Thank you for your question regarding earth systems for different voltage transformers, it is our pleasure to help.
Yes, the earthing systems not only should be bonded together, but they must be bonded together under the National Electrical Code (NEC) and many other standards and regulations.
It may seem counter-intuitive, but even when you have isolation transformers, isolated earthing, DC and AC systems, multiple voltages, Delta, Wye, solidly earthed, resistive earthed, and other non-current carrying equipotential planes, they must all be bonded back together to form a common earthing system. The trick is in how you do the bonding. Series vs parallel connections and other bonding techniques are needed to properly ensure that the above-grade earthing systems are bonded to the below-grade earthing systems.
Frankly, it can get quite complex. Feel free to give us a call at 310-318-7151 California time and someone will be happy to discuss your project with you free of charge.
The Engineering Team at E&S Earthing Solutions
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