View Single Post
  #3  
Old March 30th 17, 06:39 PM posted to uk.tech.digital-tv
Johnny B Good[_2_]
external usenet poster
 
Posts: 387
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