Wiesserfalke:
I want to measure a current on a 3 kV device with the Arduino without killing it
"A current" is a raisin like fruit that is, typically, a little bigger than a grain of rice. To measure it, using an Arduino in the vicinity of 3 kV is rather a challenge. I would use surveying techniques -- that way you won't have to invest in expensive protection gear, because you can do the measurement at a distance.
All you will need is a telescope, a stepper motor, a way to mount the stepper motor to a tripod, a platform that will mount to the top of the stepper motor, a laser tuned to a frequency for optimal current reflectivity, and a photodiode that will respond to the laser frequency. Place the tripod a know distance from the current, then mount the stepper motor on the tripod, then the platform, leveled, on the stepper motor, then place the photodiode over the telescope eyepiece, and mount the laser next to the scope, so it points in the same direction as the viewing direction of the scope, and make sure the laser beam is at the same level as the current, and that the scope has a view of the laser spot.
We will use the Arduino to step the motor so the platform sweeps out an arc that will include the length of the current. As the platform turns, an Arduino Analog input will monitor the photodiode and start a counter when it senses photons reflected back from the current. The counter will be advanced on each step, until loss of current signal from the photodiode. Apply a bit of trigonometry, and voila! The current is safely measured!
BTW: The current is already dead.
You can find a list of sources for the materials you will need at:
Note: Just a demonstration of how a vague question can be misinterpreted ;D