That is a pet peeve of mine.
The "isolated" or "clean" bus.
I have been watching the misinformation of a "clean" ground bus develop over the last 60 years.
60 years ago, one of the first "ground' issues to appear was problems with data links when Point Of Sale cash registers started to transition to terminals connected to a central computer.
The electronic designers, however brilliant they may have been had no experience with the reality of field wiring as it existed in the 60s and 70s.
The electronic engineers wrongly assumed that there was zero impedance between any and all grounds.
They assumed that any ground symbol on a drawing was a perfect ground.
They saved a wire by always using a ground return for data signals.
What could go wrong?
Plenty.
Equipment grounding in commercial buildings was often non-existent or ineffective.
Grounding methods in the 50s were a work in progress.
The the equipment grounding methods in those old buildings, (the newer buildings that did have equipment grounds) were prone to failure over time.
The code at that time required commercial wiring to be metal enclosed.
The common methods were Electrical Metallic Tubing (EMT) or BX (An early form of armoured cable).
EMT.
A common method of coupling EMT joints was a sleeve that was indented in four places on each tube.
When the contractor finished his work this provided a good ground, but some years later the grounding path would not be dependable.
BX
The old BX, was a spiral armour of galvanized steel or aluminum.
There was no grounding conductor.
In time, and a little surface corrosion, and the turn to turn contact would be lost.
A line to ground short may or may not pass enough current to trip a breaker, but data didn't fare well.
The data signal would spiral down the armour rather than taking a straight path across the joints.
So, there would be a high inductive impedance at data frequencies.
When there was a data transfer issue in an installation, an electronic engineer would often "scope" the ground at the terminal.
He would often see a lot of hash, rather than a straight zero line.
Hence the erroneous assumption that he had a "dirty" ground.
Actually, he had no ground or a high impedance ground that was picking up induced noise.
Interestingly, the code at that time stated that "No device shall depend on a ground connection for it's operation." (There were a couple of obvious exceptions, such as ground metering equipment.)
A simple rule in the code was no match for the data transfer industry culture.
With my career with multiple contractors, I was always in the field and encountered multiple instances of bad grounds.
It was never a "dirty" ground, it was always poor grounding integrity or, more often, something not related to grounding.
By the 70s, code grounding methods had developed to the point that a good code ground had adequate integrity for data transfer.
But back to industrial instrumentation.
There is still a lot of confusion based on old myths concerning dirty grounds.
In many of the large plants that I have worked at, there may be several kilometers of "high line".
That is pipe trains, or elevated piping structures.
These structures are supported on screw piles.
The screw piles form the main plant ground.
In a large plant there may be several hundreds of tons of steel forming the main ground.
In addition, it is common for equipment pads and structures to be surrounded by a buried bare copped conductor as a guard against touch potentials in the event of a major ground fault.
Add to that the instruments ground, The "magic triangle".
That is three ten foot ground rods spaced in a ten foot triangle.
This does provide secondary protection in the event that the main ground is lost for some reason, but does nothing day to day.
Best practice:
Use the instrument ground bus for all instrument data and shields, but not instrument cases. This reduces the possibility that a major fault elsewhere will cause a potential difference between instruments.
The power electricians will, according to code, install a heavy copper jumper between the dedicated bus and the equipment ground bus.
This gives you the ground protection of tons of steel.
In the event that this connection becomes compromised, the "Magic triangle" will provide back-up protection.
"Dirty Ground". I consider that a misnomer used to describe a misdiagnosed poor or missing ground connection.
OP said:
Should there also be an isolated instrument ground bus?
Not a bad idea.
The safety ground bus or equipment grounding bus should be used to ground all instrument cases.
Data circuits, if grounded should be grounded to the "isolated instrument ground bus".
Drain wires of shielded cables should be grounded to the "isolated instrument ground bus".
The equipment ground bus must be connected to the equipment grounding bus by a heavy copper conductor. Possible 2/0.
And, using the "isolated instrument ground bus" will avoid any criticism from any "Old School" "Dirty ground" believers.
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Ohm's law
Not just a good idea;
It's the LAW!
