Current on shielding of TV coax

[Halfgaar]

Hi list,

Someone told me that it is normal for devices (TV's etc) to put a current on
the shielding of the TV-cable. Is this true? Is this also true for
certified devices?

Halfgaar
--
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[Harry Conover]

Halfgaar <voor_achter@yahoo.remove.com> wrote in message news:<bl4jr7$793$1@news1.tilbu1.nb.home.nl>...

Hi list,

Someone told me that it is normal for devices (TV's etc) to put a current on
the shielding of the TV-cable. Is this true? Is this also true for
certified devices?

Huh? I must be missing something, because I can make no sense out of your post.

Do you understand how a coaxial cable functions?

Harry C.

 

[Michael A. Terrell]

Harry Conover wrote:

Halfgaar <voor_achter@yahoo.remove.com> wrote in message news:<bl4jr7$793$1@news1.tilbu1.nb.home.nl>...
Hi list,

Someone told me that it is normal for devices (TV's etc) to put a current on
the shielding of the TV-cable. Is this true? Is this also true for
certified devices?

Huh? I must be missing something, because I can make no sense out of your post.

Do you understand how a coaxial cable functions?

Harry C.

Leakage current from the equipment, flowing to ground through the shield
of the coaxial cable.
--


Michael A. Terrell
Central Florida

 

[w_tom]

Appliances should never put current on coaxial cables. For
example, the TV antenna input is typically galvanically
isolated. Only the high frequency signals - trivial current -
should pass between TV and cable.

However leakage currents can exist. Currents on the order
of microamps. But coax typically connects to same earth
ground that AC electric uses - as required by code. Therefore
leakage currents are grounded - no voltage should be apparent.

Halfgaar wrote:

Someone told me that it is normal for devices (TV's etc) to put a
current on the shielding of the TV-cable. Is this true? Is this
also true for certified devices?

 

[Richard Crowley]

"Halfgaar" wrote ...

Someone told me that it is normal for devices (TV's etc)
to put a current on the shielding of the TV-cable. Is this
true? Is this also true for certified devices?

No, it is not "normal", it may occurr more often than desired,
but it is NEVER inentional. In some circumstances, it is
actually LETHAL.

If you have a specific situation you are dealing with BE VERY
CAREFUL. If you are looking for responses to something that
is happening in real life, by all means ask specific questions
rather than dancing around the theory. Else you may end up
more knowledgable, but dead.

 

[Jim Thompson]

On Sat, 27 Sep 2003 23:45:00 GMT, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Harry Conover wrote:

Halfgaar <voor_achter@yahoo.remove.com> wrote in message news:<bl4jr7$793$1@news1.tilbu1.nb.home.nl>...
Hi list,

Someone told me that it is normal for devices (TV's etc) to put a current on
the shielding of the TV-cable. Is this true? Is this also true for
certified devices?

Huh? I must be missing something, because I can make no sense out of your post.

Do you understand how a coaxial cable functions?

Harry C.

Leakage current from the equipment, flowing to ground through the shield
of the coaxial cable.

I did a building-to-building link for GenRad in the late '70's using
RG8... there was only a hard ground on one end and a high value
resistor on the other end to ground, with transformer-coupling and
DC-restore. There was about 15V difference in ground potential
between the two buildings (300' apart).

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[w_tom]

Even NEC requires communication cables between buildings to
be earthed at both ends - where cable enters each building.
That 15 volts between two buildings (300' apart) demonstrates
another reason why coax must be earthed before it enters a
building.

Jim Thompson wrote:

I did a building-to-building link for GenRad in the late '70's using
RG8... there was only a hard ground on one end and a high value
resistor on the other end to ground, with transformer-coupling and
DC-restore. There was about 15V difference in ground potential
between the two buildings (300' apart).

 

[Jim Thompson]

On Sun, 28 Sep 2003 09:24:29 -0400, w_tom <w_tom1@hotmail.com> wrote:

Even NEC requires communication cables between buildings to
be earthed at both ends - where cable enters each building.
That 15 volts between two buildings (300' apart) demonstrates
another reason why coax must be earthed before it enters a
building.

Jim Thompson wrote:
I did a building-to-building link for GenRad in the late '70's using
RG8... there was only a hard ground on one end and a high value
resistor on the other end to ground, with transformer-coupling and
DC-restore. There was about 15V difference in ground potential
between the two buildings (300' apart).

This be Arizona... there is no "earth" ;-)

Touch-grounding the far end and drawing sparks caused me to choose the
method I did.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Halfgaar]

w_tom wrote:

Appliances should never put current on coaxial cables. For
example, the TV antenna input is typically galvanically
isolated. Only the high frequency signals - trivial current -
should pass between TV and cable.

However leakage currents can exist. Currents on the order
of microamps. But coax typically connects to same earth
ground that AC electric uses - as required by code. Therefore
leakage currents are grounded - no voltage should be apparent.

coax grounded to the same earth as AC by code? I don't think that's the case
here, I wouldn't know where. The cable doesn't enter the house near the
breakerbox. There is a difference of almost 0 ohms between coax ground and
AC ground, but I suppose that's because there's grounded equipment
connected to the TV-cable.

Halfgaar
--
To send me, Halfgaar, email, remove remove from my email address.

 

[Halfgaar]

Richard Crowley wrote:

If you have a specific situation you are dealing with BE VERY
CAREFUL. If you are looking for responses to something that
is happening in real life, by all means ask specific questions
rather than dancing around the theory. Else you may end up
more knowledgable, but dead.

Not to worry, I haven't been shocked by the cable here. The TV-cable here is
grounded to the AC ground, but I don't see how (see my reply to w_toms's
message).

Halfgaar
--
To send me, Halfgaar, email, remove remove from my email address.

 

[w_tom]

National Electrical Code makes these demands for human safety:
Article 820.33

The outer conductive shield of the coaxial cable shall be grounded
at the buidling premises as close to the point of cable entrance
or attachement as practicable.

Article 820.40A Grounding conductor

1) Insulation. The grounding conductor shall be insulated and
shall be listed as suitable for the purpose
2) Material. The grounding conductor shall be copper or other
corrision-resistant conducitve material, stranded or solid.
3) Size. The grounding conductor shall not be smaller than 14
AWG. It shall have a current-carrying capacity approximately
equal to that of the outer conductor of the coaxial cable.
The grounding conductor shall not be required to exceed 6 AWG.
4) Length. The grounding conductor shall be as short as
practicable. In one and two family dwellings, the grounding
conductor shall be as short as practicable, not to exceed 6.0
meters (20 ft.) in length.
5) Run in Straight Line. The grounding conductor shall run to
the grounding electrode in as straight a line as practicable.

Clearly, without visual inspection of this grounding system,
then one cannot tell if the system is or is not earthed.
Measuring alone is not sufficient. That measurement could
detect a 40 AWG metallic sliver making the connection. It
would read 0 ohms but, according to code, would be a
non-existent ground. There is no alternative to a visual
inspection.

Halfgaar wrote:

coax grounded to the same earth as AC by code? I don't think
that's the case here, I wouldn't know where. The cable doesn't
enter the house near the breakerbox. There is a difference of
almost 0 ohms between coax ground and AC ground, but I suppose
that's because there's grounded equipment connected to the
TV-cable.

 

[Halfgaar]

w_tom wrote:

Clearly, without visual inspection of this grounding system,
then one cannot tell if the system is or is not earthed.
Measuring alone is not sufficient. That measurement could
detect a 40 AWG metallic sliver making the connection. It
would read 0 ohms but, according to code, would be a
non-existent ground. There is no alternative to a visual
inspection.

As I said, I can't see that's it's grounded, so it probably isn't. I'll see
if I can ground it. Thanks for your help.

Halfgaar
--
To send me, Halfgaar, email, remove remove from my email address.