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Unjacketed Concentric Neutral Cable and Stray Voltage

Mike,

I have found and corrected several instances of privately owned high voltage systems with unjacketed primary concentric neutral (exposed bare neutral conductor on outside of the cable) where the neutral conductor corroded completely away and became nonexistent for hundreds of feet. The bare concentric neutral continued to operate for some time by feeding the neutral current into the earth and the bonded water systems inside the connected buildings. When the resistance through the earth and the water piping became high enough, and the neutral load increases, the voltage increases on the water piping. The Voltage is then often noticed between the water piping and faucets and the drain piping.

Unjacketed concentric neutral cable was a very bad idea. I believe that all exposed concentric neutral wire will eventually corrode away. The worse the PH of the soil the faster the exposed neutral will corrode away (acidic or alkali). I have found this occurring in conduit as well as direct burial. Inside conduit the water from the surrounding soil has the same PH level.

Two utilities in our area stopped using unjacketed concentric neutral cable and started using jacketed cable instead. However there is still a lot of unjacketed cable in use both private and utility owned.

Actual cases I have encountered due to corroded away primary concentric neutral on unjacketed underground cable:

  1. College dormitory where student was shocked while in shower when foot touched the floor drain and hand touched the faucet.
  2. Doctor's office where patient touched the front door knob while standing on wet concrete porch. (metal building, metal door, metal knob)
  3. Industrial building where unusual current found flowing into ground rod.
  4. College classroom building where lights operating phase to neutral changed brightness.

What often happens is that the water piping system bond will carry enough current to keep the facility operating but backs up enough voltage to be noticed somewhere. All of the above buildings were still operating when I investigated the problems. The problem is more noticeable when the padmount transformer is single phase or wye-wye connected.

Scott Yeazell, scott@triec.com
Triec Electrical Services

Mike Holt's Comment: As we have more primary neutral conductor deterioration/failure, the primary neutral touch voltage (some call stray voltage), which is impressed on all metal parts that are conductive to the electrical system, will become more of a problem. For more information on this topic, visit my "Stray Voltage Links" from my "Code Forum Page", my "Technical Page", and my "Newsletter Archives Page."


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