Ground Loop on PC USB

Hi Folks,
I wonder if someone can offer some advice on this as I have already broken one motherboard.
I have a serious problem with many of my USB devices and ground loop.
Touching the ground on the USB cable (which is plugged into a hard drive, hub, printer or any number of things) and the computer chassis gives a nice tickle. If you touch the USB plug against the chassis, there is a little spark as well.
I measured it last night and get 113V (220 is supply). I have already lost a pc as I plugged it in, it sparked and the pc went silent :(
I have swapped the pc PSU with two others, removed various devices etc, it all stays the same. It seems to be most, but not all, of the USB peripherals which have this problem. All of the peripherals use a switchmode psu (non-earthed).
My question is, how can I fix this? What is causing it? The switchmode psus?
Many thanks
Cheers, Crispin
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Crispin wrote:

That's why most manufacturers recommend that you plug all the cables together, before plugging in the AC. Put a 10 K ohm resistor across your meter, and read it again. It won't be anywhere near 113 volts.

Capactive coupling from the power line to the output side of the power supply It is normal. To get rid of it, you would need a good isolation transformer for each AC adapter, or run everything directly from batteries.
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It sounds like a possibility is a high-impedance leak between the "hot" side of your power and the chassis of one of your devices (not necessarily the PC). First, don't let the high impedance of the meter itself fool you, measure the voltage again with the meter paralleled by a 10K resistor (exact value not important). Is the voltage still close to 113V?
Do you have a real ground available? If so, measure the voltages between that ground and the supposedly grounded portions of all devices, after disconnecting them from each other and plugging them in and switching them on. Parallel the meter with the resistor. Perhaps this will find the culprit.
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That could be what's known as a "phantom voltage", meaning that if one end of a wire is not connected to anything, it picks up (by induction) stray voltages. Try putting a wet finger or better, a 10k or so resistor, or any light bulb of the filament type across the meter leads (I use flashlight bulbs) and measure it. If the voltage goes away, or the bulb doesn't blow, or doesn't light, then it's a phantom voltage and more akin to static voltage than anything else. If it does not go away, then there is a serious problem somewhere that may be dangerous and needs to be tracked down, regardless of anything else.

The above is puzzling. Looking within the PC system & architecture, there needs to be a common earthing. In other words, any exposed metal of any component should show continuity between them. Only a Class II (NA term) would not have this requirement. So, the earth (not neutral) pin of any cable or connector should be zero ohms measured to any other such connector, plug or cable of chassis.
You don't give your location, but in Europe, UK, Australia, NZ and parts of the Pac Rim, desktop computers are earthed while laptops may be ClassII but will still be earthed within themselves. Your switchmode supplies are also earthed if they are to support more than 2.5A or wahtever the number is. The earth connections from all connectors inthe PC should also be continuous to the wall outlet.
Is the above all true? If not, and it's legally wired, please explain so I can get a better head around this?

No, the switchmode psu's won't be the cause of that; those are pretty much standardized designs and as long as they carry a Safety Approval or rating label/certificate, they are fine. If they are not marked as such then all bets are off and they are going to be of an unknown/illegal design most likely.
My guess at the moment is that you have some devices with 2-pin polarized plugs that are reversed, or the wiring within the outlet may be reversed (hot-neutral relationship). IF it's possible, you might try reversing the plugs one at a time and see if the sparking problem goes away (assuming the first part above has been settled out). Or a component (wall wart, whatever) is miswired. Or a component is breaking down and has leakages above what's allowed to exist. You say you get a "tingle": It's obvious then that the voltage isn't actually 113Vac or it'd knock you on your butt! You wouldn't describe it as a "tingle", but a full blown shock. So, that's probably turned out to be a phantom voltage that goes away if any load is applied for it.
Try disconnecting all the USB devices and make sure there isn't any spark. If there isn't, then plug the rest of them in, one at a time, looking for one that causes the ark to occur - I'm betting there will only be one at the moment. That's the one that is miswired or needs replacing. If none are detected, then repeat at the other various USB connectors; it could be one of those, also.
HTH, I know it's a bit wordy plus I'm in a hurry and need to get somewhere, but that's my 2 for now.<g>
Twayne

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Non-earthed switchmode power supplies have some leakage from mains side to the low voltage side through RFI filtering capacitors between high voltage and low voltage sides. This causes the leakage that gives you that voltage you measured. The voltage you see is normally around half mains voltage when measured with high impedance multimeter (10 megaohms normally), but the voltage will quicly drop if you measure the voltage with lower imepedance meter or over some resistor because the leakage current is limited to be to a low value (considerably less than 1 mA) for safety reasons.
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