Industrial Control Panel - Ground Bus

[Designer_82]

I'm working on a project with industrial control panels housing plc's, I/O modules etc.

There is a safety ground bus on these panels.

Should there also be an isolated instrument ground bus?

 

[davidbeach]

Ground is ground. Lots of interesting, but wrong, ideas float around about "clean" grounds, etc. But that "isolated" bus is either isolated or it's ground, but it can't be both. It can be connected to ground at a single point where any standing power system ground currents couldn't find a path through the instruments. But ground is ground.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

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.

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.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

@ Mr waross (Electrical)23 Mar 23 19:00
"....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..."
1. I agreed in full to the learned advice.
2. There is a typo error ? [ The equipment ground bus must be connected to the equipment grounding bus....]
3. In all cases, the distance between the "isolated instrument ground bus" and the " equipment ground bus " should be kept as short as possible.
3.1 Possible AWG 2/0 (67.43 mm[sup]2[/sup] ) is fine as it is of very short length. I am of the other extreme that a say AWG 13 (2.63 mm[sup]2[/sup]) would fulfill the purpose?
Please advise
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che;
Your suggestion is defensible and valid in many instances, from an engineering viewpoint.
However the code prevails..
Our code specifies the size of a grounding conductor by the ampacity of the largest ungrounded conductor.
Grounding conductors for instruments fed by #14 AWG or #12 AWG copper conductors may be grounded by the use of a #14 AWG copper conductor.
But when an equipment grounding bus is supplied for general equipment grounding, it is sized on the by the ampacity of the largest supply conductor.
For busbars,

Ampacity of largest
ungrounded busbar
not exceeding... (1000 Amps)

For 1000 Amp busbars, the table requires #2/0 AWG copper cable.
The dedicated instrument grounding bus is considered an extension of the equipment grounding bus and must be grounded by 2/0 AWG copper.
We may discuss the code requirements, but arguing with the code is futile.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Mr waross (Electrical)24 Mar 23 01:01
".....The dedicated instrument grounding bus is considered an extension of the equipment grounding bus and must be grounded by 2/0 AWG copper.... "
In this case:
(a) the "isolated instrument ground bus" shall be of the same size as the " equipment ground bus " ?,
(b) the conductor connecting the "isolating instrument ground bus" to the "equipment ground bus" shall be sized equal to the "equipment ground bus", instead of AWG 2/0 ?
Note: the "isolated instrument ground bus" is not expected to carry GF short circuit current.
Please advise
Che Kuan Yau (Singapore)

 

[waross]

The choice of ampacity for a ground bus conductor is a matter for both code and local practice.
For illustration, consider a motor fed by 3c #12 AWG TECK cable or tray cable.
Such a cable typically includes a #14 AWG equipment grounding conductor.
In the event of a grounded motor winding, the fault circuit becomes a #12 AWG phase conductor in series with an effectively equal length of #14 AWG equipment grounding conductor.
In Canada 600/347 Volts is a common industrial supply voltage.
In the event of a grounded motor winding, the motor housing will rise to a voltage greater than 347/2 Volts or greater than 173.5 Volts.
In a large plant, the cable run to a motor may be so long that a fault current is not enough to clear on the instantaneous trip, but rather hold in until the thermal elements trip.
For that reason, in the local petro-chemical plants and heavy oil upgraders, it is common practice to augment the code ground with a second larger grounding conductor.

With this in mind, consider the case of a grounding bus bar in the event of a fault current.
There will be a voltage rise on the bus bar proportional to the impedance of the conductor from the system ground to the equipment grounding bus bar.
This will manifest as a common mode voltage rise on the cases of all instruments and equipment grounded to the equipment grounding bus.
You may argue from an engineering point of view that the voltage drop (or rise) across a short bus bar grounding jumper will be negligible and voltage rise will only be dangerous in the instance of long, undersized bus bar grounding conductors.
Fair comment, but;
The code does not consider the length of the bus bar grounding conductor.
The grounding conductor must be selected from the code table, based on the supply ampacity, regardless of the type or size of the equipment intended to be connected to said bus bar.
It's the code, and local engineering practice.
The code is a minimum.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

I think you could successfully argue that in a panel the grounding point for some instruments doesn't need to be 2/0 or anything close to that size.

 

[waross]

Yes, Lionel. for example, in a 100 Amp panel, the grounding bus size is based on the 100 Ampere rated ungrounded conductors.
Our code table #16B requires a minimum #8 AWG for bonding a 100 Amp ground bus.
I am referring to an installation where a dedicated instrument ground bus is daisy-chained from the ground bus of a 1000 Amp MCC.
In this instance, looking upstream to the source rather than downstream to the equipment may justify the larger jumper.
Also, in the petroleum plants that I have seen it is common to see grounding conductors many times larger than the code minimum.
This larger grounding jumper on motors is not an NEC requirement and is not governed by the NEC.
It is typically connected to the skid of which the motor is a component.
It has a low enough impedance to hold touch potentials on a faulted motor housing to very low levels.
As I have pointed out, a code compliant ground on a motor may allow the touch voltage on the motor housing to approach or reach over 1/2 of the voltage to ground of the supply until the circuit is cleared.

I have been involved in years of installing and troubleshooting petro-chemical installations, but while I have done design work, I have not done designs to American Petroleum Industry code or standards.
When I have queried grounding jumper standards, I have been referred to the API codes.
I don't have a copy of the API code and I am not going to spend $250 for a copy just to illustrate a point on a free website.
I wouldn't expect you to do so either.

And in addition to code minimums, one of the stated purposes of grounding and bonding is to limit touch and step voltages to safe levels in the event of a ground fault on a circuit.
It may easily be shown there are examples of code compliant grounding that WILL NOT limit touch voltages to safe levels on 480/277 and 600/347 Volt installations.
In final explanation of your argument:
From a 100 Amp supply, minimum #8 AWG jumpers.
From a 1000 Amp supply, a minimum of #2/0 jumpers.
Per Canadian Electrical Code, table 16B.
Is there a similar table or rule in the NEC?
Respectfully, Lionel and Mr Che;
Thank you for giving me the opportunity to more fully explain my previous post.
Yours
Bill






--------------------
Ohm's law
Not just a good idea;
It's the LAW!