Grounding screened control cables

Hi guy's, interesting topic of grounding. If you were to run a control cable, that uses screened leads, from a plc to a drive would you ground at both ends? I know it is industry practice, if the cable, ie instrumentation cable, contains a bare wire "what i like to call a drain wire" even if it's inside a foil screen, should be grounded only at one side. ie. ground at transmitter leave floating back at panel.
If grounded at both ends it could cause a ground loop which will cause you headaches. I just read a manuel that recomended grounding the screen at both ends of the cable. What do you think of this?

Is it saying ground it on both ends or connect it to the "drain" terminal on the device

it says " Use screened leads for connections to the control circuitry. Ground the screen at both ends."

and says the same for the power cables.

I would say that is a big NO NO. I think I would call for an explaination.

Of course, I rarely read the instruction unless it doesn't work

It`s the fundamentals of EMC

EMC-Electromagnetic Compatibility

In stalling a PLC with a drive there are ff rules:

1. Make sure there is a perfectly functioning reference ground
( Central point )
- This is for protection of PLC from externel influence
- Shield electromagnetic fields resulting from inductors
( Transformers, motors, contactor coils )

It`s the fundamentals of EMC ( Electromagnetic compatability )

There are essential to observed the ff rules when installing a
PLC to achive EMC

Make sure there is a perfectly functioning refernce ground
-Shield electromagnetic fields resulting from inductors
( transformers, motors, contactor coils )
-Data transmission cables should be screened & connected at
both ends
-Analog cables should be screened & the screen connected at one
both ends.

In other words for electromagnetic interferance purpose.

Very Interesting.

can you give me an example. maybe a quick drawing of what you mean. I've always been told not to ground at both ends because it causes ground loops. This can cause EM interference especilly for circuits with lets say 4-20 ma.which are quite common. Can you explain how grounding on both ends would prevent this? So if you were going to run an anolog i/p from a transmitter, are you saying that you would ground the screen on both ends?

I think we should discuss what exactly we are refering to as the screen. Is it the external braid found on for example marine cables etc.? Is it the foil wrap inside cable wrapped around each individual conductor? IS it the bare wire often ran with the individual cables or pairs of cables "the Drain"? What is it?

Any suggestions?

I could be wrong, but the rule of thumb that I learned was to only ground one end for Analog Signals, and ground both ends for Digital Signals.

Hope this helps.

That's interesting. Can anyone else confirm that?

I've also only grounded at one end of an analog circuit to prevent ground loops. My practice is to float the transmitter end, terminate the PLC end, which passes any induction etc.. to the PLC ground, which should be connected to the main plant grounding grid so only one path exists. I've worked for companies that ground the transmitter end, assuming the associated piping would take care of the grounding. We repeatedly had problems with ground loops and changed the practice.

To comply with EMC regs:

Shielded analog cables must be earthed at one end, however if the cable is very long it is possible to earth the other end via a capacitor.

The shields of digital control cables should be earthed at both ends.

With regard to earth loops IEC 61000 says:
"earth Loop are allowed ... earth loops are effective mitigation measures against interferance caused by currents and EM fields from external sources"

These are euro regs, Im sure similar regs stand in the US

benjo said:

With regard to earth loops IEC 61000 says:
"earth Loop are allowed ... earth loops are effective mitigation measures against interferance caused by currents and EM fields from external sources"

Click to expand...

Hmmm. Yesterday was Pi day, and I recall that the state of Indiana legislated Pi = 3.000 in the late 1800's. A classic attempt by politicians and their fellow travelers, bureaucrats, to regulate the universe . . . .

I suppose the European regulation could, in the regulators' infinite wisdom, not allow earth loops, and somehow declare that electrons will not move through a conductor betweee two different potentials because that's what bureaucrats are good at.

But for those of us that have to deal with the physics of what happens when one grounds a long run at both ends, the question arises, OK, earth loops are allowed, what happens to the signals associated with the grounding?

My experience replicates cntrlfrk's explanation. I concur with his statements.

Dan

sirhiss2 said:

Very Interesting.

can you give me an example. maybe a quick drawing of what you mean. I've always been told not to ground at both ends because it causes ground loops. This can cause EM interference especilly for circuits with lets say 4-20 ma.which are quite common. Can you explain how grounding on both ends would prevent this? So if you were going to run an anolog i/p from a transmitter, are you saying that you would ground the screen on both ends?

Click to expand...

Cable Screening

Screened cables must be connected at both ends to the grounding bar with a large-area and if possible directly at the cabinet inlet.Good attenuation of all conducted frequencies can only be achieved by connecting at both ends.

The ff must be observed when handling the screen:

1. Use metal cable clamps to secure the braided screens with
a large-area contact
2. Avoid the use of cable with foil screens since the foil can
be easily damage by tension or pressure when fitting,thus
leading to a poorer screen effect.

Note: An equalizing current may flow via the screen connected
at both ends in the case of variations in the ground
potential.Use an additional equipotential bonding
conductor in this case.

In certain cases the screen can also be connected at only one end.Only the lower frequencies are then attenuated.Connection of the screen at one end may be more favorable if:

1. An equipotential bonding conductor cannot be laid
2. Analog signal (several mV or microA ) are transmitted

I hope this help!!!

This isn't helping me a bit.

Is there no definate answer to this. Personally I only ground one end but I have absolutely no basis for this, just what I have always heard.