Q: The intrinsic safety spec I’m reading calls for an isolated ground. Isolated from what?

Answer: A true isolated ground is not connected to any ground that can ever carry fault current from unrelated parts of the electrical system. It is best to run it directly to grounded building structural steel, an underground metal water pipe, or a separate grounding electrode from the building electrical service as described in Article 250 of the National Electrical Code. However, many grounds that claim to be “isolated” are actually just separate wires run back to the ground bar on the nearest panelboard.  At best, they are run all the way back to the service entrance ground. In either of these cases, a high-current ground fault in the electrical system can raise the potential of the ground wire to destructive levels. True isolation is important for sensitive electronic devices, and is especially important in intrinsically safe systems where an explosion could result from a high voltage appearing on a ground conductor.