonging to my utility were dug into on approximately 50 separate occasions. Contractors who were installing sewer or water lines and couldn't be bothered with contacting us to request that we mark the location of our cables for them cut most of these.

Mike, though the projects that I deal with fall within the NESC realm instead of the NEC, I enjoy your columns and newsletters and feel like I do get a lot out of them.

Robert Maxwell
robert.maxwell@union.ncemcs.com


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