Ground wire in custom capacitance-dimmable metal lighting fixture
You can not ground the sensor wire - that will render the capacitive sensing useless.

In order to ensure safety (i.e. don't kill anyone) you can use double insulation. I.e. use a double insulated wire inside the pipe. This works well at least according to European rules.

Let me point out that according to my previous capsense experiences, having a long tube as a sensor will not work, since it will pick up too much noise from the environment.

You also have to be careful since you need to mount at least one end of the tube to some solid surface. That surface must be insulated from the ground, and shall not be metallic at all (i.e. a metal shelf is not a good choice). Otherwise the mounting base will be also part of the touch sensor.

Another idea you can try is to cover the tube with a thin layer of plastic wrap. These self-adhesive wallpapers could work, but also simple electrical insulation tape works well. The capacitive sensing does work trough a non-conductive material as well (e.g. you can have your sensor plate made of wood, plastic, glass, leather). (Note that if your sensor is not capacitive but works by picking up AC noise, you do need a metalic contact).