Seriously weird!

Ok this is VERY weird so I'm going to put it out there to see if anyone else has come across this.

I just plastered a wall and stapled some metal corners around an edge. I just touched the still wet plaster and got a slight tingle off of it. Odd I thought so I went and got my digital multimeter and sure enough there's 30V on the plaster next to the corner I cut down to form a strip (Due to location, that edge is just a reveal). Maybe I'd stapled through a neutral or something, yet they were short staples into hardwood, and I'm pretty sure I know where all the wires are. There's a double light switch (hanging out from the plaster, so I switched the lights off and the voltage dropped to 23V. Push the wires back into the wall and it dropped further!

SO! I'm left to deduce that a voltage is being introduced into this strip from something somewhere. I'm wondering if anyone else has ever come across this, what caused it, and what the end result was? At the end of the day it's only 30V but it seems awfully strange to me (no high power transmitters or anything I know of around here).

Hazarding a guess, I'd say you have put a staple into the stud near where the wire has been passed through it (the stud) and have either hit the wire inside the electrical flex or just partially penetrated it. Whatever the case, you are going to have to find it and fix it. If it gets a full charge someone is in for a zapping. :oo:

Is there a water pipe or amplified TV antenna cable in the wall by any chance?

Just occurred to me that someone in the past may have done an illegal connection to extend the reach of the switch, any chance of that?

bit of a funny one, stapling a cable seems the most likely , the only times ive seen induced voltage are over long runs with high voltage, eg a fence under a power line.
i would get a sparky to check the cables going down the wall.
possibly a very dangerous situation.
from memory tv amps are around 14 or 17 volts so the 30v seems high.

Agreed, if I were just reading this I'd suggest the same thing, wire through a cable. BUT it's really not giving those symptoms (ie it would either be full charge or nothing, not 30V and certainly not varying). Also the way I stapled would have made hitting a wire almost impossible as the staples were only 1". In fact after pinning I switched to 1/2 inch through gyprock into a hardwood stud. Indeed I planed the stud considerably to straighten it so know it intimately ;)

Since posting I had a poke around with the DMM with one side to GPO earth, and many "things" around the area register a voltage (the ladder 17v for example). Digital multimeters have such a high impedance they will easily pick up this sort of thing but I'm surprised I was able to detect it. Antenna has masthead amp but comes up from the floor. No chance of hitting that. Photographed the wall before hanging the gyprock so I know where all the cables run. No shonky "wiring extensions".

Will explore more tomorrow and post results.

Wet plaster could be also more capable of carrying voltage. Still 30 volts after drying? Is pretty weird.:oo:

Pete what colour is the cable

Ok I checked my photographs (good hint for those who don't do it; photograph studs/wiring with some sort of scale, I use my levels, before lining. Worth gold after the sheets go on). Absolutely no way any wire could physically be hit.

I've now thoroughly checked this and think I may have been jumping at shadows a bit, sorry people. Not sure what caused me to feel the initial "tingle" in my finger (I was feeling the wet plaster to see what state it was in, you know, how you just can't resist poking something even though you know you shouldn't). Since now BOTH metal angles measure the same voltage to both GPO ground and a gas line BUT there is no current at all. Not a sausage. Further, the plaster is certainly no longer "wet" but if I earth one angle the other drops to near zero volts. Conclusion: It's just noise I'm picking up on the DMM. One of the reasons I'm "old school" and still prefer the old analogue meters.

Many years ago I got badly zapped from something that was "live" when it shouldn't have been due shonky wiring. Even though I went on to work in electronics and got electrocuted, nuked, irradiated, microwaved, and my favourite, RF burnt, seemingly daily, I still have a very healthy respect for the old electrons. It's not the first time I've seen a nail or screw go through a wire (or a gas line), fortunately not this time. Sorry to waste your time but thanks to all those who offered suggestions.

PS. Oh I just re-read my initial post, it contained a typo which was very misleading. I meant to say an INDUCED voltage, not "introduced".

If the cable is the old black rubber stuff the insulation could be breaking down

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Originally Posted by nev25

If the cable is the old black rubber stuff the insulation could be breaking down

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No I reckon the house has been rewired, it's standard 2 core white/red lighting cable. All wiring is in good condition and seems to have been done well. Strangely however they didn't fit an RCD, even though the box is relatively new. I think the clipsal RCDs will simply replace the main clipsal breaker on the same rails right? So I just need to find a sparky who is willing to do a ten minute job :)

Actually that was one of my main concerns from this. Long after this job is forgotten, trying to track down why the RCD I have fitted keeps tripping.

Pete,
I reckon the clues are wet plaster and a light switch. I got a tingle once while wallpapering and what it turned out to be was a small sliver of wet wall paper which had slipped behind the switch and was resting on the active :o.

Maybe a bit of moisture trickled down the cable - anyway I doubt it's "induced" voltage and your unintentional typo is probably closer to the truth.

Regards

Ian

Digital Volt Meter will have a very high (min 10 Mohm) input resistance, and hence will offer limited 'loading' to the voltage that you are detecting.This can be highly desirable when working through an operating circuit, but can be misleading when looking at something that has been charged and disconnected from the charging source.

An amplified (FET) analogue will have a similar input resistance, again with minimal loading.

A non amplified multimeter will use a current meter for the movement, and the input resistance will be rated in Kohm per Volt. A junky unit with a 1mA movement will rate as 1Kohm/volt, input resistance is 2Kohm on 2V scale, 200 Kohm on 200V scale etc. A higher quality unit may be based on a 10microamp movement, would rate as 100Kohm/Volt or 200Kohm on 2V scale, 2Mohm on 200V scale. Analogues fell from favour because the 'load' the circuit being tested, giving somewhat erronious results.

As an electronics engineer, I learned to cope with these issues fairly early on, and have a range of meters in both analogue and digital formats so i can select as I need. My cheaper digitals can often "find' a volt or two when they are on and both leads are unconnected (floating). The top notch one can sometimes manage about 4 V under the same conditions.

Incidently, Pete, you mentioned that you stapled the strip on. Any chance that you might have used an double insulated electric staple gun to do this? If so the feed and hammer system would be floating (not ground referenced) and the impulse in the hammer coil when firing could induce some voltage onto the strip, leaving it charged.

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Originally Posted by Pete F

Ok this is VERY weird so I'm going to put it out there to see if anyone else has come across this.

I just plastered a wall and stapled some metal corners around an edge. I just touched the still wet plaster and got a slight tingle off of it. Odd I thought so I went and got my digital multimeter and sure enough there's 30V on the plaster next to the corner I cut down to form a strip

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Pete,
I know you are not concerned about this any more but when you say there's 30V on the plaster what are you using as a reference? Exactly what are you touching with the two meter leads?

Malb, correct, see my reference to the input impedance (it's AC so actually as you know, technically more correct than resistance) above. It was a case of "expectancy"; I thought I got a tingle so expected to see a voltage, which I did. Unfortunately a lot of my electronics equipment was stolen some time ago and I didn't bother replacing most of it, just a good quality DMM and an oscilloscope.

I'm not sure I agree with you regarding the strip somehow "remaining charged" but anyway, no, it was an air stapler.

NC, as above, I tested to both GPO earth and a gas line. Also checked the voltage potential between the two grounds (zero) and the resistance between the two grounds (very low) both as you would expect. At one stage I was wondering (given how humid/rainy it's been) if the whole house was actually "floating" at a slightly different voltage to earth (as used by GPO and gas), and the moisture in the wood was allowing this voltage to be detected. A bit of an earth loop in other words. Funny how sometimes we can over-complicated even the most basic things at times :doh:

As above, thanks again for the input. The answer to the "puzzle" was there was no question to begin with. There was no voltage, just a paranoid operator :q

Oh Well. Better safe than sorry!!:cool: