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.
Well, can't say that I am suprised by this scenario - just didn't even think about it happening!
Makes sense though!
These testers use the Capacitive charge that is established on any circuit when the conductors are charged. Normally, there's an equal level of Potential between the conductors and grounded components, so the Capacitive charges are built up to that level. Since they exist between an insulated conductor and air, these two things become the Dielectric between them. Even an open circuit [one that's connected to the power supply, but has no connected loads] will have this preliminary charge created on it.
Since the water contains metallic particles within it, they act as conductive paths, which draw charged currents better than the original setups through the Dielectrics, so the Capacitive Charge Potentials exist highest across these points of current flows.
Have you tried this test with a certain level of current flowing through the extension cord also?? The results might be different - you might get a reading real close to the ungrounded conductor with some level of current flowing through it.
Glad that you posted this message to the group!! It could very well save someone's life!!!
I personally do not rely on any other testing than direct contact meters for voltage. Even then I judge the circuit in question as if it was still live.
Have seen many others in the trade use and even rely on these proximity effect type circuit testers. They're great for troubleshooting, but they should not be the only means used for testing the state of circuits.
Keep us informed to the final outcome!!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
#899 - 03/30/0109:13 AMRe: Non-Contact Voltage Detector Concerns
Scott, I agree that these units should not be used as a final testing device, particularly where life-safety is an issue. However, I , like many, sometimes yield to haste or habit. Hindsight is 20-20, and in the future, hopefully, we'll remember this incident. By the way, the cable in question was at approxinately shin-level, and my brother was kneeling in moist grass at the time. If the cable was already the proper length, he would have been stripping it instead of cutting it, and I believe he would have been killed.
#901 - 03/30/0110:01 AMRe: Non-Contact Voltage Detector Concerns
Yes...I've noticed this happening to both my GB and Amprobe brand volt probes. NMC under a house (damp area) wouldn't register on the probe even though I could see the that the light was on! I learned this the easy way, and remembered it. I guess I sort of thought that everybody figured this out on their own... I told my boss, but he wasn't terribly impressed with the info.
I see the moist paper acting as a shield...just to simplify things and stay away from quantum mechanics (sorry, Scott) a probe won't work on shielded cable or SE cable (where the neutral is wrapped around the hots), and I consider the wet paper as acting as a grounded shield (it touches the EGC).
I don't trust my probe very much, it tells me there's voltage three feet away from wires sometimes (floating voltage?) and as you have described, even worse when it "lies" to you and tells you a circuit is dead when it ain't. I try to test the situation at every job and confirm in more than one way if I need to make d*** sure the circuit is off. I've been known to kill a main when things are really iffy on the readings.
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI
#903 - 03/30/0101:26 PMRe: Non-Contact Voltage Detector Concerns
Use *any* voltage tester with an clear understanding of the device's limitations, and when it is likely to give false readings (or no reading). Experience is a good teacher. The concept is valid and used just about everywhere—a lot of utilities use devices like Modiewark testers at 230kV+. There are no absolutes in this world. We take risks every day, but mostly they are ‘informed’ risks—there may be consequences. The non-contact tester increases job safety, but know when to question its readings—where another type of tester may do a better job. I like the various models EXCEPT those using “button cells” (hearing-aid batteries)…stupid!
[This message has been edited by Bjarney (edited 04-17-2002).]
#905 - 04/17/0207:06 PMRe: Non-Contact Voltage Detector Concerns
Scott, Thanks for the info on Capacitive testers. I use a Fluke probe, I always thought it worked on induction as well. My question is, if it works on capacitance, how does it discharge so quickly when you pull it off the source. (many times I'll touch the wire and remove and replace it repeatedly to confirm that the wire is hot and it instantly goes off when pulled away from the wire)? Does the current just discharge into...where? Thanks, Brian