Allastairasks us: A structure has two separate earth systems: a
safety earth and an electronics earth. The safety earth is connected to an
earth-mat located 100m away from the structure. The safety earth is locally
earthed. Is this a safe configuration, in terms of lightning safety? Explain
your answer. If it is a problem, propose a solution.
Thank you for your question regarding separate earthing systems, it is our pleasure to help.
You may not have truly isolated grounding systems, these two systems must be bonded together in at least one place. Even when a specification calls out for an isolated or dedicated ground, it still must be bonded back to the first service disconnect at some point. This is primarily for safety reasons as you do not want to generate differences in potential from one ground system to another.
If you were to test your two grounding systems by conducting a simple point-to-point resistance test, they must show that they are positively bonded to each other. Now, when the bond is temporarily removed, you should see a very high resistance. This indicates that the two ground systems are isolated from each other. But are still systems that are bonded in parallel.
We hope this helps.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions