Q: grounding of cable shield

[Jon Juhlin]

I have a simple question regarding shielded cable. I take it for granted
that the proper way to ground a cable shield is at the end where the power
source is. I assume this is true. I understand why it is wrong to ground at
both ends but I can't explain why it is proper to ground at the end nearest
the power source. I have learned from experience that grounding at the end
away from the power source can cause interference.

In my particular application I have a single 12VDC proximity sensor mounted
on a rock crusher. All other electrical components on the crusher and other
equipment in the plant are 120VAC or 460VAC 3ph. Electrical power around
portable rock crushing plants is typically provided by a generator set and
various parts of the plant are connected via a maze of cables lying on the
ground or strung through conveyor supports. It is an inherently electrically
noisy environment.

I have a 12VDC power source mounted in a remotely mounted push button
control enclosure which is tied by a multiconductor cable to another
enclosure where the 120VAC control power enters the control assembly. The
pushbutton control enclosure is grounded via a grounding conductor in the
cable to the chassis ground that comes in with the control power. The
proximity sensor on the crusher is connected to the push button enclosure
via a shielded cable. This sounds more complicated than it is.

My question is: Is it proper to ground the shield at the push button
enclosure (and hence via the grounding conductor to the control power
ground)? If so why exactly? It seems intuitively obvious but I can't really
explain why.

Jon Juhlin

 

[CWatters]

How does the 12V power get produced? Are you running 240V down the same
cable to a 12V power supply in the the pushbutton enclosure?

The way I like to think of it is that the cable shields are just an
extension of the enclosure. eg use the shield to extend the "power
enclosure" to include the "push button enclosure" making one big enclosure.
That overall enclosure should only be connected to ground/0V at one point
only (eg at the power supply in the power enclosure).

This would appear to make the "grounding conductor" you mention redundant -
however you need to consider safety - eg The cable screen may be a good
earth for noise reasons but not for safety earth continuity reasons (EN60950
or whatever the magic number is).

Having said all that, my experience is that things rarely go by the book.
What works in some situations fails hopelessly when used again in an
aparently identical situation for no good reason. You frequently just have
to try it and see.


Colin


"Jon Juhlin" <j_juhlin@efn.org> wrote in message
news:105fmdrkijlnsa6@corp.supernews.com...

I have a simple question regarding shielded cable. I take it for granted
that the proper way to ground a cable shield is at the end where the power
source is. I assume this is true. I understand why it is wrong to ground
at
both ends but I can't explain why it is proper to ground at the end
nearest
the power source. I have learned from experience that grounding at the end
away from the power source can cause interference.

In my particular application I have a single 12VDC proximity sensor
mounted
on a rock crusher. All other electrical components on the crusher and
other
equipment in the plant are 120VAC or 460VAC 3ph. Electrical power around
portable rock crushing plants is typically provided by a generator set and
various parts of the plant are connected via a maze of cables lying on the
ground or strung through conveyor supports. It is an inherently
electrically
noisy environment.

I have a 12VDC power source mounted in a remotely mounted push button
control enclosure which is tied by a multiconductor cable to another
enclosure where the 120VAC control power enters the control assembly. The
pushbutton control enclosure is grounded via a grounding conductor in the
cable to the chassis ground that comes in with the control power. The
proximity sensor on the crusher is connected to the push button enclosure
via a shielded cable. This sounds more complicated than it is.

My question is: Is it proper to ground the shield at the push button
enclosure (and hence via the grounding conductor to the control power
ground)? If so why exactly? It seems intuitively obvious but I can't
really
explain why.

Jon Juhlin

 

[Jon Juhlin]

The 12VDC power supply is powered by 120vac along with other components in
the enclosure. The grounding conductor is provided because in our
applications the push button enclosure may be located outside and so the
grounding conductor is the only path to ground other than the person
pressing the buttons.

The crusher and other equipment are independently provided earth grounds via
driven grounding rods. Hence there can quite easily be a difference in
potential between the earth ground at any particular piece of equipment and
the line power ground at the service entrance.

Jon Juhlin

"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:qmU5c.37986$sy5.2524302@phobos.telenet-ops.be...

How does the 12V power get produced? Are you running 240V down the same
cable to a 12V power supply in the the pushbutton enclosure?

The way I like to think of it is that the cable shields are just an
extension of the enclosure. eg use the shield to extend the "power
enclosure" to include the "push button enclosure" making one big
enclosure.
That overall enclosure should only be connected to ground/0V at one point
only (eg at the power supply in the power enclosure).

This would appear to make the "grounding conductor" you mention
redundant -
however you need to consider safety - eg The cable screen may be a good
earth for noise reasons but not for safety earth continuity reasons
(EN60950
or whatever the magic number is).

Having said all that, my experience is that things rarely go by the book.
What works in some situations fails hopelessly when used again in an
aparently identical situation for no good reason. You frequently just have
to try it and see.


Colin


"Jon Juhlin" <j_juhlin@efn.org> wrote in message
news:105fmdrkijlnsa6@corp.supernews.com...
I have a simple question regarding shielded cable. I take it for granted
that the proper way to ground a cable shield is at the end where the
power
source is. I assume this is true. I understand why it is wrong to ground
at
both ends but I can't explain why it is proper to ground at the end
nearest
the power source. I have learned from experience that grounding at the
end
away from the power source can cause interference.

In my particular application I have a single 12VDC proximity sensor
mounted
on a rock crusher. All other electrical components on the crusher and
other
equipment in the plant are 120VAC or 460VAC 3ph. Electrical power around
portable rock crushing plants is typically provided by a generator set
and
various parts of the plant are connected via a maze of cables lying on
the
ground or strung through conveyor supports. It is an inherently
electrically
noisy environment.

I have a 12VDC power source mounted in a remotely mounted push button
control enclosure which is tied by a multiconductor cable to another
enclosure where the 120VAC control power enters the control assembly.
The
pushbutton control enclosure is grounded via a grounding conductor in
the
cable to the chassis ground that comes in with the control power. The
proximity sensor on the crusher is connected to the push button
enclosure
via a shielded cable. This sounds more complicated than it is.

My question is: Is it proper to ground the shield at the push button
enclosure (and hence via the grounding conductor to the control power
ground)? If so why exactly? It seems intuitively obvious but I can't
really
explain why.

Jon Juhlin