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 SET

Redsy;
an interesting experiment,one that i'll have to copy if the opportunity arises,..let us know if the manufacture' follows thru !

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.

Redsy,

I am so happy to see that no one was hurt!!

Scott SET

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.

it's interesting in that the capacitive buildup is bled off by the metallic mierals in h20. it makes sense in the old story about pure h2o being a bad conductor.

in the same vien, would conductors in close proximty of metallic substances also create this prob??

i have notice that these "induction pens"
( why not capactive pens?) ring easier in the presence of a heavy load.

in any event, one should practice certain safety measures when metering, or connecting a load.

unrelated, how many out there energize a load at arms length, once you have a firm grip, look away, then make contact? this is how i was taught.

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).]

In the summer I wear just a T-shirt for work and will clip my volt pen inside the shirt collar.

After a long sweaty day the pen will often not work dependably. The moisture seems to be the common link.

FWIW

GJ

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

Scott, are you absolutely certain that pocket testers such as the Fluke and Amprobe work a the electrostatic field and not the inductive coupling? By the way, since I went to work building a new 345KV switchyard's control room, and wiring up four big ABB GCB's (SF6 gas filled circuit breakers)any control wiring in the yard, including the 3 phase 240v. convenience outlets, only a meter will reliably confirm a circuit is dead. The "static" is so high that any length conductor is going to show "hot" even though it is not. It is tough to even get up on a 6' ladder and touch a grounded box, or get out of a work truck and not get bit by the "static". The reason I'm using parenthesis around the word "static" is that the linemen and high-voltage industry commonly referes to it as static, but in reality it is electromagnetic induction.

I'm not sure how they work either, but as long as they work...
I did tell my employees about the testers not working in relationship to moisture.
One of my men was working outside the other day and sweating. He also keeps his GB "ticker" clipped to the inside of his T-shirt. He said when he was working outside that day he couldn't get the tester to work reliably. Thanks for sharing the information guys, it may save someone some day.
I always tell my guys that the testers always tell you if the power is on but not necessarily if the power is off...use a meter.
Also, I know that if you are under high-tension / high-voltage wires that if you hold your tester in the air it will beep like there is constant voltage.

The effects on the tester with the water soaked NM points towards a Capacitive Coupling Effect, more than an Inductive Coupling effect, so that's where I formed the basis of Cap effect [like Cap Proxy].

Inductive Coupling would be an answer to Steve's tester having a stronger signal with higher loads, but could also fall back to Capacitive Coupling just as easilly.

Seems the tester would have to be self calibrating to sense Inductively on a circuit which could be idle or drawing 100 amps. At idle, there's very little Xl present [or Inductive Coupling between active circuit conductors and the enviroment surrounding them]. At high load, there's quite a lot of Inductive coupling between the active conductors, High Inductance surrounding the conductors, and higher coupling to surrounding enviroment [due to higher mag fields surrounding the conductors].
This should yield variable test responses at different distances - per load conditions - but not have a draining effect from water.

If you have other information regarding this, please pass it on!!!

I am going on theories only. Personally I do not use these types of testers, only have seen others use them [and seen contridictary reports from the tests! making me unsure of their use].

Scott SET

hi,
these type testers only work on AC circuits...not ANY circuit. they detect a the magnetic field created around the conductor when AC is applied.
-regards

frodo

[This message has been edited by frodo (edited 04-21-2002).]

I was under the impression that a magnetic field was present only when there was current flow.If there is no load on the ckt,thus no current flow,the detector will show the presence of voltage through the electro static field(capacitive coupling).
I THINK!

Chris

man it's early, didn't realize that the last post on here was from 2002! oops!

Hey, threads like this are timeless, that doesn't matter! What was your original comment?

Luketrician

I’m glad you bumped this tread up
I for one fall into the "use the volt pen from habit" group. Though I use it mostly in the classroom for fault finding when the students get circuits wrong.
I have never used it out in damp places. So I for one have never came across this situation but I will now rethink some of my faultfinding techniques

Kenny