Where to wire analog input cable shield

Thread Starter

alexfrey

Joined Feb 23, 2019
22
I am new in automation industry and trying to learn fundamentals. In a project we have several analog inputs. My friend told me to wire cable shielding to earth terminal in one end. I know it is advised to wire it to earth but what happens if I wire it to power supply common ? What is the difference or it may make same effect ?
Thanks.
 

Wolframore

Joined Jan 21, 2019
1,748
I have seen positive ground in old British cars. Many issues related to that design. Just use ground for shielding. Its much safer...
 

Bernard

Joined Aug 7, 2008
5,605
Connect to equipment ground. Best if 2 signal wires encased in a shield. Might look at
some of the comments below.
 
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Yaakov

Joined Jan 27, 2019
1,612
I am new in automation industry and trying to learn fundamentals. In a project we have several analog inputs. My friend told me to wire cable shielding to earth terminal in one end. I know it is advised to wire it to earth but what happens if I wire it to power supply common ? What is the difference or it may make same effect ?
Thanks.
The reason for connecting the shield to one side is to prevent ground loops. If there is a potential between grounds on the two devices, current will flow on the shield. The function of the shield is to prevent induced interference, it shouldn't be used to carry a signal. The connection to a good earth ground on one side allows it to function, doing that on both sides allows it to be a signal path.
 

Yaakov

Joined Jan 27, 2019
1,612
There is a tendancy now to connect both ends of shielded cables, especially when equi-potential bonding has been applied.
See Ch6 of the PDF.
Max.
So long as the equipment is all well bonded to a proper common earth, there's no reason to protect against ground loops that can't happen.
 

MaxHeadRoom

Joined Jul 18, 2013
21,187
Years ago I remember that everyone stressed the need for only connecting shield to one end, since the suggested practice of equi-potential bonding, ideas have changed.
This is what I started doing in my installations.
Max.
 
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Yaakov

Joined Jan 27, 2019
1,612
Years ago I remember that everyone stressed the need for only connecting shield to one end, since the suggested practice of equi-potential bonding, ideas have changed.
This is what I stated doing in my installations.
Max.
In recording studios, particularly in the past, it was extremely difficult to ensure no potential among grounds so even tough I always used a star ground, with my own earth system when possible, I still grounded the shield on the input side only. I often had to remediate hum in studios we bought time in.
 

MaxHeadRoom

Joined Jul 18, 2013
21,187
The OP mentions he is working in the automation industry which is the one I worked in and the PDF tends to apply to.
The intimation by the OP of the power supply common not being connected to earth ground is one option method, I always subscribed to the opposite method of earth grounding all power supply commons where possible.
Max.
 
Remember a couple of things:

1. Shielding reduces RFI
2. Twisting reduces EMI

So a lot of stuff is twisted pair shielded.

That said for the shield to work, it should be at earth potential. If you have the option to pick which end, use the sourcing end.

Grounding the shielded at one end helps eliminate ground loops. Believe me, you want to design a ground loop out of your circuit and not find and fix later.

I was using instrumentation in a lab and it's easy to find multi-channel pseudo differential voltage measuring things.
Now what happens when that instrument wants a setpoint too. The easiest way out is to use a current source and a resistor at the device's voltage input. Nothing really has to be isolated and no ground loops will exist providing there is a path to ground.

Now take something messy, 4-6 channels of Mass flow controllers that require +-15 to operate. One power supply for 4.

Now each controller is off in subspace somewhere. For potentiometer setpoints the pot is referenced to the MFC. There is a 5V supply to the MFC.

Everything works UNTIL you want a computer to supply this 0-5V setpoint signal. Yep, I missed that.

That particular MFC box was in-house made and was used successfully manually. I added calibration pots and decimal selects to make calibration easier. I also added a switch that set the readout to full scale which made calibration a snap.
Just missed the ground referenced input signal. Pseudo-differential readouts would have worked.
 

MaxHeadRoom

Joined Jul 18, 2013
21,187
Grounding the shielded at one end helps eliminate ground loops. Believe me, you want to design a ground loop out of your circuit and not find and fix later.
What comments do you have regarding the mention of it in the Ch6 PDF, if read.?

Quote:
When is simple shielding sufficient?
In most industrial plants, simple-shielded lines are sufficient for the reliability of the
plant.
The double-sided connection of the shield to the earth is thereby the most
effective shielding measure.
When is double shielding necessary?
Double shielding is advantageous for signal transmission in a particularly
interference-prone environment.

Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,187
In the industry:
- shield of data cable is grounded on both ends
- shield of motor cable is grounded on one end
Although when wiring a VFD to motor, the shielded cable instruction is to ensure both ends connected to earth ground, also this is automatically done if using metallic conduit, flexible or otherwise.
Max.
 
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