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Old 1st February 2008, 11:21 PM   #1
bob_v5 is offline bob_v5  England
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Default Random electrocution?

"This is reinforced by you saying that there are missing connections inside the player and you get tingles from certain parts of the insides."
This made me think. When mucking about with a/v stuff, (dvd players etc) I often get electric shocks when swapping interconects between differnt devices when they are plugged in and running. (maybe not a wise thing to do?) I dont belive it is static, as it lasts for as long as I leave my finger on whatever is zapping me. Anyway, I was wondering if anybody could tell me why this happens. As far as I understand it, there is just not enough power flowing in an interconnect to hurt me. This seems to happen a lot, even on gear at other people houses that I have never taken apart! I read somewhere that it is something to do with switch mode supplys. I dont want to think about what would happen if I had a pacemaker!
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Old 2nd February 2008, 12:43 AM   #2
gareth is offline gareth  Wales
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Default Re: Random electrocution?



I think it could be either bad ground problems on your interconnects, wherein you are becoming part of the circuit. This is perhaps unlikely when there is no more than 2volt and some milliamps running through them. Having said that only 30mA is enough, under the right conditions, to kill. (BS7671 wiring regulations).

Another possibilty is that a static charge is being built up on the cables perhaps through stray capacitance or something . There are other things that can cause this aswell.

You say that this happens when you touch other peoples equipment too. Do you wear lots of man-made clothing? Or walk on many man-made objects? This may sound silly but all man-made objects have the ability to produce static charge.

Humans tend to be negatively charged and thus we tend to attract electricity, we are also made of carbon and contain lots of water. All of us have varying resistance to electostatic discharge and electrocution. If you can get your hands on a good Fluke multimeter you can measure resistance over different areas of your body and compare readings.

I hope this kind of answers your question.
Gareth

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Old 2nd February 2008, 12:49 AM   #3
OzMikeH is offline OzMikeH  Australia
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If you check with your multimeter you'll see it's half of your mains voltage. It's only a couple of mA to ground but it's very unpleasant when you're on a wet concrete floor with bare feet. Forcing an involuntary breath out through clenched teeth kind of unpleasant.

It's something to do with a slightly dodgy mains filter in double insulated appliances. (no earth pin)

I have an LG DVD/VCR combo in a spare room that livens the whole antenna distribution up in my house. You could see the sparks when you plugged the antenna into a grounded device.

I connected the outer of the coax to the building frame and it stopped the shocks and tingles unless I'm messing with the DVD player and it's antenna is not connected.
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Old 2nd February 2008, 12:53 AM   #4
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There is capacitance between pretty much any two things in the universe. That includes your transformer windings, and the rest of the chassis. And you. If left to its own devices, the chassis will assume some voltage, depending on the capacitive and resistive leakage from the transformer windings, and the difference is what you feel if you lightly run your finger across the chassis. This assumes no safety ground. Once you tie the chassis to ground, there should be no stray voltage on it. So, do all your components have a safety ground? Didn't think so. FWIW, the chassis of components having 2-wire plugs is often at 40-60 VAC, until it gets connected to something to bleed this off. There isn't much current available, but it's not a good situation. It used to be common to reverse the plugs of each piece of equipment, measuring to ground, to find the orientation with the least chassis voltage. The assumption was that this would give minimal current flow in the cable shields, and the best hum performance. I've also seen surprisingly high voltages on cable TV shields that weren't properly grounded. IMO, there are some issues between getting the absolute best hum performance from audio equipment, vs. having everything properly safety grounded. Generally it's not a big problem unless you have very high gain, or components plugged into distant outlets.
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Old 2nd February 2008, 12:59 AM   #5
gareth is offline gareth  Wales
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Quote:
Originally posted by OzMikeH
If you check with your multimeter you'll see it's half of your mains voltage. It's only a couple of mA to ground but it's very unpleasant when you're on a wet concrete floor with bare feet. Forcing an involuntary breath out through clenched teeth kind of unpleasant.

It's something to do with a slightly dodgy mains filter in double insulated appliances. (no earth pin)

I have an LG DVD/VCR combo in a spare room that livens the whole antenna distribution up in my house. You could see the sparks when you plugged the antenna into a grounded device.

I connected the outer of the coax to the building frame and it stopped the shocks and tingles unless I'm messing with the DVD player and it's antenna is not connected.

If you are seeing sparks when you plug your antennae in then this is more likely to be some kind of inductance, possibly from the transformer. In theory there should be nothing of the kind though as double insulation takes the circuit earth from the centre tap of the secondary side of the transformer. The centre tap in theory being 0 volts.

This may suggest some sort of fault in the power supply area of your LG combi and is probably worth looking at.

Try measuring your antennae cable with a good meter. You should be in the region of 12 volts and with close to 75 ohm provided the cable is properly terminated.

Thanks
Gareth
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Old 2nd February 2008, 01:24 AM   #6
OzMikeH is offline OzMikeH  Australia
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I was talking about voltage between cable screen and protective earth. There's a thread talking about

DC resistance will most likely be anything but 75 ohms, but that is another discussion.

About multimeters: to Misquote Croc Dundee.
"That's not a meter........
(Whips out a HP974a and HP8920)
"THATS a meter."
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Old 2nd February 2008, 01:31 AM   #7
gareth is offline gareth  Wales
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This is co-axial cable ? Why don't you try to measure any voltage/current flowing in the outer braid then? Surely you can work back to where the fault lies ?

Have you tried new cable to connect your combi to your splitter. I would have thought you have. It could be a breakdown of the inner insulation. (25kV rating though).

As to your choice of multimeters, hmmm....not expensive enough.

Gareth
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Old 2nd February 2008, 05:41 AM   #8
Eva is offline Eva  Spain
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Please stop non-sense.

Every off-line switching mode power supply has at least one small capacitor (100pF to 1nF, safety rated) connected between primary side ground, which is mains live, and secondary side ground, which is signal ground too in A/V stuff.

This capacitor serves EMI filtering purposes and allows some AC current to leak from mains line to signal ground in the process. This capacitor is almost mandatory, EMI would be too high without it, except with some specially designed low-power SMPS transformers.

Furthermore, bigger switching mode power supplies witn a safety earh connection also include two more EMI filtering capacitors that are similar to the previously mentioned one but of a slightly higher value. They are connected from line to earth and from neutral to earth. Since safety earth is usually connected to the metal case of appliances and to secondary side ground (also signal ground in A/V stuff), if the safety earth wire in the plug is left unconnected, this will cause half of mains potential to appear at signal ground and the metal case.

Fortunately, the AC current leaking through these capacitors at mains frequencies isn't high enough to do you any harm. This is unless the signal grounds of several pieces of equipment without safety earth are connected together, because leakage current is cummulative and it may become high enough to shock you.

Power supplies employing conventional transformers are not free from mains leakage either. All transformers will exhibit some leakage capacitance (cummulative too) and some appliances will still include EMI filtering capacitors from line and neutral to safety earth.
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Old 2nd February 2008, 06:01 AM   #9
Nordic is offline Nordic  South Africa
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I am so sick and tired of the satelite receiver and dvd player shocking me..........!!!!!!!!!!!!!!!!

Its not the shocking that bothers me.... well actualy it does.... but the nicking of knuckles against nearby appliances and shelves from the reflex.
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Old 2nd February 2008, 06:05 AM   #10
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I found that I was getting a mild shock from the shield of the coax from the roof top antenna, it was picking up AC - a wire from the grounding lug on the splitter to a convenient copper pipe fixed it.
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