You are not the first one to have such a doubt. The undersigned is one more.

Much of your concerns can be answered if you can explain about that another line that is called Neutral. If in your installation there is no such thing as a Neutral and Earth - well most of your concerns ARE true. You are faced with a huge problem of ground noise. The Nuetral is supposed to be a return path for the Phase currents and since you do not want to lug a huge cable all the way back to the power generation station, you bury a "good" conductor just outside your installation and use mother earth to do the rest. That is Neutral.

The "Ground" is supposed to serve as a safety "sink" and it is so well connected to everything that is metallic ( iron reinforcements inside flooring/walls, water pipes, etc )that it has such a low impedance and it is impossible for any voltage to exist on it.( You have a 10^10 liter tank full of water and you add a drop of water with a syringe. How much level will it raise ...? ). Thus it will be the "Zero" reference for any measurements that you make.

So in case if you have no separate Earth and Neutral you can do this. Create a fresh "Instrumentation earth" near the equipment with a good earth rod ( use a certified one or have one such contractor do it for you ) and use this as your ground for measurement. Use a heavy cable or flat to link this with your DC ground. And use the existing earth for safety reasons as usual.

Note: I have used the above method in some very electrically dirty environments and have had good improvements in the overall noise level. Experts may have a difference of opinion in this method...

( I wish Prof.Kai responds to this)

Raghu