Instrument Earth and Plant Earth


Thread Starter


Could you someone please advise whether there are any guidance related to grounding/earthing cable routing inside the control cabinets room?

1) Is it allowed to route instrument/plant earthing cable in the same tray with instrument cables (non-I.S, 24V)? The instrument cable have screen and armour. If not what shall be the minimum distance?

2) Is it allowed to route instrument and plant earthing cables (insulated) in same cable tray?The cables are going to be route from control cabinets to the instrument and plant earth busbar in the control room. If not what shall be the minimum distance between instrument and plant earthing?

Plant earthing cables and instrument earthing cables are going to be connected to plant earth busbar and instrument earth busbar respectively.

In all of the plants and control rooms and marshalling cabinets/rooms I have worked in, the earthing cables attached to the cabinets and ground bus bars were allowed to be in the same cable tray <i>in the same panel as the instrument cables.</i> In other words, for a very short distance. Every effort was made to try to make the entrance of the earthing cables into the control panels as perpendicular as possible to the instrument cables/cable trays, but where necessary--and only for short distances (1 meter or less, usually) the earthing cables were allowed to be in close proximity to the instrument cables.

Earthing cables were not generally run in the same cable trays outside the control panels as instrument cables. Again, usually the earthing cables were run separately, and where they "crossed" instrument cable trays they were usually made to cross at a right angle (perpendicular to) the cable trays.

Most earthing cables come out of the floor/concrete and are run up to the bus bars or cabinets, again, usually at right angles to the instrument cables. This is true of false floor and vault designs (under the control panels). I have rarely seen earthing cables run in cable trays, and when it was done it was usually done for short runs and the earthing cables and other cables in the tray were separated as much as possible (so, usually, cable trays with as few cables/conductors as possible were selected.

I have seen plant earthing cables bonded to cables trays, on the outside of the cable tray (not inside the cable tray, and particularly not if the cable trays have metal covers).

Finally, in most cases I was not aware of any governmental or regulatory body standard--only the standards in the plant or the Customer's standards, and just good common-sense construction practices.

Hope this helps! It's probably more anecdotal than you were looking for, but it based on more than three decades of experience. (As a member of commissioning teams, or the commissioning person, I was never usually on site during construction. But I'm party to more than one (dozens) of meetings about grounding practices--AFTER the fact, unfortunately. It's something that's generally not well managed and often overlooked during construction--until it becomes a problem during commissioning or afterwards during the warranty period.
I don't believe that there is any code or standard that directly addresses this. Grounding issues are always complicated by many factors. You are not providing enough information so that we may help you.

1. Why do you want to route these cables in an instrument tray?

2. What kind of cable tray are you talking about? Under the floor or other?

3. Is your plant earthing ground an equipment ground or a grounding electrode conductor? Cabinet ground?

4. Were do the ground cables come from?

5. Where are the plant earth busbar and instrument earth busbar located in reference to the "source" of the grounding cables?

6. How are the instrument cable's armor and screen grounded? To what in relationship to the two grounding cables?

William (Bill) L. Mostia, Jr. PE
ISA Life Fellow,
Winner of the 2018 ISA Raymond D. Molloy Award
Sr. Functional Safety Consultant
SIS-SILverstone, LLC.

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