hi friend

i have one confusion: why we should not did grounding on both side of shiled thermocouple? i heard that if we did it that then ground loop currunt flow in side matel. what is effect of this groundloop current?

thank you

 

Basically all grounds are not created equal. If you connect both ends of a shield to ground, current will circulate in the shield due to the difference in potential between the two different grounds. This current can induce noise on the low level thermocouple signals as well as on other types of signals. The voltage gradient across the shield may also defeat the intent of the shield to provide a shield at zero potential relative to the signal wires. If there is sufficient current, say under lightning strike conditions, you may even burn up or damage a cable.

Bill Mostia
=================================
William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation
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These opinions are my own and are offered on the basis of Caveat Emptor.

Bill Mostia

 

Electrical signals which pass between one area of this plant and the other where this huge ground loop of yours has formed, will probably not work too well, however many places you ground the screen. Good engineering practice requires the use of equi-potential bonding conductors between potential islands in the electrical control system, to equalize potential differences like the one you have described. If you take a look at the Emerald book series, you will find one covering the subject of earthing and bonding of sensitive electronic equipment and it covers the principles of earthing system design.

It is commonly known that the EMC inducted at high frequencies is not attenuated appreciably by earthing at one end of the screened cable only, however in a pure analog environment this is unlikely to cause a significant problem, however if the analog signal is connected to a PLC it may very well introduce a source of high frequency noise into the PLC cubicle, and this may very well result in the malfunction of the PLC and other high speed digital equipment in this environment.

With the use of high speed digital processing equipment it is advisable to assume that high frequency (30mhz and up) noise is always present and if you don’t take precautions to earth properly regardless of whether signals are analog or digital then sooner or later you will pay the price, this is one instance where prevention is far cheaper than cure. I am still not advising you to earth your analog signals at both ends, as this is generally unnecessary however what you are well advised to do is to get assistance in designing a sound earthing system, from someone who does this sort of thing, you only have to do it properly once to get the idea of how to think about these problems, and the investment in a bit of tutoring will ultimately pay off.

And by the way the conductance of the earth is far better understood as the conductance of ground water within the earth, ever seen an electrical instrumentation contractor knocking meter upon meter of copper rod into the ground, well he wasn't looking for a piece of conductive turf, I promise you, and by the way, if the original builder of the plant had followed good engineering practice in the first place and layed down a good earth mat when the plant was built, the contractor wouldn't be there in the second place, a common scenario in plant refurbishment in the 80's as control systems turned more and more to digital.

Regards Donald Pittendrigh

 

Ground looping is current flowing through cable[comunnication] connected beteween two sites with different ground s potential. This current flow and disturbes communication. Solutions- connect between sites with fiberoptic cables.

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