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Old 11-04-2009, 12:55 AM   #1
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Getting small shock from household electronics cases (in Asia). Why?


Hi,

In my apartment, if I touch the bare metal case of certain electronics with a bare finger (in particular the back of a knuckle) while standing on the floor with bare feet, I get a tiny shock.

It is absolutely not static electricity. It is a never-ending potential difference between the floor of my apartment and the casing of various appliances. It doesn't feel like static felt, it doesn't make a sound, it only happens with electronics cases and not other metal in the house that isn't hooked up to an outlet. When I unplug the appliance, it ceases to give me the shock, which returns when I plug it back in. There is no carpet in the entire house. The humidity is constantly above 80% and the temperature never drops below 75 F. I have never experienced static electricity in this environment, but this "shock" will happen over and over again upon touching the appliance and never stop, while I am sitting down and not moving around.

This happens with the metal case of a DVD player, the bare metal of a TV antenna, the bare metal of a laptop case, and the bare metal of a capacitive touch volume control on a set of speakers.

I should emphasize that it is extremely unlikely that this is a problem with the appliances themselves, since they are of various types and from various global manufacturers. This is a problem with the wiring in my apartment.

I live in Asia, and we have 230V power with type F outlets.
Some of the electronics that give these tiny shocks are connected to the ground (earth) pins of the outlets, while some are not.

I did 2 tests with a multimeter:
1) I have bare feet on the floor. I hold one probe in one hand, and touch the other probe to the appliance case. Result: about 0.3V AC. If I take my feet off of the floor, the voltage drops to 0.

2) I have bare feet on the floor. I hold one probe in one hand, and the other probe in the other hand, and touch one of the probes (which is also touching my hand) to the appliance case. My hand is not directly touching the appliance case. Result: about 9-12V AC. If I take my feet off of the floor, the voltage drops to 0.3.

Any other tests that I should try?

Note: the appliance tested here has an external AC-to-DC power supply and thus AC power does not enter the body of it. The measured voltages exist with the AC plug plugged in both possible orientations (polarities). I don't understand why/how a DC device can have an AC voltage.

So what is causing this? Is it that the AC neutral wire in my apartment is at a different voltage than the earth? Or is it that the ground/earth wire in my apartment is not actually connected to the earth? Or could it have something to do with incorrect polarization (plugging the live AC wire into neutral), since type F outlets are not polarized?

As a related question, how does grounding work with external AC-to-DC power supplies? So for example if my laptop's power supply AC plug has a connected ground pin, but the output DC connector is just 2 pins (plus and minus), is one of these 2 DC pins connected to the metal case of the laptop? Which one? And is one of these 2 DC pins somehow connected to ground? How?

I think it's an international practice to wire neutral to a case (or any exposed conductor that is part of the circuit) if there is no ground (in the case of a 2-prong plug). So how does this work with an item that uses an external AC-to-DC adapter? Is the case/enclosure of such an item somehow designed to be wired to neutral?

I am just unclear on how enclosure-to-neutral or enclosure-to-ground works through an external AC-to-DC adapter.

Note: some appliances here do not have 3-pin (i.e. grounded) plugs, but some do

See a Type F connector: http://electricaloutlet.org/type-f

Thanks

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Old 11-04-2009, 09:53 AM   #2
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Getting small shock from household electronics cases (in Asia). Why?


The case should not be connected to neutral.

There is some electronic equipment (not U.S.A. UL approved) where the chassis is connected to one side of the line but should not have any exposed metal parts making contact with the chassis. It should have a "polarized" plug so the chassis is always on the neutral side. This equipment is safe if connected via an isolation or step-down or step-up transformer neither of whose secondary terminals is connected to the primary. Do a continuity check (set the multimeter to ohms) between each plug prong in turn and the metal case.

Also, phantom (induced) voltage can be present in the cases or chassis of electronic equipment just because live parts or wires are juxtaposed although not touching.

If you are standing with bare feet on a not so dry floor or a concrete floor, you are more likely to feel the effects as you describe.

It is a good idea to earth (to ground) the cases and/or chassis by stringing a bare wire (suggest 14 or 16 gauge) from one piece of equipment to the next, attaching with a screw that penetrates through to metal. Connect the far end of the wire to a known ground, which could be a water pipe if that is not plastic part of the way down to the basement and on out of the house.

It is possible that the ground pin on a receptacle is not really a good connection (this happens in the U.S. also) in which case running the bare wire as described above to a "better" ground connection will help.

Grounding (earthing) the case or chassis or enclosure with the separate bare wire works whether or not you are using an AC/DC adapter or a transformer.

About the equipment with the AC/DC adapter, do a continuity check between the power plug on one end and the power plug/socket at the other end to verify that there is no defect allowing AC to reach the equipment as evidenced by no continuity. Four tests: prong 1 to shell, prong 2 to shell, prong 1 to hole, prong 2 to hole. There could be a defect where only DC and not AC flows unless someone touches the case in which situation the bare ground wire could short out the adapter.

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Last edited by AllanJ; 11-04-2009 at 10:15 AM.
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Old 11-04-2009, 11:15 AM   #3
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Getting small shock from household electronics cases (in Asia). Why?


I tested 2 different devices for chassis-to-plug continuity as you suggested.

Device 1: Uses an AC-to-DC adapter where the AC plug does not have a grounding pin.

Result: No continuity with either of the AC plug pins and the chassis or any of the DC connection points.
-----------
Device 2: Uses an AC-to-DC adapter where the AC plug does have a grounding pin. In addition, the DC connector actually has 3 connection points, not the usual 2.

Result: There is continuity between one of the 3 DC connection points and the AC grounding pin only. This also has continuity with the device chassis. No other continuity exists between other combinations of pins.
-----------

Both devices yield the tiny shock when their cases are touched with bare feet on the ground.

Does that mean that this is induced/phantom voltage and not a scenario where the cases of the devices are actually hot AC? Can I somehow distinguish between the 2 scenarios using multimeter tests?

Is it typically possible to feel this phantom voltage with the body like this? From reading about it elsewhere, I got the idea it only showed up when using digital multimeters.

Is my situation caused by lack of a good ground? Does fixing the ground connection eliminate the phantom voltage?

I guess I am confused about whether having no real ground connection causes the phantom voltage, an actual hot AC leak that can carry current, or both. Although if there is no continuity between the cases of these devices and either of the AC current-carrying pins, then I don't see how it could be anything but the phantom voltage. (unless somehow my ground wire is actually connected to one of the AC hot/neutral pins, and in the case of Device 1, there is no ground pin)

The floor is made out of 1 foot square hard tiles. They are not wet or damp, but it is quite humid. I cannot tell what material they are.
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Old 11-04-2009, 12:43 PM   #4
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Getting small shock from household electronics cases (in Asia). Why?


From your descriptions of the power supplies of device 1 and device 2, they are both in good condition and not a shock hazard.

Yes, a good ground gets rid of phantom voltage by dissipating it (shorting it).

If the ground wire in a receptacle or other ground wire or element is not adequately grounded, it (itself) could have phantom voltage on it.

Normally, electrical systems with metal outlet boxes connected by metal conduit are considered to be properly grounded. Except we cannot rule out oxidation of the joints for example where conduits are attached to the boxes resulting in less than perfect grounding. In the U.S. nonmetallic cables such as Romex are supposed to have all ground wires fastened together in outlet boxes but in years past, the ground wires were often twisted together and once in awhile became loose or oxidized. Nowadays they must have a ring crimped around them or be wire nutted together.
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Last edited by AllanJ; 11-04-2009 at 12:56 PM.
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Old 11-04-2009, 12:53 PM   #5
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Getting small shock from household electronics cases (in Asia). Why?


I built a Heathkit which had a resistor/capacitor network connecting one side of the line to the metal chassis. I called them and they explained the current flow wasn't dangerous.

Possibly most people have a skin resistance, wet or dry, of between 3000 and 300 ohms. If you can get more than 20 mA to flow through resistors of these values from your appliances, you have a problem.

Last edited by Yoyizit; 11-04-2009 at 07:40 PM.
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Old 11-04-2009, 06:24 PM   #6
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Getting small shock from household electronics cases (in Asia). Why?


If you connect resistors and/or capacitors between one side of the line and the chassis and the plug happens to be put into the wall receptacle so that the side in question is hot, then you will get a measurable voltage on the chassis unless the chassis is grounded.

What voltage you can measure depends on the number of amperes or milliamperes drawn by the load which could be a voltmeter itself or your body from finger to bare foot. One of today's digital voltmeters may draw so little current that the voltage measured is close to line voltage.

While the number of milliamperes you can draw may be not enough to hurt you, you may still be able to feel it.
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Old 11-05-2009, 02:18 AM   #7
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by Yoyizit View Post
I built a Heathkit which had a resistor/capacitor network connecting one side of the line to the metal chassis. I called them and they explained the current flow wasn't dangerous.
What line? Who did you call? And what current flow?
I guess a Heathkit is some sort of educational build-it-yourself kit where you build some sort of electrical device. Are you saying that you once built a Heathkit that connected one of the 2 AC pins of a wall outlet to the metal chassis of the kit itself through some resistors and capacitors? And then you detected current flow between the chassis and earth ground? And you called up the Heathkit company and they said it wasn't dangerous? Is this a similar situation to the one I am experiencing? It seems as if your situation involved one of the 2 AC wires being connected to a chassis, where my situation does not.

In fact, as AllanJ said, it seems as if your situation could have in fact been dangerous if it was the hot wire and your resistors/capacitors weren't sufficient to reduce the potential.

Quote:
Originally Posted by AllanJ View Post
What voltage you can measure depends on the number of amperes or milliamperes drawn by the load which could be a voltmeter itself or your body from finger to bare foot. One of today's digital voltmeters may draw so little current that the voltage measured is close to line voltage.

While the number of milliamperes you can draw may be not enough to hurt you, you may still be able to feel it.
AllanJ, is this in response to my situation or Yoyizit's?
Because they seem quite different, mine involving phantom voltage and Yoyizit's involving "normal" voltage.
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Old 11-05-2009, 08:27 AM   #8
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Getting small shock from household electronics cases (in Asia). Why?


>> see below

Quote:
Originally Posted by scootley View Post
What line?

>>one side of the AC line

Who did you call?

>>Heathkit

And what current flow?

>>Since I felt it but it was not painful, I'd say between 1 and 10 mA AC.

I guess a Heathkit is some sort of educational build-it-yourself kit where you build some sort of electrical device. Are you saying that you once built a Heathkit that connected one of the 2 AC pins of a wall outlet to the metal chassis of the kit itself through some resistors and capacitors?

>>Yes.

And then you detected current flow between the chassis and earth ground?

>>Yes, by getting a shock

And you called up the Heathkit company and they said it wasn't dangerous?

>>Yes.


Is this a similar situation to the one I am experiencing?

>>I guess it could be

It seems as if your situation involved one of the 2 AC wires being connected to a chassis, where my situation does not.

In fact, as AllanJ said, it seems as if your situation could have in fact been dangerous if it was the hot wire and your resistors/capacitors weren't sufficient to reduce the potential.

>>I imagine this circuit was a compromise between safety and some desired circuit performance.

AllanJ, is this in response to my situation or Yoyizit's?
Because they seem quite different, mine involving phantom voltage and Yoyizit's involving "normal" voltage.
As I recall it was a Heathkit AR-109 AM/FM receiver, from the 60s. I gave it [the receiver, that is ] to a girlfriend from the 70s.
If you can find the schematic online you can see the RC network and its connection for yourself.

Last edited by Yoyizit; 11-05-2009 at 08:33 AM.
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Old 11-05-2009, 11:20 AM   #9
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Getting small shock from household electronics cases (in Asia). Why?


>>> ... AllanJ, is this in response to my situation or Yoyizit's?
Because they seem quite different, mine involving phantom voltage and Yoyizit's involving "normal" voltage ...

Both. When electricians refer to phantom voltage that means voltage induced in a supposedly isolated conductor by a live conductor juxtaposed with but not touching it. The effect is the same as connecting a capacitor between the two conductors.

If the capacitor is big enough, a very large amount of current can flow if/when a load is connected between the "isolated" conductor and ground (for example someone touches both at the same time).

A "direct connection" applying normal voltage is achieved if a resistor (or just a plain wire) is connected. If the resistor has a high enough value the amount of current that can get across is too small to harm a person.

The amount of (AC) current that can flow through a capacitor of a given value is less when the frequency is lower. (The "resistance" more correctly impedance of a capacitor is less the higher the frequency is.) Capacitors do not pass DC. The amount of current a resistor can pass is not dependent on frequency.

(Actual for resistors, oversimplified but conceptually correct for capacitors) The amount of current that flows is equal to the voltage on the live conductor divided by the total resistance in ohms (including that of a person's body) between the live conductor and ground.

In the situation at hand, there are two "resistances", First is the capacitance between the juxtaposed conductors allowing the phantom voltage to appear OR the resistor/capacitor grouping in the Heathkit. Second is the person touching the case or chassis of the piece of equipment OR the voltmeter used to conduct tests. If the First resistance is large compared with the Second, then the voltage measured at the chassis (to ground) will be a small fraction of the hot conductor voltage. More precisely (requires calculus and complex numbers to compute and express) the voltage measured at the chassis (to ground) is equal to the Second resistance divided by the sum of the First and Second resistances.
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Old 11-05-2009, 11:32 AM   #10
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Getting small shock from household electronics cases (in Asia). Why?


I see. So is there a definitive test that I can do to verify that my situation is in fact a phantom/induced voltage?
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Old 11-05-2009, 12:37 PM   #11
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by scootley View Post
I see. So is there a definitive test that I can do to verify that my situation is in fact a phantom/induced voltage?
Real voltages have low source impedances, so they act like Voltage Sources.
Phantoms have high source impedances, so they act like Current Sources.
Besides phantom voltages, the only current sources commonly used outside laboratories are fluorescent lamp ballasts.

For household wiring, load the voltage with a 10A load; if the voltage drops 5% or so it's a voltage source.

Last edited by Yoyizit; 11-05-2009 at 12:40 PM.
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Old 11-05-2009, 02:41 PM   #12
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by Yoyizit View Post
For household wiring, load the voltage with a 10A load; if the voltage drops 5% or so it's a voltage source.
Can you elaborate in more of a plain English and in a way that is specific to my situation, since I am not very experienced with this?

Are you suggesting that I should take something like, say, a light bulb that I think would draw 10A from this voltage (which is let's say 9V AC)? So that is I guess a 90W bulb?

So I connect one terminal on the bulb to the metal chassis of an appliance in question, and what do I do with the other terminal? Hold it in my hand? (Since the voltage of 9V was measured with a multimeter probe in my hand)

Then measure the voltage drop across the bulb? So put one multimeter probe on each of the 2 bulb terminals? Or take 2 measurements, each between a bulb terminal and my hand?

And you say it drops 5% if it's a voltage source. What does it do if it is a phantom? Not drop at all?

Are you suggesting that a 5% drop vs a 0% drop is "definitive" when one of the voltage reference points is the human body? That does not seem definitive to me.
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Old 11-05-2009, 03:39 PM   #13
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by scootley View Post
I see. So is there a definitive test that I can do to verify that my situation is in fact a phantom/induced voltage?
There is no intrinsic difference (other than number of watts you can draw) between a phantom/induced voltage and a directly applied voltage.

This is what I would consider a definitive test. Take the external ground wire I was suggesting, connect it to a good known ground first, and see if you can light a small wattage incandescent light such as a 7 watt night light using the chassis or case of the electronic equipment as one terminal and the ground wire as the other terminal. With the light connected, measure the voltage. If the voltage did not go to zero then you should investigate to be sure you don't have an actual defect or bad connection making the chassis live.

Now you could simply ground the chassis with the aforementioned ground wire and not worry about it any more. But if there was measureable current flow, that would show up in your electric bill.
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Old 11-05-2009, 04:16 PM   #14
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by scootley View Post
Can you elaborate in more of a plain English and in a way that is specific to my situation, since I am not very experienced with this?
Procedures can be in plain English but understanding (which seems to be what you also want) means using technical terms which are defined with other technical terms, and so on back down to very basic terms.
What also makes elec. hard to understand is one can't see it, unlike a machine that has moving parts that one
can see how inputs lead to an output.

Try Wikipedia for the definition of Voltage and Current Sources. This is the most basic difference between phantom and real voltages. But, even defining where a real-world approximation of a voltage source [e.g., a car battery] leaves off and a current source [e.g., a fluorescent ballast] picks up is kind of elusive.

I'd also get some resistors if you want to experiment with this. Using the human body as a circuit element to learn about elec. will make anybody crazy and can be dangerous. The resistance is unpredictable except within a wide range, and there is some capacitance, which can affect AC measurements.

Incand. bulbs are resistors but their value varies with the current through them in a non-linear fashion, so this is also confusing.
Non-electronic hair dryers and toasters, and heating elements for water heaters and ovens and cooktops act like fixed-value, linear resistors, but they have low values of resistance.

Last edited by Yoyizit; 11-05-2009 at 04:23 PM.
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Old 11-11-2009, 06:29 AM   #15
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Getting small shock from household electronics cases (in Asia). Why?


Quote:
Originally Posted by AllanJ View Post
This is what I would consider a definitive test. Take the external ground wire I was suggesting, connect it to a good known ground first, and see if you can light a small wattage incandescent light such as a 7 watt night light using the chassis or case of the electronic equipment as one terminal and the ground wire as the other terminal. With the light connected, measure the voltage. If the voltage did not go to zero then you should investigate to be sure you don't have an actual defect or bad connection making the chassis live.
I connected one end of a wire to a water supply pipe. I connected the other end to one terminal of a small light bulb rated that said "12V 5W" on it.

I connected the other terminal of this bulb to a known appliance chassis that produced the "tiny shock" in question.

The light does not illuminate.

With the light connected, I measured the AC voltage between the 2 terminals of the light, which reads 0V.

Is this the measurement you were talking about?

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