@User: No, you don't have clamps on ground rails. Think of the power supply as a loop, from supply in to ground return. The clamp will shunt current directly into the ground return at the pad in the case of an ESD event. So the clamp is an unconnected circuit resource in the chip; it has only two pins, one connected to the supply and one to ground, and if you want to use the clamp you need to connect both pins.