That Tingling Feeling!

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This morning I am decorating the bedroom and in order to do so I have to take down the shelf holding LCD TV, VCR and DVD player. As I was unplugging everything I noticed whilst holding the aerial lead (metal part of the plug) I brushed the inside of my forearm against the DVD player case (metal) I got a tingling sensation, it didn't hurt but just a sensation. I have checked the earth connection in the DVD player 3 pin plug and it appears OK. I tried the 'setup' in other sockets with the same result. I haven't taken the cover off the DVD player to investigate if the cover is grounded inside yet. Anybody got any ideas where or what the problem could be. We have other metal cased items in the house and there doesn't appear to be a problem on those so I think it is unlikely to be in the mains wiring but then again I am not a Sparkie!
TIA
John
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Lots of devices use high value resistors between line and ground for RF suppression. These are of such a value to limit any current to a safe figure. But if the conditions are right - like no ground connection at the time - you can get a tingle. It's quite normal.
--
*Why are a wise man and a wise guy opposites? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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This puzzles me also.
We were given a Panasonic VCR last week. It has a metal cover (fixing screws touchable), lots of 'earthed' screens inside, all connected together. But the mains lead to the VCR is only 2-wire?
Ok, so it might earthed via the coax cable to the TV.
But the mains lead to the TV is also only 2-wire?
There's no chance that it could be earthed by the aerial.
So I seem to have mains-powered boxes with metal bits that are touchable, with none of those metal bits safety-earthed??
--
Tony Williams.

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Tony Williams wrote:

this is common.
NT
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Well, yes, we seem to all agree on that. However, it does beg the question*why* is it normal.
We don't accept minor tingles from anything else, and accept it as 'normal'. Why should we accept it in this case?
Recently, we criticised a householder who electrocuted himself by coming between between a live kitchen shelf who's mounting screws had punctured a wire in the wall, and a nearby earthed appliance. He was criticised for not realising something was up when they'd been getting a tingle from the shelf previously.
I don't know what 'class2' 'double insuated' actually means, ( Yes, I know I could google it... ) but in this case, it doesn't appear to amount to much if there are exposed metal parts which routinely deliver a shock!
Just seems fundamentally wrong to me.
All it would take is single failure of a filter cap for the partially-exposed chassis to become properly live, and with no earth, over-current protection would not trip.
--
Ron




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Ron Lowe wrote:

A class2 device is unlikely to give you a tingle unless you can feel some form of inductive coupling to the casework...
With a normally earthed bit of eletronic kit then you can get a tingle if you have a relaively high earth loop impedance (e.g. as would be typical in houses with TT (local earth rod) earthing). The RFI filter on the input can then in effect hold the casework at half mains voltage (although with a very high impedance).

I would expect with a double isolated device the casework would not be connected to the RFI filter.
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Cheers,

John.

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Or touch the aerial inlet connector, as discussed in this thread...

I can see how that could happen. However, for the purpose of this thread, I'd like to focus on what's acceptable practice in the design of a double-insulated appliance with a 2-core mains lead, and why it's acceptable. Here, someone made the actual choice to design the high ( infinite! ) earth impedance, with the consequences.

I'd expect that too.
But how about the shell of connectors which are externally accessible? Is it regarded as acceptable for those to give you a tingle? If so, why is it acceptable?
Getting shocks off other kit is not normally regarded as acceptable. Is it OK for an electric shower to give you a little tingle, so long as it's just the pipe outlet and nothing more, and the tingle is high-impedance so not many mA flows?
Is there a rule about how much externally-touchable surface is OK to be giving shocks? Or how big those shocks are allowed to be?
It just seems wrong to me that *any* device should be, by design, giving *any* shocks, however mild.
--
Ron




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Any shock can make you jump and hurt something as a result. I found 56 volts on my coax (digital meter)
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Ron Lowe wrote:

it means metal cased equipment that is to a fair extent single insulated.

you havent had a shock. A tingle is not a shock.

its acceptable to have single insulation, mark it as double insulated, and its ok if you get a slight bite off the aerial.

why not? Why would something be prevented if it isnt causing a problem?

yes, the consequences that it cost a little less.

true. But not the point. The point is not what runs through someones imagination, but what actually happens in the real world. The reality is that the safety record is excellent.

double isolated? I dont think so.

So you expected wrongly. Thats all.

why not?

no. A shower needs secure earthing not to be dangerous, so tingling isnt an option.

There are insulation requirements for both class I and class II goods. Whether theyre always followed is open to debate I suppose.

there you go assuming again. I'd agree its not best practice, as its long been known that touching an aerial that bites when youre standing on a roof, no matter how slight the shock, can be enough for a person to fall off and die.

cos its not killing people, as far as we know, and its cheaper.

why not?

quite, if a metal shelf gives you electricity its fairly obvious somethings wrong.

you mean its not the way you'd assumed. Every aspect of life is like that. Single insulated goods are called double insulated to inspire public confidence in safety, and to distinguish them from goods with none of the requirements of so-called double insulation. Theres nowt new about political spin and public miseducation.
NT
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As others have said - it's normal.
It's caused by one of the devices ( TV / Digibox / VCR etc ) connected to the antenna circuit using only a 2-wire mains lead and also having internal filters which by design cause small amounts of leakage current to flow to their internal chassis. Since the internal chassis is not grounded ( 2-wire mains lead ), the chassis floats at around half mains voltage. This slightly-live internal chassis is not exposed anywhere except the aerial inlet, and this is is where you get the tingle from.
You get the tingle when you touch both the slightly-live chassis, and something with a true ground, at the same time.
Also, this 'floating' voltage will be back-fed from the offending appliance to your RF distribution, meaning that quite possibly all the aerial outlets in your house will also have this voltage present on them. The first many people know about this is when they get a tuner card for their PC, and as they plug the aerial in, they feel the tingle between the aerial wire and the PC case ( which is grounded via a 3-pin plug. ) They often assume the PC is faulty and 'live', but usually this is not the case. The PC is correctly earthed, it is the aerial cable screen that it 'live'.
I know someone who has run a short earth wire between the screen connection in the back of the UHF wall-plate to the earth lug in an adjacent 13A outlet, basically earthing these floating chassis devices via their aerial wires. This was intended to allow this leakage to bleed away, since he thought this may be causing issues with the tuner card in his PC. I don't know whether he has had any issues with ths or not.
--
Ron





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Thanks for all the input guys. I'll leave it then and just make sure I don't suck the aerial lead whilst putting a disc in the DVD!
Cheers
John
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We've got a Panasonic TV that does this and as said consequently 'livened' up the RF distribution system. At the time I thought that it was possible that line and neutral had been swapped in the TV and I put it on an extension lead with the supplies reversed, and the leakage went away.
I meant to chase it up with the manufacturers but that never happened and when some time later and the warranty was over, and I was fitting a digibox, I took the back off and changed the mains connector to the pcb around.
Rob
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robgraham wrote:

Remember that with many countries electrical systems their sockets do not preserve polarisation. So the devices are designed to not be phased (pun intended) by reversal of the supply. This is also why they normally insert a capacitive filter to earth from both phase and neutral.

It may suggest on your set there is only a filter between one leg of the supply and earth.
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Cheers,

John.

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Hi all seems no one has picked up on the fact that john held the aerial lead if it was still attached to the tv then there is your answer . The tv sends a small radio carrier out through the aerial in order to better capture the radio signal ,if its sat' ie sky then the LNB power is sent via the cable. As any radio guy will tell you you can get a heck of a belt of an aerial . This also manifests itself in aluminium coax plugs as the plug itself greying and becoming very flakey also if dark sometimes a small spark can be seen when fitting/removing. None of this is dangerous in normal cases and as with any electrical appliance always unplug before moving .
HTH CJ
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Well, sort of. Actually, the local oscillator signal leaks out of the aerial - it's not designed to do so, but it's not worth designing it out in cheapo domestic equipment like TVs.
Look up "superhet receivers" as to why.
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Plus the SWR is never 1-1 and s'hets & hetrodines were killed by fet and lsi'
Techno-babble is great but I thought this was a laymans thread.
CJ
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cj wrote:

Laymans should be warned that the statement
"The tv sends a small radio carrier out through the aerial in order to better capture the radio signal"
sounds a bit like BS
-- Adrian C
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I have this image of a carrier signal modulated with small hooks - a bit like Velcro...
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unplugging
plug)
got
inside
Enjoyed this thread immensely - but not understood a chuffing word :-) I'll get me coat!
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Basically it means you buy an invention of the Devil called a PC with a TV card in it. You extend the aerial to go in the PC and while holding the aerial, you touch the PC case and get a shock.
The immediate reaction is OHFUCKIAMGOINGTODIEANDELECTRICIANSARECHRISTMASCANCELLED.
So you do the next best thing and drag it in here. :))
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