Shock from TV aerial socket

[Robert C Monsen]

"Dr Engelbert Buxbaum" <engelbert_buxbaum@hotmail.com> wrote in message
news:bulvj4$28q$02$1@news.t-online.com...

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.

The third prong is there to ground the case, not any electronics inside the
case. The idea is that if a 'hot' wire touches the case, it shunts to ground
through the third wire, not through your body. That supposedly causes the
circuit breaker to trip.

The 'big' plug (in the US) is Neutral, which is grounded at the breaker, so
it shouldn't be that far away from earth ground. Measurements around my
house yield a maximum voltage at 5V differential with ground.

Thus, if you are getting 80VDC, there is something wrong, like the plug is
inverted, or the Neutral plug is actually Hot. You might want to test the
plug with an AC voltmeter to make sure the big hole is near the round hole.

Regards,
Bob Monsen

 

[Roy McCammon]

Dr Engelbert Buxbaum 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.


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.

Its been a while since I read the applicable safety standards (UL etc.)
for consumer equipment. My recollection is that they are intended to
prevent only dangerous electric shock. That means it would be lethal
or cause involuntary muscle movement. Perceptible shock is allowed.

Medical standards are much more restrictive. And considering litigation
some manufactures' internal standards require no perceptible shock.

 

[Dr Engelbert Buxbaum]

Michael A. Terrell wrote:

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.

Current was definetly very low as this was a high internal resistance
voltage source. After all the case could be grounded (and hence that
voltage short-circuited) via the stereo system without sending the
sparks flying.

But nevertheless it was sufficient for a tingling sensation. And I did
feel a bit uneasy about that.

 

[Dr Engelbert Buxbaum]

Robert C Monsen 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.

The third prong is there to ground the case, not any electronics inside the
case. The idea is that if a 'hot' wire touches the case, it shunts to ground
through the third wire, not through your body. That supposedly causes the
circuit breaker to trip.

Of course. But at the same time connecting to safety ground will bring
the voltage of the case to near zero, and prevent the bildup of that
sort of "leakage-voltage" (for lack of a better word).

The 'big' plug (in the US) is Neutral, which is grounded at the breaker, so
it shouldn't be that far away from earth ground. Measurements around my
house yield a maximum voltage at 5V differential with ground.

But there should not be a connection between neutral (or hot, for that
matter) and the case. Only the safety ground (if present, in HiFi it
ain't) should be connected.

Thus, if you are getting 80VDC, there is something wrong,

Exactly my point. But note that Center-European plugs are symetrical,
i.e. there is no such thing as inversion of hot and cold end.