Proper connections of cable shields, and how they work

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Hi - can somebody explain the concept of cable shields to me? As in, why do they work, and how do you connect them? For my current purposes, I'm interested in shielding the outside world from what's going on inside of my cable.

Generally speaking, my understanding is that you typically connect one side of your shield to ground, and leave one side floating. However, I have seen both sides connected to ground, one side grounded and the other AC coupled, and various other combinations. I believe the shield blocks electric fields from leaving the cable, but why?

At first I was thinking that it was due to charge redistribution on the shield. Something along the lines of how a conductor will redistribute charge such that there is no electric field within the conductor. But that made me realize that it'd essentially just be re-radiating the signal. (yes no field within the conductor, but the charge distribution on the outside of the shield would be changing to match the changes inside the shield).

So I'm confused. It's been too long since school. Can somebody remind me how a shield works?

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Quote:
the shield blocks electric fields from leaving the cable
Mostly it's the other way around, it stops noise from getting INTO the cable.

Do you own an amp with an AUX input or better a mic input? What happens if you plug a lead that is not shielded into the socket? Hummmmmmm....

Quote:
Can somebody remind me how a shield works?
I'd ask captain Kirk. :-)

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
Quote:
the shield blocks electric fields from leaving the cable
Mostly it's the other way around, it stops noise from getting INTO the cable.
Its both ways. Shielding works as well stopping ingoing and outgoing radio waves. And it is commonly used for both.

And the shield shall be connected on both sides. Some say that its better to connect on one side, and it might seem like that is a good idéa in the real world when you are standing there by your machine and it starts working when you disconnect the shield on one side. But in the case it starts working when disconnecting one side, it only means that you have a problem somewere else that can be fixed. And haveing a shield connected on one side is the same as not having it at all. It will only make the shield an antenna.

There are only two things you can do to stop radio interference. Shield or filter. If you shield, you must shield all that is causing the interference. Like putting it in a bag of metal with no openings larger than the frequency you want to contain (inside or outisde). If you filter, you must do so when the cable leaves the shielding and connect the ground of the filter to the shield in that place. If you do not shield or filter, the cable going trough the shield will transport the interference trough the shield.

If you follow this fully, you will have no problem with radio interference getting into, or out of the shield. If it is very inpractical to follow the rules, try using it as much as possible. And then test if it works. Note that things inside the shielding might interfere with each other.

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If I have three different ground; power supply ground, digital ground connected from one point to power supply ground and analog ground also connected from one point to power suppy ground, shielded cable from load cell, load cell ground connected to analog ground. How shielding should be connected, to analog ground or to power supply ground, or to metallic enclosure which is conneted to power supply ground.

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Connect to metallic enclosure. But is the load cell analogue ground connected to the loadcell enclosure?

The best to avoid ground currents in the shield is to only have connections between ground and shield in one place.

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Quote:
The best to avoid ground currents in the shield is to only have connections between ground and shield in one place.

But your first post you write:

Quote:
And the shield shall be connected on both sides.

Which one?

Jim

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I think that you are talking about two different things. (1) shield as a device to reduce signal coupling from the outside world into a sensitive circuit (a wire being part of a circuit) and (2) the use of a "shield" to establish a common (ground) between two different circuits.

In the first case, a shield reduces interference due to the characteristic of electrical fields NOT penetrating a conductive enclosure (sphere, box, cylinder, what-ever). The woven shield on a wire is simply an imperfect attempt to create a conductive cylinder into which external fields do not penetrate. It is imperfect because it is not a perfect conducting cylinder. The metal has finite resistivity AND it is not solid (as a pipe would be) since it is woven. To understand THIS, you need to go back to your Electromagnetic Fields class!

Now, for the end connections. A shield does not make a tight enclosure. The ends are open. Without this, it is possible for the shield to couple, capacitively, to the inner conductors. Connecting an end to the circuit provides a path for induced currents to flow and keeps that shield at the same potential as circuit ground.

Whether or not to connect BOTH ends depends on several things. Often, the shield is used as though it is a heavy wire, and this makes a convenient way to establish a common ground between two different circuits. This can, in return, reduce noise, but this effect has nothing to do with shielding. Instead, it has to do with minimizing voltage differences between two circuits.

Summarizing, the use of shields, especially in cables, tends to be a Black Art. People who say that they understand it, including me, likely do not, fully. Its far more a matter of what works. If connecting both ends of a cable shield helps in your situation do it. Otherwise, don't.

Jim

 

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To understand THIS, you need to go back to your Electromagnetic Fields class!

That's the truth! When stuck on an EMC problem, I often recall something a professor said 35 years prior, that did not make complete sense at the time, however it is the solution to my problem. When I went to college about 50%, could not pass EMF classes, they went to other fields, the rest became EE's. Today there are EE's who have not had a single class in EMF.

It all starts with a mental vision.

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In the case of audio one needs to consider the current paths. At times connecting the shield at both ends creates an "earth loop" (no problems on other planets... :? ) producing audible hum.

In other cases it is better to have the shield be the common ground link between pieces of equiment as long as it doesn't carry heavy current with possible AC components.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Hallo to all.
There are some conductive paints for walls, that suppose to protect the human body of the magnetic fields created around electrical cables, from inside the wall. After painting the wall, someone can ground the paint to the main ground line of the house. As i know magnetic component passes through grounded shields without problem. Then how those paints can protect someone who i.e is living in a very close vicinity with an electrical power transformer?

Do you think is a good idea to make big coils around opposite walls in a big 8 shape to cancel each out in an external magnetic variation?

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jgmdesign wrote:
Quote:
The best to avoid ground currents in the shield is to only have connections between ground and shield in one place.

But your first post you write:

Quote:
And the shield shall be connected on both sides.

Which one?

Jim

Perhaps ka7ehk answered this, but to make it clear. The shield shall be connected on both sides to shield, not on both sides to GND. So a cable between two metal boxes shall have the shield connected in both ends to its metal box. That does not mean that GND is connected in both ends. Only in one of the boxes the metal of the box is connected to its contained electronics, and in one point/area. The electronics in the other box is connected to the other boxes electronics trough cable.

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Quote:
Can somebody remind me how a shield works?

If you can recall the underlying maths, Maxwell's equations & Lorentz laws cover it very concisely!
That would make a good job interview question actually! We must make sure we cover it in the AVRFreaks University syllabus. :wink:

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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how shielding works is basically making a faradays cage. you make sure that all external interferences get passed aroound the things you want to shield. As this also works the other way around you also keep the 'garbage' that is in the system internal and thus shielded form the outside world.

How to connect the shielding depends on the system it self and thus there is no general rule saying you alsways do this or always do that.

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Quote:
Then how those paints can protect someone who i.e is living in a very close vicinity with an electrical power transformer?

The principle is to create a faraday cage of your walls, BUT this power transfomers works on 60 Hz, right? So you calculate the skin deepth and you will see that just this paint on your wall isn´t enough to block the eletromagnetic waves from this power transformers...

Quote:
So a cable between two metal boxes shall have the shield connected in both ends to its metal box. That does not mean that GND is connected in both ends. Only in one of the boxes the metal of the box is connected to its contained electronics, and in one point/area.

Nice... BUT how Jim said, depends the situation, you could have a situation where the box that aren´t connected to the GND creates a voltage diferential between the metal box and the GND of the circuit, so it´s better to connect the GND to the metal box..

Regards,

Bruno Muswieck