Hi N R Metha,
Thank you for your question regarding the grounding of Programmable Logic Controllers (PLC) and the safe distance from other grounding systems. It is our pleasure to help.
First of all, it is important to remember that the nearby transformers and your PLC’s must be bonded to the same common grounding system. There may not be separate grounding/earthing systems within your facility. Now if you are trying to install what is called an “Isolated Ground” for you PLC’s, you will still need to have a grounding conductor that runs back to the first service disconnect to ensure that Over Current Protection Devices (OCPD) such as circuit breakers and fuses will function properly. If an isolated ground system is what you are trying to do, then you will need to do the following:
- Isolate all of the above-grade grounding conductors associated with the isolated ground system from contact with any metallic object from the grounding electrode, to the device, and all the way back to the first service disconnect. This can be a very challenging task, as you may be required to run the ground conductor in metallic conduit. This metallic conduit will need to be isolated from the walls and clamps via rubber gaskets and you will need to bond both ends of the conductor back to the metallic conduit to prevent impedance imbalances which can cause a ‘chocking’ effect allowing electrical noise to form on the conductor.
- You will need to move the grounding electrode as far away as possible from other electrodes to prevent stray currents from coming back up your ground system. Without soil resistivity data, it is impossible to know how far to keep these electrodes apart from each other. Some facilities that deal with high-end electronics have a 1,000,000-ohm resistance requirement between their sensitive electronic grounding (earthing) systems and their normal grounding (earthing) systems. This tested using a mega ohmmeter. Systems that we have tested in the past had their sensitive electronic grounding systems placed hundreds of feet away from the facility by routing the grounding conductors in PVC conduit to an electrode array placed in an adjacent field.
Also to create a “clean” power source for PLC’s and other instruments sometime a UPS or power filter is added to the HOT line of the power source.
Now, all that said, I don’t believe that your PLC’s are going to be that sensitive that they need special grounding. Programmable Logic Controllers are known to be very robust, good above-grade grounding techniques, combined with an effective low-impedance below-grade earthing system should be satisfactory for both your transformers and your PLC’s.
If you should need any help calculating safe distances, soil resistivity, impedance-to-ground of electrodes, or should you have any further questions, please do not hesitate to give us a call at 310-318-7151.
The Engineering Team at E&S Grounding Solutions
Photo credit: http://upload.wikimedia.org/wikipedia/commons/2/25/BMA_Automation_Allen_Bradley_PLC_3.JPG