I see what you're looking for now Bman. It's just that you wouldn't be using relays in series to do what you need. One bummer is that it is hard to sense for a big ground fault, without causing a little ground fault of your own in your detection equipment.
So you can sense the corners to ground as Paul has described. But think about another possible method... What if you connect a properly sized and rated fuse and series resistance from each corner into a common node in the middle. Then use any of a number of methods to sense between this node and ground. Choices here are driven by how high a value of ground resistance you are trying to detect as well as how much ground leakage you are willing to impose with your sensing circuit.
Joe
