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TV socket charge up



 
 
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  #1  
Old March 30th 17, 05:59 PM posted to uk.tech.digital-tv
Brian-Gaff
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Posts: 566
Default TV socket charge up

I was plugging the aerial back in the back of a reasonably modern telly last
week and got a bit of a shock, well quite a big one actually off the socket
and the plug. the plug goes straight to an aerial, no amps or anything so
the whole thing has to be electrostatic I guess. I had assumed once crts
went that none of this sort of charging would happen but it seems not.
Brian

--
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Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)


  #2  
Old March 30th 17, 07:02 PM posted to uk.tech.digital-tv
Adrian Caspersz
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Posts: 276
Default TV socket charge up

On 30/03/17 17:59, Brian-Gaff wrote:
I was plugging the aerial back in the back of a reasonably modern telly last
week and got a bit of a shock, well quite a big one actually off the socket
and the plug. the plug goes straight to an aerial, no amps or anything so
the whole thing has to be electrostatic I guess. I had assumed once crts
went that none of this sort of charging would happen but it seems not.
Brian


Is this in the same place that you have problems with USB sound cards
and noises?

--
Adrian C
  #3  
Old March 30th 17, 07:39 PM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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Posts: 453
Default TV socket charge up

On Thu, 30 Mar 2017 17:59:03 +0100, Brian-Gaff wrote:

I was plugging the aerial back in the back of a reasonably modern telly
last week and got a bit of a shock, well quite a big one actually off
the socket and the plug. the plug goes straight to an aerial, no amps or
anything so the whole thing has to be electrostatic I guess. I had
assumed once crts went that none of this sort of charging would happen
but it seems not.


I doubt it's static build up, Brian. More likely simply the quite common
effect of EMC filtering used by double insulated mains kit where the
chassis or 'common ground' is left floating at half mains voltage
courtesy of the high impedance capacitive voltage divider effect provided
by the EMC filtering circuit.

A typical aerial installation will be characterised by a high resistance
path to local ground rather than a perfectly insulated lightning
conductor capable of accumulating several hundreds of volts before a
breakdown of the insulation halts any further build up of static voltage
into the kilovolts and tens of kilovolts region.

The aerial leakage path to earth can vary from as low as a hundred ohms
or so to as high as a few hundred kilo ohms depending on how the aerial
was actually installed and varies with the local weather conditions,
notably the difference between a hot sunny day after a spell of dry
weather or a day of prolonged rainfall after several such damp days.

The chances are quite high that you're acting as a bridge between the
high impedance half mains voltage contact source of electric shock
provided by your modern TV's aerial socket and a significantly leaky
enough path to earth via the aerial plug.

Incidentally, when you have several double insulated items of kit linked
together via signal interconnect cables, the Hi-Z half mains voltage
sources from each item's 'common ground' are effectively paralleled up,
reducing the effective impedance of this half mains voltage source of
shock current.

Unless one or more of the items has an actual galvanic connection to
earth, the severity of the shock will keep increasing as you add
additional mains powered double insulated kit to the mix. The best and
simplest solution to this problem is to connect a safety earth to one of
the offending items, preferably the one into which the aerial is plugged
into. Alternatively, failing that, just remember to wear marigolds
whenever you feel the need to fiddle about with the TV aerial
connections. :-)

--
Johnny B Good
  #4  
Old March 30th 17, 08:23 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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Posts: 148
Default TV socket charge up


"Johnny B Good" wrote in message
...
On Thu, 30 Mar 2017 17:59:03 +0100, Brian-Gaff wrote:

I was plugging the aerial back in the back of a reasonably modern telly
last week and got a bit of a shock, well quite a big one actually off
the socket and the plug. the plug goes straight to an aerial, no amps or
anything so the whole thing has to be electrostatic I guess. I had
assumed once crts went that none of this sort of charging would happen
but it seems not.


I doubt it's static build up, Brian. More likely simply the quite common
effect of EMC filtering used by double insulated mains kit where the
chassis or 'common ground' is left floating at half mains voltage
courtesy of the high impedance capacitive voltage divider effect provided
by the EMC filtering circuit.


Can't remember the last time I saw one "left floating" - AFAICR: they always
have a tapped series pair of filter capacitors, and usually a resistor
voltage divider which bleed any charge from the capacitors.

The capacitors are rarely bigger than 4n7 and the resistors anywhere from
10M to 22M. The divider sets the double insulated chassis ground to half
mains with very high resistance - until you connect more
devices.................

The aerial socket is grounded to chassis ground, which is connected to other
devices by SCART or RCA leads etc.


---
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  #5  
Old March 30th 17, 08:24 PM posted to uk.tech.digital-tv
NY
external usenet poster
 
Posts: 1,231
Default TV socket charge up

"Johnny B Good" wrote in message
...
On Thu, 30 Mar 2017 17:59:03 +0100, Brian-Gaff wrote:

I was plugging the aerial back in the back of a reasonably modern telly
last week and got a bit of a shock, well quite a big one actually off
the socket and the plug. the plug goes straight to an aerial, no amps or
anything so the whole thing has to be electrostatic I guess. I had
assumed once crts went that none of this sort of charging would happen
but it seems not.


I doubt it's static build up, Brian. More likely simply the quite common
effect of EMC filtering used by double insulated mains kit where the
chassis or 'common ground' is left floating at half mains voltage
courtesy of the high impedance capacitive voltage divider effect provided
by the EMC filtering circuit.


Been there, had the shock. In my case it happened when I unplugged the TV
aerial to my PC's DVB-T card, holding the metal screen of the aerial plug
with one hand and with the other hand on the earthed case of the PC. It was
quite a belt!

The screen of the aerial was at about 120 V wrt mains earth (the PC case).
After unplugging everything else that was plugged into the aerial (eg TV,
VCR) or was connected to the TV (eg via phono audio cables), I identified
the TV as the culprit.

I measured my body resistance (about 300 kilohms) and found resistors to
this value, and then connected them between TV aerial and mains earth, as if
it were my body (I didn't want to repeat the shock - it was that bad) and
measured the voltage again. With a British Standard human body in place, the
voltage was about 80 V - less than the high-resistance-voltmeter reading but
still quite noticeable.

Amazing really that TVs are allowed to have their aerial/phono screen at
half mains voltage, even via a high resistance.

  #6  
Old March 30th 17, 09:28 PM posted to uk.tech.digital-tv
Paul Ratcliffe
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Posts: 2,473
Default TV socket charge up

On Thu, 30 Mar 2017 20:23:15 +0100, Benderthe.evilrobot
wrote:

The aerial socket is grounded to chassis ground, which is connected to other
devices by SCART or RCA leads etc.


Which are all floating as well.

Or were you expecting something to get earthed by a SCART or phono lead? Eek!
(RCA is for Americans.)
  #7  
Old March 30th 17, 09:52 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 2,073
Default TV socket charge up

On 30/03/2017 19:39, Johnny B Good wrote:

The aerial leakage path to earth can vary from as low as a hundred ohms
or so to as high as a few hundred kilo ohms depending on how the aerial
was actually installed and varies with the local weather conditions,
notably the difference between a hot sunny day after a spell of dry
weather or a day of prolonged rainfall after several such damp days.


It depends a lot on the design of the aerial. Some don't have a
connection between the coax and the supporting mast (etc) because the
dipole isn't folded and thus isn't secured at the mid-point. Also some
mast clamps have a plastic insert.
Aerials screwed to a plastic fascia can be effectively isolated from
ground.
Aerials in lofts ditto.
For aerials on chimney or wall brackets on masonry, assuming the coax is
connected to the mast, it all depends on how wet the masonry is.
However, there is such a thing as conductive mortar, I'm told.


Unless one or more of the items has an actual galvanic connection to
earth, the severity of the shock will keep increasing as you add
additional mains powered double insulated kit to the mix. The best and
simplest solution to this problem is to connect a safety earth to one of
the offending items, preferably the one into which the aerial is plugged
into. Alternatively, failing that, just remember to wear marigolds
whenever you feel the need to fiddle about with the TV aerial
connections. :-)

Yes try climbing an aluminium ladder in the rain and then touching the
dish of a communal system that's been incorrectly installed with no
earth bond and has 30 Sky boxes connected to it.

Bill
  #8  
Old March 30th 17, 10:10 PM posted to uk.tech.digital-tv
Benderthe.evilrobot
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Posts: 148
Default TV socket charge up


"Paul Ratcliffe" wrote in message
...
On Thu, 30 Mar 2017 20:23:15 +0100, Benderthe.evilrobot
wrote:

The aerial socket is grounded to chassis ground, which is connected to
other
devices by SCART or RCA leads etc.


Which are all floating as well.

Or were you expecting something to get earthed by a SCART or phono lead?
Eek!
(RCA is for Americans.)


I wasn't - were you?

An external HiFi amp probably would, but I've never used one on a TV and
don't know anyone who has.


---
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http://www.avg.com

  #9  
Old March 30th 17, 10:23 PM posted to uk.tech.digital-tv
Graham.[_12_]
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Posts: 422
Default TV socket charge up

On Thu, 30 Mar 2017 21:52:11 +0100, Bill Wright
wrote:

On 30/03/2017 19:39, Johnny B Good wrote:

The aerial leakage path to earth can vary from as low as a hundred ohms
or so to as high as a few hundred kilo ohms depending on how the aerial
was actually installed and varies with the local weather conditions,
notably the difference between a hot sunny day after a spell of dry
weather or a day of prolonged rainfall after several such damp days.


It depends a lot on the design of the aerial. Some don't have a
connection between the coax and the supporting mast (etc) because the
dipole isn't folded and thus isn't secured at the mid-point. Also some
mast clamps have a plastic insert.
Aerials screwed to a plastic fascia can be effectively isolated from
ground.
Aerials in lofts ditto.
For aerials on chimney or wall brackets on masonry, assuming the coax is
connected to the mast, it all depends on how wet the masonry is.
However, there is such a thing as conductive mortar, I'm told.


Unless one or more of the items has an actual galvanic connection to
earth, the severity of the shock will keep increasing as you add
additional mains powered double insulated kit to the mix. The best and
simplest solution to this problem is to connect a safety earth to one of
the offending items, preferably the one into which the aerial is plugged
into. Alternatively, failing that, just remember to wear marigolds
whenever you feel the need to fiddle about with the TV aerial
connections. :-)

Yes try climbing an aluminium ladder in the rain and then touching the
dish of a communal system that's been incorrectly installed with no
earth bond and has 30 Sky boxes connected to it.

Bill


The original Sky Digital box, the Amstrad DRX100 had its chassis
earthed via the three core mains lead.
I think there was a slim Sony box that was also earthed.

I wonder why Sky specified this for some early boxes, then embraced
the Class II paradigm thereafter?

--

Graham.
%Profound_observation%
  #10  
Old March 30th 17, 10:47 PM posted to uk.tech.digital-tv
Max Demian
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Posts: 3,857
Default TV socket charge up

On 30/03/2017 21:28, Paul Ratcliffe wrote:
On Thu, 30 Mar 2017 20:23:15 +0100, Benderthe.evilrobot
wrote:

The aerial socket is grounded to chassis ground, which is connected to other
devices by SCART or RCA leads etc.


Which are all floating as well.

Or were you expecting something to get earthed by a SCART or phono lead? Eek!
(RCA is for Americans.)


'Phono' is short for 'phonograph', American for 'gramophone'. Which is
the only audio/visual device I have that is earthed, i.e. the turntable
(or rather its cartridge), which has a separate earth connection to the
amplifier.

--
Max Demian
 




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