When I took my NFPA 70E class the instructor stated that a "pen tester" is not a acceptable means of verifying zero volts. (Yellow fluke tester push the button in if there is power the end lights up red and beeps) I received a email today about mounting voltage testers on the front of our panels. They use the pen tester to verify there is no voltage before you open the panel. The company states that the tester or they referred to it as a "NCVD" Non contact voltage detector is acceptable by the NFPA. Which is correct?