What is the working principle of a pen tester. The voltage is present in three phases but the pen tester is not giving indication in one phase . Why?

Please put more than just "electrical" in the title. It gets hard to figure which thread is which.

Here is description of a proximity (pen) tester:

http://www.elect.com.au/Attachments/ProximityTesters.pdf

Use one only to detect the presence of voltage.

Do not use one to perform troubleshooting.