Does shielded cable shield need to be earthed?

[ScottishGasMan]

Hi all.

For ELV (20v) Shielded cable (2 Core-0.75) used for bus communication (20Vdc power and signal) need to have one or both ends of the shielding connected to earth to make it effective at preventing induced currents from nearby mains cables?

 

[Jackrae]

Only one end. If you 'earth' both ends you induce circulating current or voltage noise which defeats the purpose of the shielding (with respect to noise)
The 'earthing' should be at the end where the signals are tuned/amplified/conditioned and the connection should be to the point defined as the "signal ground" which may well be the chassis of the equipment.

 

[SUNRAY]

Only one end. If you 'earth' both ends you induce circulating current or voltage noise which defeats the purpose of the shielding (with respect to noise)
The 'earthing' should be at the end where the signals are tuned/amplified/conditioned and the connection should be to the point defined as the "signal ground" which may well be the chassis of the equipment.

That's a very broad assumption you're making.

The answer is to check the MI for the equipment, it is possible the screen is a functional wire, especially as it's providing the 20V power.

 

[Jackrae]

That's a very broad assumption you're making.

The answer is to check the MI for the equipment, it is possible the screen is a functional wire, especially as it's providing the 20V power.

It is indeed a broad assumption; but if the shield is used as part of a power supply circuit then it won't act as a 'screen' to eliminate noise pick-up from adjacent cables. If anything, it'll act as an antenna to pick up noise.

 

[mattylad]

The answer is yes and no.
Different people have different views about it because they are considering different things.

I have just been through the very same question about the shielded cables we use at work to try and get a company policy on what we should do and in the end we either do what the customer requests, follow the manufacturers instructions or use grounding the shield at the source end only.

If you google it there are some very technical explanations about why to ground it both ends and the same why not to.
It is suggested that simply grounding it at one end can sometimes be incorrect.

For audio signals it is suggested to ground at only 1 end to prevent ground loops that then cause noise on the signal.

For EMC and if it is not audio and there is also a ground wire in it then it is suggested that best to ground it at both ends, some manufacturers also specify this for their equipment, I Had a Variable Frequency Drive this week that did.

I often install gas usage meters with metal connectors on that need the shield connected to it, this is then passed into the meter itself.

So it's all dependent on the individual situation, of which only you can make that decision.

When grounding at 1 end only, this must be at the controlling/source device end and not the far outer end.
The screen should never be used as a functional wire, only as a screen.

Research it some more - it's a minefield.

 

[Jackrae]

If SGM gave a fuller description (with diagrams) on what he's working on then we might be better placed to give a definitive answer

 

[SUNRAY]

It is indeed a broad assumption; but if the shield is used as part of a power supply circuit then it won't act as a 'screen' to eliminate noise pick-up from adjacent cables. If anything, it'll act as an antenna to pick up noise.

Possibly, possibly not.
As a simple example in entertainment we use intercoms, I'm sure we've all seen camera operators on TV wearing a headset?, that is run on a twisted pair cable with volts on one wire, unbalanced audio on the other and a functional screen. Those are looped from pack to pack and on a job could easily have a dozen or more and hundreds of meters of cable involved.

As I mentioned straightaway check the MI's for the kit concerned.

 

[ScottishGasMan]

Well I'm glad it's not as cut and dry as I thought it would be and not just me being thick.

FYI. It's for heating system controls. Its a 2 Core bus cable, that runs at 20Vdc to power the displays of the controls, but that also sends signal down the same two cores (Basically 20v power, that will jump between 20v, and 15/17v or there abouts to send the 1s and 0s of signal) If there run alongside mains cable for too much of a distance, the signal can be affected, I assume because of induced voltage but possibly I'm getting my terms mixed up and "noise/interference" is something different.

The shielded part for the cable is purely to cut down on this noise/interference/induced voltage if the bus cable and mains cables cannot be separated by a proffered distance.

The bus cable caries 2-way signals so the controls talk back and forth with the boiler, but generally the boiler is the power supply for the bus signal.

 

[JohnW2]

FYI. It's for heating system controls. Its a 2 Core bus cable, that runs at 20Vdc to power the displays of the controls, but that also sends signal down the same two cores (Basically 20v power, that will jump between 20v, and 15/17v or there abouts to send the 1s and 0s of signal) If there run alongside mains cable for too much of a distance, the signal can be affected, I assume because of induced voltage ....

Have you actually experienced such problems? From what you describe, I would be rather surprised.

Kind Regards, john

 

[Jackrae]

Based on your description, and lack of manufacturer installation instructions, shield at boiler end only.

Others may disagree

 

[Simon35]

Have you actually experienced such problems? From what you describe, I would be rather surprised.

Kind Regards, john

Same. I imagine this is pretty robust comms with or without shielded cable.

 

[Harry Bloomfield]

Have you actually experienced such problems? From what you describe, I would be rather surprised.

Kind Regards, john

Me too, they are not exactly high speed comms and the signalling is fairly robust, so no special need for screening, but if screening is used then earth at one end the boiler end.

 

[Harry Bloomfield]

Me too, they are not exactly high speed comms and the signalling is fairly robust, so no special need for screening, but if screening is used then earth at one end the boiler end.

Standard practice is to have the braid cut at the far/remote end, then if an earth is needed is to add a separate wire just for the earth.

 

[SUNRAY]

Standard practice is to have the braid cut at the far/remote end, then if an earth is needed is to add a separate wire just for the earth.

In what environment is that standard practice?

 

[Harry Bloomfield]

In what environment is that standard practice?

Any environment where ground loops along screened sensitive cable can pickup interference.