a lot of misinformation out there about grounding

ganutenator

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I was once asked a question in a job interview. "Do you ground both ends of a shield?"
I answered: "You only ground one end".
Apparently that was the answer that he was looking for.
But is it ALWAYS the correct answer.
I recently had a customer ground both ends and solve his electrical noise issue.
Not before installing ferrite chokes to no avail.
 
A topic of longstanding discussion. See here and here for threads of yesteryear.

I thought I recalled a fairly recent thread as well, but was unable to find it quickly.
 
I was once asked a question in a job interview. "Do you ground both ends of a shield?"
I answered: "You only ground one end".
Apparently that was the answer that he was looking for.
But is it ALWAYS the correct answer.
I recently had a customer ground both ends and solve his electrical noise issue.
Not before installing ferrite chokes to no avail.

Its not the wrong answer...its the wrong question.
For RS-422, the answer is actually YES.
For some signals, the answer is Why shield in the first place? (Re: three-twists-per-inch unshielded instrumentation wiring.
.
So, yes the arguments continue.
 
You can't look at grounding the shield of signal wires in isolation, pun intended :). We assume that the earth in one box is the same as the earth in the next box, earth should be earth should be earth, but it isn't. It depends on the quality of the earth bonding between the two enclosures/machines. No voltage difference would be the goal, but it depends on many things for that to happen. It also depends on the type of voltage difference, is it smooth DC, smooth AC or a very noisy difference. Try using at least a multimeter and preferably an oscilloscope and measure what is happening in the shield circuit. Then you may understand why grounding both ends is a good idea sometimes, and a very bad idea at others.

Just to throw in one last thought. We talk about grounding or not grounding as the only two options, there is a whole range of resistances/capacitances/inductances between those two that can be used.


So the answer to the interview question is 'it depends on site conditions'.
 
A VFD cable shield should be grounded at both ends. Every other application I would say NO!

Correct. I shield VFD cables at both ends. We have some wet location motors that induces noise to the sensitive electronics in the room and we can only get the noise out by grounding both ends of the cable.
 
Shielded cable Grounding or Bonding as some call it has been a source of many conversation and speculation for years and I suspect that it will continue to be for a long time to come.

The best and most excepted rule for bonding is to bond one end only and that end should always be the end that is the source of the signal.
For output from a plc this is simple it would be in the plc panel, however if the output module has a bonding terminal then it should be used for that purpose. Note: I haven’t seen one that has the bonding terminal for many years I generally bond the shield on the terminal strip for the wires exiting the panel and run shielded cable from there to the plc output.

For interments the source of the signal would be in that interment and the bond should be in that same interment. What that means is that in the panel you will see some shielded cables bonded in the panel and some are not, that would be normal.

If the shielded cable passes through a terminal panel the shield of the cable should not be bonded in that panel. But saying that I have run into situations where the shield was bonded only in the terminal panel, this would be expectable as long as the only point of the bond is in that panel but this tends to confuse trouble shooter and should be avoided.
It was common to install shrink tubing over the free end of the shield to make sure that it doesn’t accidently come in contact with any other potentials and cause a problem but I don’t see it much anymore.

To help with understanding this you have to understand a few things
The source of electrical noise in an interment cable is magnetic fields generated by other cables in close proximity to the cable. The shield is there to drain those fields to ground so the effect of magnetic noise on the internal wires is minimized.
The shield is a conductor and if that conductor is connected to at both ends and the terminal point on each end are at a different paternal , then a current will flow through the conductor from one terminal to the other.
Note: There will always be a different potential between the 2 points. This will happen even on a properly grounded system.
The larger the potential difference the higher the current flow between them and the larger the noise problem. And that electrical noise in concentrated more heavily in the wires inside the shield.

I have actually seen a few times where they bonded the shield at both ends and the normal bonding wire for the panel or device got broken, somebody accidently cut the cable or the terminal just worked loose. The potential between the panels went up. (electricity is funny this way it will find another path and maybe not the one you want )as a result a large current is passed through the shield, the shielded cable catches fire and burns other wire in close proximity are also burned through the full length of the run. I think you see how quickly this can become a major very costly repair.
And for those that said that bonding both ends of the shield solved a problem I would say the original problem was a broken shield ( usually at a midpoint terminal ) so the shield was not continuous, you should go back and have a new look because it will give you problems in the future.
 
Gary S:
Accepted and excepted are opposites.

"The best and most excepted rule for bonding is to bond one end" is opposite of
The best and most accepted rule for bonding is to bond one end.
 

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