Thank you for your question regarding separate earthing systems for electric and communication grounding at substations, it is our pleasure to help.
It is very counterintuitive, but you must tie your expensive communications equipment into the same grounding system that handles the substations power grounding. Not only is this code in every country in the world, but it is also the only safe way to protect your equipment.
While it seems that you would not want sensitive and expensive communication gear exposed to high-voltage currents, by bonding these systems together the difference in potential is reduced to near zero. If you have an illegal separate ground source for your communication gear, you have actually increased the difference in potential between the two systems, which will allow hazardous voltages and currents to form.
Most codes require a fiber-optic telecommunications connection to the substation with a transition point from copper-to-fiber called the Copper-Fiber Junction of CFJ. The purpose of the optical fiber is to prevent transient voltages from traveling up the copper and to the Telco data center where it could damage the center. The CFJ is commonly placed just outside of what is called the 300-volt line.
The 300-volt line is distance from the substation where during an electrical fault, the earth voltage will drop below 300 volts p-p (or 212 volts RMS). By bringing the copper wires no closer than the 300-volt line, the Telco data center is protected. For example, if the 300-volt line for your substation was calculated to be 500-ft (~150 meters), you would place the CFJ at that distance away from the substation. The copper Telco lines would go to the CFJ, where the signal would be converted to optical and sent down fiber cable to the substation. The plastic fiber will not conduct electrical energy. However, once inside the substation, the signal must be converted back from optical to standard electrical signal and the cables become copper again. This copper communication system must be bonded to the same ground system as the rest of the substation. If you need assistance calculating the 300-volt line for your site, please do not hesitate to call 310-318-7151 California time.
It is important to note that your substations Human Safety grounding systems (Step & Touch Voltage) also must be bonded to a common grounding system within the substation. If you need assistance calculating fibrillation currents and eliminating voltages hazardous to the safety of your personnel, please do not hesitate to call 310-318-7151 California time.
In conclusion, you must bond all your metallic systems within the substation to a single common grounding system. This includes, utility power, Telco grounding, building steel, water pipe, gas pipe, lighting protection, equipment grounds, human safety grounding, communication grounding, and any other grounding system found at the site.
Please keep in mind that most countries have laws mandating the engineering analysis of the grounding systems for substations, to protect the personnel working at the site. In America that law is 29 CFR 1910.269. If you should need any assistance in this regard, please do not hesitate to contact us at 310-318-7151 California time.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions