Shock from TV aerial socket

[James]

I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

 

[Michael A. Terrell]

James wrote:

I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

Measure the current, not the voltage.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

[Baphomet]

"James" <james@unknown111.demon.co.uk> wrote in message
news:gpvd00p911vuaike19hd8o88pasjbb9fnp@4ax.com...

I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

"He said its very common but OK" - Tell that to the patient with an
implanted catheter. It's definitely NOT o.k. That is what micro-shock is all
about and why major hospitals are required to have bio-engineering
departments. 10 micro-amps leakage current is all that is allowed from any
electrical appliance (beds included) in any hospital, especially those
having CCU or ICU units. That is why hospitals have floating power systems
in operating rooms and why patient leads on electronic diagnostic equipment
are now required to be isolated.

 

[Earl]

It's o.k. in the sense that all t.v's have this phenomenon, namely
when you touch the aerial connection with a bare hand you will
receive a small annoying shock. I am not an engineer nor am
I talking about hospitals, but all the t.v.'s I've come across have
this problem. btw. this is why you should never connect any audio
video equipment into your t.v, always connect them to the vcr.
I have no idea why this high voltage occurs, maybee someone will
explain. I do know though that the chasis of a t.v. is live, so never
touch it. I guess this is because there is no step down transformer
in a t.v. since it needs all the voltage it can muster in order to
generate the 20000V it needs for the tube.

 

On Thu, 15 Jan 2004 22:40:20 GMT Michael A. Terrell <mike.terrell@earthlink.net> wrote:
| James wrote:
|>
|> I wonder if someone could explain something to me. Im an electrical
|> technician in a hospital, and the other day i got a shock from the
|> back of a tv, namely the aerial socket (outer side) to earth. I was
|> very curious about this and done some tests. I measured 115v ac to
|> earth, and 80v dc to earth. This i found on a few TV's so made a call
|> to a TV engineer i know. He said its very common but OK. He said its
|> to do with a floating earth and is OK because its isolated in the
|> power supply. Correct me if im wrong, but surely if it's isolated,
|> then a connection between live and earth would not produce this kind
|> of shock.
|>
|> Could someone maybe help me out on explaining this to me.
|>
|> Thanks
|
| Measure the current, not the voltage.

If there is enough current available to feel the shock, it is WAY too much
in a hospital setting. Back when TVs used 300 ohm balanced in and the
distribution was 75 ohm, the little transformer to convert it isolated the
DC and low frequency AC. Now days we plug 75 ohm coax straight into the
TV and don't think about what might be on there. Normally TVs in hospital
are out of reach and patients use remote controls. But I would find a way
to isolate them in that environment. Also keep in mind it could be the
distribution amplifier putting that voltage on there.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

On Fri, 16 Jan 2004 00:51:41 -0000 Earl <qqq@qqqzzzqqq.com> wrote:
| It's o.k. in the sense that all t.v's have this phenomenon, namely
| when you touch the aerial connection with a bare hand you will
| receive a small annoying shock. I am not an engineer nor am
| I talking about hospitals, but all the t.v.'s I've come across have
| this problem. btw. this is why you should never connect any audio
| video equipment into your t.v, always connect them to the vcr.
| I have no idea why this high voltage occurs, maybee someone will
| explain. I do know though that the chasis of a t.v. is live, so never
| touch it. I guess this is because there is no step down transformer
| in a t.v. since it needs all the voltage it can muster in order to
| generate the 20000V it needs for the tube.

Modern TVs tend to use solid state power supplies. But I do remember
lots of older TVs from the 1950's and 1960's that had big hefty power
transformers. The primary side had the power switch and the safety
cord connector (the one that was pulled off when the back was removed).
Several voltages came out, including filament for the CRT, filaments
for the various tubes, around 60 volts or so to power most circuits,
and 400 or so to feed the flyback circuits that generated the higher
voltage to drive the CRT. At least that's what I saw in schematics of
the ones a friend of mine took apart. Color sets probably had different
voltages, but transistors were starting to be common around then, and
the transformers eventually disappeared, too. Oldest set I saw was a
Philco unit made around 1949. Only 15 inch CRT in a box about 24 inch
by 24 inch by 30 inch deep, plus 8 more for the CRT yoke. The power
transformer on this beast was at least 8x8x8 inch cube. Took about 3
minutes to warm up. We had it until around 1971. And oh yeah, I did
get shocked on the antenna connection.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

[Baphomet]

<phil-news-nospam@ipal.net> wrote in message
news:bu7vjj0250c@enews2.newsguy.com...

On Thu, 15 Jan 2004 22:40:20 GMT Michael A. Terrell
mike.terrell@earthlink.net> wrote:
| James wrote:
|
|> I wonder if someone could explain something to me. Im an electrical
|> technician in a hospital, and the other day i got a shock from the
|> back of a tv, namely the aerial socket (outer side) to earth. I was
|> very curious about this and done some tests. I measured 115v ac to
|> earth, and 80v dc to earth. This i found on a few TV's so made a call
|> to a TV engineer i know. He said its very common but OK. He said its
|> to do with a floating earth and is OK because its isolated in the
|> power supply. Correct me if im wrong, but surely if it's isolated,
|> then a connection between live and earth would not produce this kind
|> of shock.
|
|> Could someone maybe help me out on explaining this to me.
|
|> Thanks
|
| Measure the current, not the voltage.

If there is enough current available to feel the shock, it is WAY too much
in a hospital setting. Back when TVs used 300 ohm balanced in and the
distribution was 75 ohm, the little transformer to convert it isolated the
DC and low frequency AC. Now days we plug 75 ohm coax straight into the
TV and don't think about what might be on there. Normally TVs in hospital
are out of reach and patients use remote controls. But I would find a way
to isolate them in that environment. Also keep in mind it could be the
distribution amplifier putting that voltage on there.

Which brings to mind this bit of anecdotal trivia. In the early 70's, Ralph
Nader (whom I admire but think he was way off base on this one) was running
around the country declaring that 10,000 hospital patients per year were
dying of accidental micro shock. Coincidentally, NASA had just had its
budget slashed and there were a lot of out of work engineers. Miraculously,
many of these engineers were now hired to fill available slots in the
bio-electronics departments then being implemented in major metropolitan
hospitals around the country.

While it was true that much of the equipment including beds was heavy on
A.C. leakage current, it was also true that it was difficult to document
which equipment was contributory toward a particular shock incident; the
medical staff often got so spooked after an incident of this nature, they
frantically shuttled machinery around to eliminate faulty or suspect
equipment. By the time an engineer or tech could arrive on the scene, most
suspect equipment could no longer be traced.

Several years later after outlets were rountinely tested for polarity,
equipment checked for leakage currents and wiring switched to minimize it,
plugs switched to hospital grade, isolation transformers installed on beds,
and patient leads and critical power systems isolated (all at a huge
expenditure of public monies), the leakage problem (if indeed one really
existed to justify these expenditures) was gotten under control.

The moral of this story is that if an on-site team of experts couldn't come
up with sufficient documentation to determine which equipment was causative
of a particular micro shock incident, how could someone off-site who wasn't
even an expert in the field.

 

[Ian Stirling]

Baphomet <no.spam@no.spam.us> wrote:

"James" <james@unknown111.demon.co.uk> wrote in message
news:gpvd00p911vuaike19hd8o88pasjbb9fnp@4ax.com...
I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

"He said its very common but OK" - Tell that to the patient with an
implanted catheter. It's definitely NOT o.k. That is what micro-shock is all

Why would such be messing with the antenna connector?

 

[Baphomet]

"Ian Stirling" <root@mauve.demon.co.uk> wrote in message
news:%qRNb.15634$tQ6.582710@wards.force9.net...

Baphomet <no.spam@no.spam.us> wrote:

"James" <james@unknown111.demon.co.uk> wrote in message
news:gpvd00p911vuaike19hd8o88pasjbb9fnp@4ax.com...
I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

"He said its very common but OK" - Tell that to the patient with an
implanted catheter. It's definitely NOT o.k. That is what micro-shock is
all

Why would such be messing with the antenna connector?

Actually, they wouldn't have to directly. Assume that a tech was touching
the antenna terminal and also the bed frame. Further assume that the bed
wasn't grounded because of a bad outlet or broken ground lead. Further
assume that the patient catheter was contacting the bed frame. ZAP! One
always has to assume absolutely worst case conditions in these types of
critical or sensitive environments, if for no reason other than liability
which is not insubstantial.

 

[Michael A. Terrell]

phil-news-nospam@ipal.net wrote:

If there is enough current available to feel the shock, it is WAY too much
in a hospital setting. Back when TVs used 300 ohm balanced in and the
distribution was 75 ohm, the little transformer to convert it isolated the
DC and low frequency AC. Now days we plug 75 ohm coax straight into the
TV and don't think about what might be on there. Normally TVs in hospital
are out of reach and patients use remote controls. But I would find a way
to isolate them in that environment. Also keep in mind it could be the
distribution amplifier putting that voltage on there.

First of all, TV sets are not built for "hospital settings". The are
common household appliances, and the standard is for leakage current,
not open circuit voltage.

Second, when you connect the coax, the voltage is pulled into the low
millivolt range.

Third, any current you would get by touching the coax and another
ground should fall well below your safety requirement, which you didn't
state.

Fourth, I have never seen a TV the patient can reach in any hospital.
They are hung from the wall or ceiling and use a remote control, either
with a standard remote, or through a membrane keyboard on the hospital
bed. I have seen too many hospital rooms over the years watching as
family and friends lay there in pain.

Last of all, tell us which hospital you work for. I don't want to be
in one without proper written standards for their electrical or
electronic systems.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

[James]

Ok, firstly, thanks for all the reply's.

Anyway..

Today i set up a test to measure the current from the aerial socket to
earth. I used a 1k resistor, paralled up with a .15uF cap for the
meausurement test circuit (This i took from my notes many years ago
when i used to repair medical equipment). 1mV / uA of current. I
measured 200 uA which is below the threshold of perception, and also
below the maximum value for a piece of type B medical equipment which
is about 500uA from memory. It does seem strange though that i got a
shock from this. You are correct in assuming that patients should not
be able to touch a TV, but in the instance of our wards, the TV aerial
goes behind the patients beds, because that is where the socket is.
These Tv's should not even be here these days because all beds in the
NHS should well be on their way to having their own TV's which ours
do. Removing the bay TV's though is very political as you can imagine.

Thanks all for the replys, im sure this discussion will go on with
much interest. Other peoples opinions in this situation surely makes
you think, especially what you may have forgotten, and even
overlooked.

James







On Fri, 16 Jan 2004 06:07:04 -0800, "Baphomet" <no.spam@no.spam.us>
wrote:

"Ian Stirling" <root@mauve.demon.co.uk> wrote in message
news:%qRNb.15634$tQ6.582710@wards.force9.net...
Baphomet <no.spam@no.spam.us> wrote:

"James" <james@unknown111.demon.co.uk> wrote in message
news:gpvd00p911vuaike19hd8o88pasjbb9fnp@4ax.com...
I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

Could someone maybe help me out on explaining this to me.

Thanks

"He said its very common but OK" - Tell that to the patient with an
implanted catheter. It's definitely NOT o.k. That is what micro-shock is
all

Why would such be messing with the antenna connector?

Actually, they wouldn't have to directly. Assume that a tech was touching
the antenna terminal and also the bed frame. Further assume that the bed
wasn't grounded because of a bad outlet or broken ground lead. Further
assume that the patient catheter was contacting the bed frame. ZAP! One
always has to assume absolutely worst case conditions in these types of
critical or sensitive environments, if for no reason other than liability
which is not insubstantial.

 

[Earl]

o.k.

now to sum up:

the shock which is received is only when toucing the aerial connection
if there is no wire there, or if there is a wire, by touching the other
bare end of the wire.

we had many responses, no one has explained why this occurs.

all the stories about the hospitals is not relevant and are a figment of
imagination of the OP and some of the commentators.

this prooves that you can most of the time write on usenet almost anything
you like, and get away with it. People comment not because they
necessarily know the answers, but because they want to talk.

have a wonderful sunny day.

 

[Michael A. Terrell]

Earl wrote:

o.k.

now to sum up:

the shock which is received is only when toucing the aerial connection
if there is no wire there, or if there is a wire, by touching the other
bare end of the wire.

we had many responses, no one has explained why this occurs.

It was explained, but you missed it. It is leakage current if you are
touching two items. If you only touch one item, then it is a simple
static discharge. USe Google and do some research like "Tutorial leakage
current" or "Measuring leakage current".

all the stories about the hospitals is not relevant and are a figment of
imagination of the OP and some of the commentators.

How is the discussion of the leakage current between the antenna
connector and the coax not relevant?

this prooves that you can most of the time write on usenet almost anything
you like, and get away with it. People comment not because they
necessarily know the answers, but because they want to talk.

You must be speaking about what you post.

have a wonderful sunny day.

Its supposed to rain.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

[Baphomet]

"Earl" <qqq@qqqzzzqqq.com> wrote in message
news:bub90v$lv3$1@news8.svr.pol.co.uk...

o.k.

now to sum up:

the shock which is received is only when toucing the aerial connection
if there is no wire there, or if there is a wire, by touching the other
bare end of the wire.

we had many responses, no one has explained why this occurs.

all the stories about the hospitals is not relevant and are a figment of
imagination of the OP and some of the commentators.

Should I forget, please remind me never to hire you for electronics safety
related work in a hospital setting. Clearly, if you don't think the
responses about the hazard that this type of situation poses in critical
environments is relevant, you are inhabiting a planet other than terra
firma.

this prooves that you can most of the time write on usenet almost anything
you like, and get away with it. People comment not because they
necessarily know the answers, but because they want to talk.

Just making sure you don't get away with your nonsensical uninformed
blather.

have a wonderful sunny day.

and you ;-)

 

[R. Anton Rave]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message news:<40080456.B8FAAEAA@earthlink.net>...

First of all, TV sets are not built for "hospital settings".
They are common household appliances, and the standard is for
leakage current, not open circuit voltage.

Then what were all those Zeniths I saw back in the 1980s with 3-wire
cords? I saw them only in hospitals and nursing homes, while hotels
had almost the same model with 2-wire cords.

 

[Michael A. Terrell]

"R. Anton Rave" wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message news:<40080456.B8FAAEAA@earthlink.net>...

First of all, TV sets are not built for "hospital settings".
They are common household appliances, and the standard is for
leakage current, not open circuit voltage.

Then what were all those Zeniths I saw back in the 1980s with 3-wire
cords? I saw them only in hospitals and nursing homes, while hotels
had almost the same model with 2-wire cords.

They were also used in classrooms and air traffic control towers, but
they are long extinct. They were built before the leakage standards were
tightened, and even then, they were not to be used in a room where a
patient was connected to monitoring equipment. They were a standard
consumer TV chassis, with a three wire cord added, and a metal cabinet
for rough service. I repaired a lot of them at Ft. Rucker Alabama in
1972 & 1973. I also repaired a lot of early '60s version for the
Middletown, Ohio school system that only had a two wire cord, but were
metal cased, "Institutional" TV set. They were strapped to heavy metal
carts so they could be moved from one classroom to another without
worrying that they would be damaged by rolling them into a wall or a
door frame.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

[ĺnřn˙mřu§]

On Fri, 16 Jan 2004 17:25:52 +0000, James <james@unknown111.demon.co.uk> wrote:

Thanks all for the replys, im sure this discussion will go on with
much interest. Other peoples opinions in this situation surely makes
you think, especially what you may have forgotten, and even
overlooked.
There is in my opinion only three sources for the problem.

One of your TV's has a problem and you'll need to disconnect them one at a time to find it.

or

Something is shorting to the TV's input cable from within the wall or service boxes.

or

They're a problem with the amplifier that is driving the system.

 

[Michael A. Terrell]

"ĺnřn˙mřu§" wrote:

On Fri, 16 Jan 2004 17:25:52 +0000, James <james@unknown111.demon.co.uk> wrote:

Thanks all for the replys, im sure this discussion will go on with
much interest. Other peoples opinions in this situation surely makes
you think, especially what you may have forgotten, and even
overlooked.
There is in my opinion only three sources for the problem.

One of your TV's has a problem and you'll need to disconnect them one at a time to find it.

or

Something is shorting to the TV's input cable from within the wall or service boxes.

or

They're a problem with the amplifier that is driving the system.

Or, you don't know what you are talking about. If there is a
"defective piece of equipment" you won't find it with this method. You
have to measure the leakage current for each item, and do it to the
manufacturers specifications. All TV sets have a low leakage current
that will appear as a high voltage when read with a high impedance volt
meter. What you are concerned with is the CURRENT that can flow. Volts
might hurt, but Current flow is what kills.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

[Dr Engelbert Buxbaum]

James wrote:

I wonder if someone could explain something to me. Im an electrical
technician in a hospital, and the other day i got a shock from the
back of a tv, namely the aerial socket (outer side) to earth. I was
very curious about this and done some tests. I measured 115v ac to
earth, and 80v dc to earth. This i found on a few TV's so made a call
to a TV engineer i know. He said its very common but OK. He said its
to do with a floating earth and is OK because its isolated in the
power supply. Correct me if im wrong, but surely if it's isolated,
then a connection between live and earth would not produce this kind
of shock.

It is astonishing what voltage some entertainment electronics has on
touchable parts. I was once setting up a Philips satelite receiver when
I got a mild shock from touching the case. It turned ot that there were
about 80 V DC towards ground. The voltage had a high source impedance,
simply connecting the audio output to a stereo system made that voltage
go away.

I think the main problem is that audio/video equipment usually does not
have a saftety ground connection (2-pole plug instead of 3), to avoid
ground loops.

Nevertheless, I was kind of surprised that equipment with such high
voltages on the case would pass safety standards.

 

[Michael A. Terrell]

Dr Engelbert Buxbaum wrote:

It is astonishing what voltage some entertainment electronics has on
touchable parts. I was once setting up a Philips satelite receiver when
I got a mild shock from touching the case. It turned ot that there were
about 80 V DC towards ground. The voltage had a high source impedance,
simply connecting the audio output to a stereo system made that voltage
go away.

I think the main problem is that audio/video equipment usually does not
have a saftety ground connection (2-pole plug instead of 3), to avoid
ground loops.

Nevertheless, I was kind of surprised that equipment with such high
voltages on the case would pass safety standards.

They do pass the safety standards, which limit the leakage current.
You can't have a current flow without a voltage to push it.

--
We now return you to our normally scheduled programming.


Michael A. Terrell
Central Florida