Grounded phase conductors
As long as the circuit has no other path to ground you can connect any point in a circuit to "ground". 

What you are really doing is making that point in the circuit a fixed reference point. All voltages in that circuit can then be referenced from that grounding point. 

It's like having three mountains. If you stand way down here at sea level, you see three heights of the mountains, all plus relative to you.

However, if you stand on top of the first mountain, the tops of the other two mountains are now at a different height with respect to your viewpoint. One may be plus, one may be minus etc. Looking back to where you started from, that is now a big negative height value to you.

Voltage is just a potential, just like height is.

You just can't stand in two places at the same time.

You also can't connect two different ground points together. Just like you can argue all day with your buddy who is still down at sea level about how high the mountains are.

Just as the mountains did not change when you climbed one, the circuit does not change either. The currents within the circuit behave exactly as they did before you grounded that corner. They simply look different from your reference.