Does shielded cable shield need to be earthed?

[SUNRAY]

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

I don't quite get where this may apply, a ground loop is a ground loop, it shouldn't make any difference if it's the cables screen or a separate wire. If a signal is reliant on a 'ground' connexion (for want of any other description), I'd have thought one would not want to dissassociate that wire from the signal path and potentially risking eddy currents/other interference not cancelling.
Do you have an example of the situation you describe please?

 

[Harry Bloomfield]

I don't quite get where this may apply, a ground loop is a ground loop, it shouldn't make any difference if it's the cables screen or a separate wire. If a signal is reliant on a 'ground' connexion (for want of any other description), I'd have thought one would not want to dissassociate that wire from the signal path and potentially risking eddy currents/other interference not cancelling.

It's common sense really - No low level signal should be screened by a cable where the screen can carry any current, because that current will induce noise onto those signal cables. If the piece of equipment at the remote end needs an earth, then you provide that earth via a separate means.

I must admit I once ignored that rule, installing prototype vibration monitors at a power station, on the big generators. They used a sort of tuning fork arrangement, inside a coil, inside a heavy duty cast box. I connected the sensor to the measuring instrument with two core screened, with the screen connected to earth at both ends. The small voltage difference between the generator and the instrument panel next to it, was enough to induce unwanted noises voltage onto the two cores and swamp the instrument. Cut the screen back - basically I nicked the screen through twice, half an inch apart, close to the sensor head, removed the screen and covered it with self amalgamate tape. Sensor head casting was earthed by being bolted to the massive generator casing.

Once that was done they worked fine and proved to be very sensitive and reliable. Idea was, if any undue vibration was detected, it triggered a warning alarm at the power station control desk.

 

[SUNRAY]

It's common sense really - No low level signal should be screened by a cable where the screen can carry any current, because that current will induce noise onto those signal cables. If the piece of equipment at the remote end needs an earth, then you provide that earth via a separate means.

I must admit I once ignored that rule, installing prototype vibration monitors at a power station, on the big generators. They used a sort of tuning fork arrangement, inside a coil, inside a heavy duty cast box. I connected the sensor to the measuring instrument with two core screened, with the screen connected to earth at both ends. The small voltage difference between the generator and the instrument panel next to it, was enough to induce unwanted noises voltage onto the two cores and swamp the instrument. Cut the screen back - basically I nicked the screen through twice, half an inch apart, close to the sensor head, removed the screen and covered it with self amalgamate tape. Sensor head casting was earthed by being bolted to the massive generator casing.

Once that was done they worked fine and proved to be very sensitive and reliable. Idea was, if any undue vibration was detected, it triggered a warning alarm at the power station control desk.

Well no it's not common sense as there are situations where the screen is connected both ends, even if both ends are earthed, one that instantly springs to mind is DMX where it may be connected to many tens or hundreds of devices, another is the intercom I described previousely however that is only connected to ground at the power supply (and attempting to add any more ground points WILL cause serious ground loop problems).

I have installed many of that style of vibration sensor and agree the screen connexion is not required at the sensor, in fact there is not normally any requirment for any earth at the sensor or for that matter a 'functional' earth terminal. I cannot of course comment on any sensors I have no experience of, particularly a prototype/specialist product.

Connecting devices together need to be individually assessed; for example 2 audio devices such at a mixer desk at the rear of an auditorium and the amplifiers nearer to the loudspeakers and the Hifi/AV system at home may be manufactured/wired very differently despite the apparent similarities.

EDIT: To make more sense

 

[SimonH2]

The screen should never be used as a functional wire, only as a screen.

Hmm, better tell the people who make and use phantom power audio equipment OK, it's (supposed to be) a balanced signal, and ideally the remote end should not be separately earthed, but the power is fed down the signal wires, with the screen as the return.

 

[Harry Bloomfield]

Hmm, better tell the people who make and use phantom power audio equipment OK, it's (supposed to be) a balanced signal, and ideally the remote end should not be separately earthed, but the power is fed down the signal wires, with the screen as the return.

Any current flowing down a screen, has the opportunity to induce voltage and current into the conductors it is supposed to be screening, therefore the obvious solution is to prevent that form happening by cutting it at the signal source end. Much depends upon the magnitude of the signals within the conductors being screened and how serious the effect of induced signals from the screen.

 

[SUNRAY]

Any current flowing down a screen, has the opportunity to induce voltage and current into the conductors it is supposed to be screening, therefore the obvious solution is to prevent that form happening by cutting it at the signal source end. Much depends upon the magnitude of the signals within the conductors being screened and how serious the effect of induced signals from the screen.

As has now been pointed out there are multiple situations where the screen forms a functional conductor by design. Interference reduction by the performance of the twisted pair far exceeds that of the screen.
Generally the interference caused by a ground or earth loop has nothing to do with the signal or wiring contained within the screen, the 'extra wire' you mentioned way back is just as likely to cause that interference, in fact one doesn't even need a signal passing between those 2 points of a system for interference caused by a ground loop.

 

[SimonH2]

And the result doesn't have to be "an 'orrible 'um". When I was at university I recall us being in the lab one day while our tutor was teaching us something or other I can't recall - must have been something to do with audio amplification and had various bits of kit on the bench linked by bits of wire. But without any input signal, it was talking to us - I was able to say that I recognised the voice as one of the local radio station presenters.

 

[JohnW2]

And the result doesn't have to be "an 'orrible 'um". When I was at university I recall us being in the lab one day while our tutor was teaching us something or other I can't recall - must have been something to do with audio amplification and had various bits of kit on the bench linked by bits of wire. But without any input signal, it was talking to us - I was able to say that I recognised the voice as one of the local radio station presenters.

Indeed, keeping Radio 1 and local radio stations out of audio equipment often used to be (maybe still is for some?) far from easy - and I've certainly known people living close to BBC transmitters for whom it was almost impossible. Similarly with amateur radio transmissions, but that's much more of a 'local' issue.

Kind Regards, John

 

[Jackrae]

I recall reading an article many years ago about the 'special' WW2 transmitter located near Penrith, whereby, when atmospheric conditions were right 'rectification' in the atmosphere resulted in audio being heard from the heavens. No wires or screening required

 

[Harry Bloomfield]

And the result doesn't have to be "an 'orrible 'um". When I was at university I recall us being in the lab one day while our tutor was teaching us something or other I can't recall - must have been something to do with audio amplification and had various bits of kit on the bench linked by bits of wire. But without any input signal, it was talking to us - I was able to say that I recognised the voice as one of the local radio station presenters.

Yep, there is a similar problem 'caused' by local radio amateurs. In fact it is usually the case that the suffering equipment has inadequate or poorly designed screening.

 

[JohnW2]

Yep, there is a similar problem 'caused' by local radio amateurs. In fact it is usually the case that the suffering equipment has inadequate or poorly designed screening.

Indeed so (as I said above), most commonly in relation to TV ("TVI"), rather than radio, reception

However, the 'inspectors' in the days of old had a horrible habit of pointing fingers at the radio amateur's equipment rather than at the receiving equipment, which was usually what was primarily to blame.

Fortunately, despite the receiving equipment being the problem, it was often aerial-fed, in which case it could often be solved by putting a filter in the aerial lead. When the problem was due to direct pickup by (as you say, inadequately screened) receiving equipment itself, that was far more difficult to address.

Kind Regards, John

 

[SUNRAY]

One day in 1983 as I pulled up outside my first house I was talking on a 2 way radio and sat there for several minutes in conversation. A neighbour came out guns blazing complaining I was wiping the picture completely off his TV, to a black screen. I suggested I had a quick look at his installation but he refused, I offered a filter which I had in the vehicle, initially he refused saying it was my faulty kit not his.
Following day I asked Post Office Interference Service (I happened to be working in the same building) to check my vehicles installation and received a clean bill of health and he even gave me a completed report form.
That evening almost the same thing happened, neighbour came out complaining, again I offered a filter and again he refused stating he would get PO on the case.
Read this says Sunray... So how does this filter work? asks a deflated neighbour...
He took it and inserted it between aerial and TV, came back out saying I just wiped a single line across the middle and he was happy with that.

Yes, Yes, Yesssss result runs through Sunrays head.
The conversation then continued with He'd been having the same problem since CB became popular... I'm not sure how that made it my fault but of course, it still was.

Some many months later I saw him and asked how he was getting on with the filter, he said his TV had stopped working and had to replace it. The old TV was still standing in the front garden sans rear cover... a 405 TV complete with the 625 converter scewed to the side. 405 line transmissions had been discontinued some 10 years or so before... and the interference was my fault?

 

[mattylad]

I did mention something about this subject being a minefield and lots of different situations call for different answer to the question of where to bond the screen.
They may all be valid.
Then you may step on a mine...

 

[SimonH2]

Going back "a bit" I had a friend who was a radio amateur, and lived in a terrace. Similar to the above, his neighbour had TVI problems - each time he came banging on my friend's door he'd direct him to the Post Office to fill in a complaint form.
This went on for a bit, and eventually my friend got fed up and filled in a form on the neighbour's behalf. The neighbour was much more uoset then - he didn't have a TV licence !
As a postscript, my friend was in the post office about a week later - and the lasy behild the counter casually mentioned that she'd sold a dozen TV licences in his street that week.

 

[RandomGrinch]

I am now reminded of this :

Internet: Old TV caused village broadband outages for 18 months

Every morning at 7am when the second hand TV was switched on, the village internet went down.

www.bbc.co.uk