AC Power, Multimeters, Voltage 'Ticks' and Benzodiazepines

[TrentO]

I replaced a faulty (worn) timer-switch in house today, and noticed that-- by simply connecting one side of my Sears multimeter onto the hot supply wire, I would actually see ~30 volts (meter set to A/C 200,) even before I had a chance to touch the other side to neutral or ground... Could someone explain this phenomena to me? In detail, I hope not just "yeah it's the inductance in the line" or something like that, but really tell me how the multimeter measures voltage in the first place and how the electrons are moving around. Of course you could tell me that my REAL problem was holding on to the bare-wire exposed on my multimeter probes, and the tub of salty water I was standing in....

Related to this is-- how do "voltage ticks" work? Those things pen-like devices that light up when you approach a 'hot' wire. These baby's light-up even when there's no load or current passing through the wire.

But the biggest question is-- could this be a fault of my not-so-cheap-at-the-time made in Korea multimeter, and would having a fancy-dancy Fluke solve this problem? I see that Fluke also makes their version of a voltage-tick... Better than the $20 GB Electrical unit that I have?

Thanks in advance,

-Trent

[JeffNichols]

Related to this is-- how do "voltage ticks" work? Those things pen-like devices that light up when you approach a 'hot' wire. These baby's light-up even when there's no load or current passing through the wire.

Those (I assume) work through mutual inductance. They've probably got a small coil inside tuned to 60Hz, and they measure the EMF coming out of it. It's basically a very inefficient transformer.

[jimmc]

A typical human body has a capacity in the order of 100pF to earth.
For a 110v 60Hz supply this will allow a current of about 4ua to flow.

A typical digital multimeter has an input resistance of 10M ohm on the voltage ranges thus a reading of 30v corresponds to a current of 3uA (3 millionths of an amp).

Nothing wrong with the meter.

Voltage tics work on a similar manner, they contain a CMOS integrated circuit which will detect currents down to pA (million millionths of an amp) this current can flow through the minute capacity between the probe tip and the live cable (must be AC), the circuit is completed through the capacity between the case and your hand (through the plastic) and hence to earth as above.
The power for the lamp comes from the internal battery.

Jim

[kc1980]

Voltage Pen Explanation:

I trust the explanation below from a Fluke employee.  

http://files.engineering.com/download.aspx?folder=6d07fb4f-6f63-42b9-9a32-86845dc18c8f&file=Prox_voltage_detector_operation.pdf

It's worth noting that an electric field will exist between the hot wire and any possible return path to the source of EMF.  For example, in an ideal capacitor, there is an electric field only between the two plates, but no where else.  Charged objects product electric fields (e.g. your body after exiting your car on a dry day), but only because they have a net charge; this phenomenon has nothing to do with how the pen works as net charge has nothing to do with the AC voltage on the hot wire.  

Also, a conductor carrying DC current generates a non-time-varying magnetic field, but not much of an electric field (if any at all).  A conductor carrying AC current generates a time-varying magnetic field AND a time-varying electric field (i.e. skin effect).  However, in both cases, there has to be current running through the wire, so there's no way that the pen depends on the electric field (that is, the pen wouldn't work on open wires with no current).

DMM Single Lead Explanation:

I'm not 100% sure about his one, but I'd imagine that the capacitance between the unconnected lead and ground/neutral is the culprit.  DMMs have a very high input impedance and low input capacitance relative to the unconnected lead (or your body), so most of the voltage probably ends up across the two leads (i.e. this is what the DMM detects and displays).  Try removing the unconnected lead from the DMM, and place the DMM as far as possible from any ground paths.  Does the detected voltage change?

[Regnirps]

However, in both cases, there has to be current running through the wire, so there's no way that the pen depends on the electric field (that is, the pen wouldn't work on open wires with no current).

Wouldn't there be a considerable electric field between an open AC line and everything else? The potential on the line is changing. Electrons are being pushed towards the end and pulled away from the end (not by very much of course). In essence the open line is one side of a capacitor and everything else is the other side. It doesn't require a complete circuit, just a small charge imbalance. The higher the frequency the easier to detect. I think that the way you put it, radio wouldn't work, though it is a different phenomena. I kind of recall that what I'm talking about is more like the fields that produce a Maxwell Induction Current.

[PetrosA]

I think the others covered the technical side. I haven't tried the GB tick tester, but I have tried Ideal, Amprobe, Greenlee, Fluke and one other (I don't remember the manufacturer - it was made in Sweden or Norway IIRC) and of those the Fluke is by far my favorite. I have two in case one gets lost.

One thing to keep in mind about the tick testers and most DMMs is that they will react to phantom voltage - IOW voltage resulting from capacitive coupling in wires. You need a solenoid tester or a LowZ meter (or you need to hook up a load inline with your DMM) to actually confirm voltage in that case. I've seen very old wiring in boxes light up the tick tester when in fact they've been disconnected for years but happen to run next to some other wiring somewhere for enough of a distance to charge up.

An interesting thing to try with a tick tester is to hold onto an insulated hot wire on the same phase/leg as the one you're testing with your other hand. The tick tester will not light or beep at that point for the hot wire, but it will light up for ground or a neutral. Sort of like reversing your polarity.

[Time]

Benzodiazepines?  What do anti-anxiety drugs have to do with anything?

[Mechatrommer]

Benzodiazepines?  What do anti-anxiety drugs have to do with anything?

maybe you should take one first before try to understand this electromagnetism.

[tyblu]

Benzodiazepines?  What do anti-anxiety drugs have to do with anything?

Believe it's a tongue-in-cheek way of insinuating the OP is seeing things and going crazy