Hi,
I was getting a bit cheesed off with receiving a shock everytime I plugged
my communal aerial into any devices connected to my tv. I have been blaming
the aerial system for the problem. So, I decided to carry out a quick
investigation with a humble neon screwdriver. I found that the scart
earthing from the tv (Sharp model 28lf92H) was live.
I am puzzled as to how this can be. Would a simple mains input reversal to
the tv solve the problem? I think I may have a pdf file of the manual for
this set somewhere if need be, but my serving skills are a bit rusty after
years of non activity. Any pointers in the right direction are most
gratefully accepted. Thanks in advance.

Best wishes,
David

On Sun, 4 Dec 2005 15:15:22 -0000, "David Sewell"
wrote:

Hi,
I was getting a bit cheesed off with receiving a shock everytime I plugged
my communal aerial into any devices connected to my tv. I have been blaming
the aerial system for the problem. So, I decided to carry out a quick
investigation with a humble neon screwdriver. I found that the scart
earthing from the tv (Sharp model 28lf92H) was live.
I am puzzled as to how this can be. Would a simple mains input reversal to
the tv solve the problem? I think I may have a pdf file of the manual for
this set somewhere if need be, but my serving skills are a bit rusty after
years of non activity. Any pointers in the right direction are most
gratefully accepted. Thanks in advance.

Best wishes,
David


TV's have a hot chassis, and the antenna/cable input is supposed to
have isolation. Your isolation has been compromised somehow.

Tom

Maybe, maybe not.
The OP used a neon bulb tester which can light up from the minimal
leakage current along with the static charge build up to start the
lamp.

The only way for sure is to actually check the leakage current, or try
and light up a 60watt light bulb between the chassis and either ground
or the neutral line.

The OP also did not indicate whether the shock was a single instance or
a continual burning/tingling sensation.

wrote in message
ups.com...
Maybe, maybe not.
The OP used a neon bulb tester which can light up from the minimal
leakage current along with the static charge build up to start the
lamp.

The only way for sure is to actually check the leakage current, or try
and light up a 60watt light bulb between the chassis and either ground
or the neutral line.

The OP also did not indicate whether the shock was a single instance or
a continual burning/tingling sensation.


Hi and thanks,
I was getting a shock by plugging in an all metal coax plug from the
communal aerial system. It is not static as it is there repeatedly. I
measured with a digital meter on ac range and get 44v, but reversing the
leads gives zero. It measures as 24v on the dc range. That's between the
tv scart earth and mains earth.
Regards,
David

You need a leakage meter then to determine what kind of current is
actually present. Simply connecting a standard current meter is a bad
idea as those have little internal resistance and if the tv is
supplying high current, then something will be damaged.

It may be within manufacturing specifications as none of your measured
voltages were high enough for a full isolation failure.
It is not the voltage that is of concern, but the leakage current. You
meter was only placing a few megaohms load between the scart ground and
mains ground.

On a 120V typical hot chassis power suuply, the voltage measured to
ground would be around 85 volts and will have sufficient current to
easily light a 40watt incandescent light bulb when connected between
the offending hot chassis and ground.

I would bet that there is only a few uA of leakage current present out
of the tv scart connector given the voltages measured across a few
megaohm .

You did not mention what was measured on the community arial connector
to earth/mains ground. Given what you measured on your scart
connector, I am betting on a problem on the ariel line coming in, not
your tv set.

wrote in message
oups.com...
You need a leakage meter then to determine what kind of current is
actually present. Simply connecting a standard current meter is a bad
idea as those have little internal resistance and if the tv is
supplying high current, then something will be damaged.

It may be within manufacturing specifications as none of your measured
voltages were high enough for a full isolation failure.
It is not the voltage that is of concern, but the leakage current. You
meter was only placing a few megaohms load between the scart ground and
mains ground.

On a 120V typical hot chassis power suuply, the voltage measured to
ground would be around 85 volts and will have sufficient current to
easily light a 40watt incandescent light bulb when connected between
the offending hot chassis and ground.

I would bet that there is only a few uA of leakage current present out
of the tv scart connector given the voltages measured across a few
megaohm .

You did not mention what was measured on the community arial connector
to earth/mains ground. Given what you measured on your scart
connector, I am betting on a problem on the ariel line coming in, not
your tv set.



Communal aerial earth to mains ground (in the UK by the way) measures zero
volts with my digital meter.

Check the mains isolator at the aerial(3 high voltage capacitor. or
1capacitor in ceramic insulator} for leakage if the chasis is' hot'.But
if the power supply is the type with 'hot &' 'cold',then check the
optoisolator,a high voltage capacitor in parallel with high resistance
[1M ]resistor between the secondary and primary of the chopper{"hot
"&" cold"} for leakage.Please use AVO meter with Resistance range at
1kohm to read for leakage.GODLUCK.

"Tom MacIntyre" wrote in message
...


TV's have a hot chassis, and the antenna/cable input is supposed to
have isolation. Your isolation has been compromised somehow.

Tom

Thanks for that. I was wondering if the aerial braiding should be earthed. I
thought not, and measured it for resistance. It measures around 15-25 ohm to
mains ground, depending which way round my meter leads are connected. Just
never thought of the aerial system as being a problem. Not with the ultra
fussy british health and safety regulations these days.

Thanks Tom,
Regards,
David

Do you have a 3-wire house wiring? i.e., live, neutral and ground wiring?
Does your tv ac plug uses 3 prongs?
If you do, is the ground (round) prong still there? If the ground prong was
removed, then when you plug the tv
to the outlet it's possible the tv's ac plug is connected the other way,
i.e., the neutral prong in the plug gets connected
to the live house wire. When this happens, the tv ground chassis may be
connected to the live wire.
Bobby

"David Sewell" wrote in message
...
Hi,
I was getting a bit cheesed off with receiving a shock everytime I plugged
my communal aerial into any devices connected to my tv. I have been
blaming
the aerial system for the problem. So, I decided to carry out a quick
investigation with a humble neon screwdriver. I found that the scart
earthing from the tv (Sharp model 28lf92H) was live.
I am puzzled as to how this can be. Would a simple mains input reversal
to
the tv solve the problem? I think I may have a pdf file of the manual for
this set somewhere if need be, but my serving skills are a bit rusty after
years of non activity. Any pointers in the right direction are most
gratefully accepted. Thanks in advance.

Best wishes,
David