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Old 04-09-2006, 07:45 AM   #1
Per Hansson
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Default Ungrounded TV set electrucuting me through Composite RCA cable

Hi, I've got a TV in another room, connected to a satellite receiver, a Dreambox, and the Dreambox via toslink (optical) to a receiver... The Dreambox and the TV is not grounded but the receiver is...

From this other room I have run a 5m RCA cable from my Dreambox second SCART connector to my computer so I can view the stuff I have on my TV in an easy way, the TV is connected to the Dreambox via SCART too...

If I hold the RCA cable in my hand and touch my properly grounded PC's chassi I get a very unconfortable chock from it... I measured with a DMM; only 170mV but the shock is real anyway...

So, I was not overly annoyed by this untill today, I was poking around with the DMM inside my 3com switch and managed to short it's mosfet, nothing bad happened, it's red failure led lit up so I removed power from it and now it works just fine...

But when I held the switch in my hand, which is connected with a LAN cable to the Dreambox I got a shock when touching the cover of my PC, which at the time was turned off and not grounded... Took me a while to figure out what was happening LOL

I.e. the floating ground in my TV or Dreambox or both was trying to level out with the ground in my switch that can find a ground through it's lancable that goes down into the basement to a much bigger 24port proplely grounded switch...

Ack, that got real lengthy

Anyway, can I get rid of this issue by grounding my TV? And if so where can I connect a ground cable safley inside it?
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Old 04-09-2006, 07:49 AM   #2
tiresias
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

Quote:
Originally Posted by Per Hansson
If I hold the RCA cable in my hand and touch my properly grounded PC's chassi I get a very unconfortable chock from it... I measured with a DMM; only 170mV but the shock is real anyway...
It's going to be much more than 170mV if you feel a zap, I can assure you of that!

I have the same problem with the RCA cables on our TV myself - I'll be looking into it and seeing what I can make of it. Many sockets at our place don't have ground wires... so I'll either have to route them through the house, or execute a "ghetto" solution, like I have done with my own 'workstation'.

Last edited by tiresias; 04-09-2006 at 07:52 AM..
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Old 04-09-2006, 09:27 AM   #3
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

The tv chassie might be grounded by the coax shield, you could brake up both inner conductor and the shield and put two caps in series with the inner conductor and shield, could be worth to try? Remember some years ago when I was mounting a tv antenna, while sitting on the metal roof I was touching the coax shield that was connected to the tv-set inside the house I got hit by a pain that felt like I was stung by a wasp on my stern :
Guess that the roof was acting like some kind of virtual ground.
Well, luckily todays tv-sets got their psu primary side separated from the secondary side.
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Old 04-09-2006, 02:54 PM   #4
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

Was your DMM set for DC or AC? I wouldn't be surprised if you were getting bit by 120VAC. I know I've gotten some "tickles" while holding onto a coax connector and accidentally touching my arm to a VCR's chassis.
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Old 04-09-2006, 08:31 PM   #5
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

There is leakage (in the range of microamps) from the Y-class capacitors between Live and Neutral and the chassis in ungrounded equipment. But this amount of current is not dangerous, although it can sting.

You should connect a wire to the chassis of a plugged-in computer to the antenna cable shield before going on the roof.
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Old 04-10-2006, 01:12 PM   #6
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

Petes, 170mV DC and ca 2.9v AC...

But it feels like the 100VAC range, almost as bad as touching 230VAC netral to live with your fingers...
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Old 04-10-2006, 02:10 PM   #7
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

Quote:
Originally Posted by Per Hansson
Petes, 170mV DC and ca 2.9v AC...

But it feels like the 100VAC range, almost as bad as touching 230VAC netral to live with your fingers...
It may well be in the range of 70V or so, Per. If I connect certain power supplies to an unearthed socket, for example - I can notice a distinctive 'tingle' if I directly touch the casing and an earth, even with dry fingers. My multimeter in the same situation will measure virtually nothing at all on the Vdc and Vac settings, and a minimal (sub-mA range) current when shorted to earth.

I recall having spoken last year or so to a colleague of mine in the UK, who mentioned he had once measured the voltage between such a coax cable from a TV and earth with an oscilloscope - and found a square-wave AC-like potential of about 80V.

Quote:
Originally Posted by Per Hansson
almost as bad as touching 230VAC netral to live with your fingers
The 230Vac gives quite a zot! Not an experience I'd intentionally repeat. AC at such low frequency 50-60Hz is also considerably more lethal than high-frequency AC or full DC, even if the latter two were 'experienced' at a higher (within reason) current. It seems that the low-frequency AC is unusually prone to disturbing the sinoatrial node, and sending the heart into VF or some other lethal rhythm...

(Nothing to do with your TV, just an offtopic remark)

Last edited by tiresias; 04-10-2006 at 02:19 PM..
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Old 04-11-2006, 02:01 PM   #8
Per Hansson
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

tiresias; yea I know, 230VAC is supposed to be bad... I don't mind the shock it gives so much though... Had a few through my past carelessness...

Getting zapped by 400VAC in a breaker box when you are standing in a pool of water and it is raining is NOT somthing I'd recommend though, I know that too from personal experience :P
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Old 04-11-2006, 02:33 PM   #9
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Default Re: Ungrounded TV set electrucuting me through Composite RCA cable

Quote:
Originally Posted by Per Hansson
tiresias; yea I know, 230VAC is supposed to be bad... I don't mind the shock it gives so much though... Had a few through my past carelessness...
That's true - it doesn't always feel particularly strong - although according to most sources 50Hz AC is as lethal as a DC shock of over five times the current!

Quote:
Originally Posted by Per Hansson
Getting zapped by 400VAC in a breaker box when you are standing in a pool of water and it is raining is NOT somthing I'd recommend though, I know that too from personal experience :P
400Vac - between two phases I take it? That's quite a trip.

Last edited by tiresias; 04-11-2006 at 02:36 PM..
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