Instrument Earth Bar Cable Size

→ British Standard 7671.

thanks for the response, does this standard actually reference functional earth cable sizing? or merely that standards ustilised for safety earthing need not apply to functional earthing?

Its back to Math and good physics!

A earth grounding mat is for SAFETY PURPOSES and required for mid-to high voltage OPERATIONS and requires testing the SOIL conductivity FIRST. All residential & commercial building must have a 8-16 foot X 3/4 inch, COOPER rod at the electric panel min. But this will not stop all back feed noise and spikes come from the power grid. Utilities are exempt from equipment damages including building fires. As we deploy more cpu chips in the buildings for all kinds of devises,TV, Appliances, a grounding mate is required. Any Ham Radio operator that has a old hand book for antannes will show you these soil conductivity testing methods & engineering math. Especially on-site power generating, like solar, wind, Fuel cells, etc. because you have become the power grid to the building. Keep in mind (black & Brown) power needs to have a place to go, or called a higher grounding potential for this bad power to go. We also do this for ocean going vessels--called a DINO plates on the bottom of the haul and the ships equipment is central grounded to this special plate--I also is math sized for the ships size and makes the special equipment thinking the ground is 100 times larger.

Have fun from Indonesia

I know what an earth mat is for. You have gone off topic with your response.

Not clear on your meaning of earth bar.Is this the neutral of the instrument panel?If it is the equipment ground, the size is determined by the size of the upstream circuit breaker feeding the circuit,likewise the neutral is determined by the size of the conductors and whether there will be large harmonic currents on the neutral.

A schematic would help me to help you.

I do not think we are on the same page here. The instrument earth bar is installed inside instrument panels to terminate screens/drain wires which form part of instrument signal cabling. It is not used to terminate cores used to supply the instrument with power.

Polerz,

I agree that a couple of the guest's comments missed the topic you are asking about: standards for a signal ground bar, not the standards for a power ground bar. I don't believe that the electrical codes will give an answer to your question. Instead, I would suggest you look carefully at the installation instructions for the various instruments you will have in your panel. Usually, for shielded (screened) signal cables, they will specify that you have a single point of connection to earth/ground, and that the cable shield be isolated at any other points where it is exposed. What you want to avoid is electronic and electrical noise being coupled onto this cable shield from other sources either within or outside of the enclosure. In most cases, you can have the instrument signal cables' shields be grounded at the same terminal strip where they enter the cabinet. I would rely on the use of grounding terminals for this, and rely on the metal of the sub-panel or cabinet to provide the continuity of this signal ground to the power and building ground.

The electrical codes (at least the NEC in the USA) permit the use of isolated ground conductors from equipment back to the building's grounding point. These conductors are in addition to any power ground conductors, and do not bond at any intermediate point. Size of the conductors is determined by the amount of current they could be expected to safely carry in the event of a fault. In most cases, this is a minimal amount.

Again, look at the manufacturer's literature and installation instructions, and let them guide you.

--JMM

From the review article, "Control System Grounding - Part 2: Ground Wiring, Shield Grounds and Power Supply Grounding" a review of the book "Control System Power and Grounding: Better Practice".

You might consider the implications of DC power supply grounding, since it directly affects the 2 wire loop powered instruments.

DC Return Ground Wiring
Ensure that DC return grounds are on separate wires from each power supply to the enclosure local ground reference point. The DC return ground connection at the power supply is the negative (") DC terminal. Where bulk power supplies feed a DC-to-DC controller and I/O power supply, the DC return is placed between the carrier ground terminal and ground reference.

The recommended wiring for these ground points is AWG 8 (8.4 mm2) copper, stranded, insulated with the lengths being as short as possible. The return ground wiring should be insulated to avoid unintentional ground loops that can occur if bare wires touch the metal enclosure frame, or each other. Proper isolation from metal should be verified by electrical measurement after installation.

The second web page deals with grounding the shields:

Shield Grounds
Proper field-wiring shield grounds help ensure proper system operation by reducing electromagnetic interference in signal wiring. A proper shield ground path is to ground the shield at one end only. Grounding at both ends can cause ground loops. The preferred end is at the carrier shield bar. If field transmitters or final control elements are well-grounded four-wire units, shield wires can be grounded to the units instead of the carrier shield bar.

Note ... The carrier shield bar is connected to the enclosure frame unless the control system occupies only one enclosure, in which case, the shield bar may be connected directly to the isolated common ground reference.

The recommended wiring between the shield bar and its ground termination on the enclosure frame, or the isolated common ground reference, is AWG 8 (8.3 mm2), copper, stranded with length as short as possible. The wiring should be insulated to avoid unintentional ground loops that can occur if bare wires touch the metal enclosure frame, or each other. Proper isolation from metal should be verified by electrical measurement after installation.

To obtain the intended electromagnetic interference (EMI) protection, it is important that the insulation on shield wiring be properly stripped at the ground end and that it is properly wrapped at the ungrounded end. Figure 4-8 illustrates proper insulation stripping and wrapping.

http://www.eetimes.com/design/automotive-design/4013766/Control-System-Grounding--Part-2-Ground-Wiring-Shield-Grounds-and-Power-Supply-Grounding

Hi!

Firstly, I would suggest you to have a separate earthing system for your instrument system to prevent it from getting buggered by electrical spikes which would damage your cards and other electronic components. That "the functional earth cables need not comply with safety earth cabling regulation" is a tricky guideline and dangerous too especially when you are connecting instrument earthing and electrical earthing together. Therefore, the size will be determined by system fault current you are referring to.

BB Raina

Thanks for the responses, seems like a controversial issue. I will ensure all supplier requirements are met with our installation.

I believe it should be big enough physically to take of any associated mechanical stress as well as to sink in all unwanted signal noise and to avoid creating secondary current loops generated by your instrumentations.