Grounding Scheme for DCS Marshalling/Rack Room

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rossbaykvlt

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Tashkent, Uzbekistan
Hi. My name is Adham and I am an I&C Engineer from Uzbekistan. We are in the process of commissioning a DCS at our plant and I have been tasked with overseeing the design of its grounding network. The DCS has two rows - row 1 is made up of the power distribution cabinet and the marshalling cabinets while row 2 is made of the system cabinets and server cabinets which house the DCS controllers and server equipment etc. The only problem is that neither I nor the contractor in this case has the adequate knowledge or experience when it comes to grounding. My only experience with grounding is the electromagnetic compatibility course I took in university. So I would request you guys to please have a look at the scheme we have developed and let me know if any improvements can be made. The scheme is as follows.

A grounding bus which will be achieved by using a series of grounding bars bonded together via 25 mm2 insulated grounding wires. The bus will be made part of the external grounding system using the same 25 mm2 insulated grounding wire. The external grounding system is a grounding rod installed just outside of the marshalling room. Said grounding bus will be installed at the periphery of the cabinets in a U shape and individual connections to each cabinet will be via 4 mm2 insulated grounding wire. The 4 mm2 in this case is a limitation because the busbar inside each cabinet can only take 4 mm2 conductors. All of the grounding infrastructure will be laid beneath the raised floor. A graphical representation of the grounding scheme is presented below:

P3w2iPg.jpg

https://i.imgur.com/P3w2iPg.png

I am using a 25 mm2 grounding wire to serve as the main bus as per NEC guidelines. The MCB upstream is rated at 300 A. I, however, have some confusion as to the 4 mm2 wire I will be using for the individual connections. Please let me know if 4 mm2 will be adequate and share your feedback on the overall grounding scheme.

Thanks for your responses in advance. I understand this may sound like too much for a first post but I am under a lot of stress and would really appreciate if somebody could point me in the right direction.
 
Thanks for the link, Ron. It has already answered a good number of my concerns. My original design involved running a 25 mm2 insulated earthing conductor from the grounding pit to the Power Distribution Cabinet and then running 2 core + GND power cables from said cabinet to each cabinet downstream. This would make it a nice startpoint ground with all the grounding connections originating from one reference in the Power Distribution Cabinet but my main concern here would be the long cable runs. So I decided to go with a grounding bus instead. I sized the main earthing conductor from the rod to the busbars using NEC Table 250-95 for a breaker rating of 300 A so roughly 25 mm2. The circuit protective conductors from the busbars that I refer to as individual connections to the cabinets earlier have been sized using the fact that our grounding system is a TT earthing system and the maximum line conductor size is 4 mm2. The use of the grounding bus is in line with guidelines by the DCS OEM but they do not go into too much detail so I was looking for some feedback from the community members here. Once again, thanks for all your responses. Really appreciate it.
 
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