Household sub-panel neutral isolated from ground

Let me think as I write. The purpose of safety ground is to have a low voltage (with respect to the Earth or any electrical path to Earth) to put between a person and the dangerous voltage available within the system. The neutral is grounded at (or as close as possible) to the source to make sure that the highest voltage in the system is minimized, or at least, determinate. At the point where the neutral is grounded, (bonded to the safety ground conductor) the impedance of the ground electrode must be low enough that expected current through this path do not produce dangerous safety ground conductor voltage with respect to local Earth. This is why good conductors, like water piping must be well tied into the ground conductor system, since they may be the first contact points with the local Earth potential. The voltage drop on the safety ground conductor is minimized by having it carry no load current, except in the case of a line to safety ground fault. By having a neutral conductor carry load current, it is expected that the far end of that conductor will have some voltage with respect to Earth, any time there is load current.

Now, applying these principles, what is the case for a sub-panel? First of all, the sub-panel, is, by definition not the closest available point to the power source, so it is not the preferred place for the ground to neutral bond. If the neutral conductor between the main panel and the were to become high resistance, and the ground and neutral were bonded at the sub-panel, Then the safety ground conductor at the sub-panel would be expected to have a voltage proportional to load current. During an ordinary line to neutral fault, this would be half of line voltage, even though the safety ground conductor were still connected. If both neutral and safety ground conductors were broken, any load at all would apply full line voltage to the distributed (from the sub-panel) safety ground.

So, unless the bonded node is also re grounded with a suitably low resistance ground electrode system to mitigate the above problems, it is much less risky to rely on the bond back at the main panel. It is still a good idea to add an additional ground electrode to the local safety ground node, since the ground potential at this locality may be a bit different that it is back at the main panel, so a compromise may be useful.

Of course, for sub-panels far enough away from the main panel, this compromise may become intolerable (for instance, during a lightning strike near one panel or the other) and at some point (that I don't know how to define) it is better to re isolate the mains and redefine the neutral connection to ground, creating a new main panel.

John Popelish wrote: (snip)

Forgot to add: This also lowers the total resistance in the ground side of the circuit during a local line to safety ground fault, keeping the peak grounded conductor voltage during the fault lower.