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TV socket charge up



 
 
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  #11  
Old March 30th 17, 10:04 PM posted to uk.tech.digital-tv
Dave W
external usenet poster
 
Posts: 249
Default TV socket charge up


"Brian-Gaff" wrote in message
news
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

My first job in a TV development lab taught me never to do this. TV's were
live chassis, but they were all run from isolating transformers in the lab.
There were neons on both transformer outputs to earth, and if the chassis
was earthed via a lead or test equipment the neon on the chassis side would
go off. Under this condition the earthed aerial could be connected without
any shock. If the chassis wasn't earthed, then you would get a shock via the
neon current, which were big ones taking 3mA. My boss said that if only
they'd made them 1mA then the shock would be no worse than a neon tester
screwdriver.

In your case the aerial will be earthed, and your TV plus everything else
connected to it will have capacitance to the mains supply. If that adds up
to about 20nF to mains live then you'll get a shock.

See JBG's reply for more info.
--
Dave W


  #12  
Old March 30th 17, 10:24 PM posted to uk.tech.digital-tv
Ian Jackson[_7_]
external usenet poster
 
Posts: 59
Default TV socket charge up

In message , Bill Wright
writes
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.

I bet that brought tears to your eyes - especially when you hit the
ground after falling off the ladder.
--
Ian
  #13  
Old March 31st 17, 12:17 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
external usenet poster
 
Posts: 1,818
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.


Some satellite receivers have a three core mains lead.

Bill

  #14  
Old March 31st 17, 08:07 AM posted to uk.tech.digital-tv
Brian Gaff
external usenet poster
 
Posts: 6,745
Default TV socket charge up

No its not, its a mile or so away. I was attempting to find out if his
signal really was crap or if the set was lying. The set was lying, since a
set top box worked on his aerial. However, it then transpired that there was
something in the socket shorting it out that had got squashed. Looked like
a bit of a milk bottle top. Some people do some strange things in their
homes but once removed normal service resumed. The telly had recently been
moved for the decorators to do their decorating thing, so I can only assume
it got in then.


However the shock is the thing, the other is a red herring. No I've never
found fish in a tv, though it is of course known as the haunted Fish tank in
some circles.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Adrian Caspersz" wrote in message
...
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



  #15  
Old March 31st 17, 08:11 AM posted to uk.tech.digital-tv
Brian Gaff
external usenet poster
 
Posts: 6,745
Default TV socket charge up

ERm thanks for that unsettling news. Time was that all hi fi kit was
earthed, but not any more, but in the main with these devices all you feel
is a 50 hz tingle as you run your finger over brushed ally parts etc like
front panels and phono socket earths. You could be right about the aerial.
Being blind I tend to feel for the socket with one hand not being able to
see it.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"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.

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



  #16  
Old March 31st 17, 08:16 AM posted to uk.tech.digital-tv
Brian Gaff
external usenet poster
 
Posts: 6,745
Default TV socket charge up

Just to put the record straight. I had removed all other plugs from the
back during the tests, so it would have had nowhere to go but me, I guess. I
doubt there was much current behind it. I suppose it was the surprise
factor, considering that unlike most old tellies which often did have the
chassis at half mains volts, I had assumed, wrongly that without high
voltages inside and using a separate PSU chunk, that this problem had gone
away now.

Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Bill Wright" wrote in message
news
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.


Some satellite receivers have a three core mains lead.

Bill



  #17  
Old March 31st 17, 08:51 AM posted to uk.tech.digital-tv
NY
external usenet poster
 
Posts: 1,134
Default TV socket charge up

"Brian Gaff" wrote in message
news
Just to put the record straight. I had removed all other plugs from the
back during the tests, so it would have had nowhere to go but me, I guess.
I doubt there was much current behind it. I suppose it was the surprise
factor, considering that unlike most old tellies which often did have the
chassis at half mains volts, I had assumed, wrongly that without high
voltages inside and using a separate PSU chunk, that this problem had gone
away now.


Yes I was surprised when I encountered it. An older colour TV from about
1990 didn't do it, but the widescreen (CRT) Panasonic that I bought in 2000
was terrible for it. I connected a wire from a radiator pipe to one of the
audio phono cable screens to earth the whole setup - making sure was the
only earth point, to avoid earth loops.

I've just checked our modern LCD TV and that's fine: less than 1 V AC
between aerial screen and mains earth.

Were old live chassis TVs full-current half-mains or did they have hefty
current-limiting resistors? Was there a risk of the aerial being at
half-mains when it was plugged in? I realise that all other exposed parts of
the chassis (eg volume and channel changing knobs on front) were big
bakelite things which would have provided plenty of insulation.

  #18  
Old March 31st 17, 09:23 AM posted to uk.tech.digital-tv
David Wade[_3_]
external usenet poster
 
Posts: 14
Default TV socket charge up



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?

PAT testing failures?
  #19  
Old March 31st 17, 09:41 AM posted to uk.tech.digital-tv
Dave W
external usenet poster
 
Posts: 249
Default TV socket charge up


"Dave W" wrote in message
news

"Brian-Gaff" wrote in message
news
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

My first job in a TV development lab taught me never to do this. TV's were
live chassis, but they were all run from isolating transformers in the
lab. There were neons on both transformer outputs to earth, and if the
chassis was earthed via a lead or test equipment the neon on the chassis
side would go off. Under this condition the earthed aerial could be
connected without any shock. If the chassis wasn't earthed, then you would
get a shock via the neon current, which were big ones taking 3mA. My boss
said that if only they'd made them 1mA then the shock would be no worse
than a neon tester screwdriver.

In your case the aerial will be earthed, and your TV plus everything else
connected to it will have capacitance to the mains supply. If that adds up
to about 20nF to mains live then you'll get a shock.

See JBG's reply for more info.


I need to backtrack a bit after seeing another post. The aerial socket had
its own capacitative isolation from the rest of the TV, but my shocks
occurred because I was holding the aerial plug in one hand and the set-side
of the capacitor in the other, not the projecting socket.
--
Dave W


  #20  
Old March 31st 17, 10:59 AM posted to uk.tech.digital-tv
R. Mark Clayton[_2_]
external usenet poster
 
Posts: 473
Default TV socket charge up

On Thursday, 30 March 2017 17:59:01 UTC+1, 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

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)


Unearthed device - chassis floats up to half mains voltage ~115V.
 




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