A week or so ago I attempted to verify the absence of voltage on a piece of NM cable that had been poked through the the siding of a residence for a few weeks. The detector(which has been functioning properly prior and since)indicated a dead ckt. Luckily, the cable was too long and had to be cut to length to install a W.P. box, because the circuit was not dead after all. Upon inspection, we noticed that the paper inside the NM cable had become damp due to exposure to the elements. We set up an experiment in which we laid a dry paper towel on top of an energized cord, and the detector beeped at approx. 2" away. Dampening the paper towel with water caused the detector to not indicate voltage at all, even when contacting the towel. We tried this with 3 different brands and all responded the same way. I believe theses units operate on electroststics, and evidently, moisture interferes with the field. I spoke to the manufacturer and they were unaware of this problem. They conducted similar tests with similar results and told me that they are going to follow through on this. Hopefully they will.
