Which end of the shield on instrument cable needs to be grounded?
Which end of the shield on instrument cable needs to be grounded?
(OP)
For example, a 4-20mA pressure transmitter in the field is wired to a PLC with a #16AWG shielded twisted pair. One end of this cable is grounded, the other is left floating.
What is the purpose of this? And which end is supposed to be grounded? Is there a document that says to do this? Is there a Code requirement?
What would happen if both ends were grounded? Neither?
What is the purpose of this? And which end is supposed to be grounded? Is there a document that says to do this? Is there a Code requirement?
What would happen if both ends were grounded? Neither?






RE: Which end of the shield on instrument cable needs to be grounded?
RE: Which end of the shield on instrument cable needs to be grounded?
2. If that isn't possible - bond.
3. Connect shields of both ends to ground. Always.
4. Ignore all fairy-tales about "hum loops" and such.
Waiting for corrective comments - which I do not intend to read
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Which end of the shield on instrument cable needs to be grounded?
If you intend to only ground the cable shield at one end it should be done at the end where the circuit wiring is ground referenced. This will only give you protection against capactive coupling at low frequencies... this is why it is not recommended.
RE: Which end of the shield on instrument cable needs to be grounded?
The 1% rule is not very good when cables are getting longer than ten or twenty feet. The general rule is that anything longer than 1" (like a pig-tail) should be avoided. And best practie would be to clamp the screen to a "clean" ground rail. Or use a 360 degree gland.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Which end of the shield on instrument cable needs to be grounded?
RE: Which end of the shield on instrument cable needs to be grounded?
RE: Which end of the shield on instrument cable needs to be grounded?
RE: Which end of the shield on instrument cable needs to be grounded?
You may find recommendations in the manufacturers literature.
That said, Gunnar may be correct about the actual results.
Bill
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"Why not the best?"
Jimmy Carter
RE: Which end of the shield on instrument cable needs to be grounded?
4-20 mA loops are inherently more or less immune to interference and can be hardened in simple ways (HF chokes, capacitors, isolation amplifiers etcetera) and that makes the "Ground only one end" possible. But I wouldn't even think of it in other applications.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Which end of the shield on instrument cable needs to be grounded?
RE: Which end of the shield on instrument cable needs to be grounded?
I question this practice of single point ground for shields on CT and control wiring in the 230 kV and 345 kV substations. I think the consensus is 50/50. Half say grounding both ends is the only way, others do as we do and say single point grounding works. We haven't been alerted to any problems with the substation wiring, but maybe that's because we haven't had a major fault that would induce the noise.
Many years ago we had noise on multi-pair communication cables for a new distributed control system using FSK communications. Each shielded pair's shield and drain wires were grounded at both ends. I was troubleshooting with a sound powered headset trying to identify individual pairs by talking with another engineer at the other end. I could not tell which pair he was connected to because the signal was just as strong on all 24 pairs. When we isolated the shields to a single point ground the cross talk disappeared and many of the communication error messages went away.
Until experience proves otherwise: I go with single point shield grounds at one end on instrumentation cables. (Almost all communications are over fiber).
RE: Which end of the shield on instrument cable needs to be grounded?
or
Keith Cress
kcress - http://www.flaminsystems.com
RE: Which end of the shield on instrument cable needs to be grounded?
but with true differential signals (one side of sig not grounded) then the single ground point adding circulating ground current voltage drop Vcm is not true.
another reason why most of today's recommendations is ground both sides.
I was taught the prevailing 1960's thinking: ground source side only, to keep noise OUT; ie., electrostatic shielding for small level control signals (60hz hum etc), vs ground both sides to keep noise IN (electromagnetic shielding) for noisy radiating things like motor leads & RF transmission lines.
Used to be we shielded against that awful 60hz hum everywhere and power sigs from SCRs switching in the 60-1800hz arena.
I believe the advent of ultra high speed switching circuits (sub 0.1 usec,, aka 10-100-300+ mhz) is what has caused the majority of folks mfgring stuff needing shielding to change from 1960 thinking to today's recommendation to shield EVERYTHING at both ends. And with stripping pvc back and wire tying exposed shield to backplane ground. Doing the math one can see a short 6-12" little #12 wire pigtail from shield to ground has a 1/4 wavelength in the area of these switching frequencies and their harmonics, meaning their resistance at those freqs are infinite so might as well not bother. Our engineers began recommending all signal and power leads grounded both ends about 10 yeara ago and we never looked back. I think this is new enough that it has not become the new standard everywhere yet, but will. And with all those older engineer books in print, the signal side grounding will still take a long time to go away. My opinion.
www.KilroyWasHere<dot>com
RE: Which end of the shield on instrument cable needs to be grounded?
from SCRs switching in the 60-1800hz arena.
meant to be:
from SCRs switching in the 60-180hz arena.
www.KilroyWasHere<dot>com