I haven't been involved in this issue since I ran an electrical division on a nuclear powered aircraft carrier. This was not just a common practice, it was required and it was strictly enforced. I can tell of one instance in which it was not practiced, and that event nearly resulted in the death of a sailor. He was reaching to place a grounding clamp on what he thought was a de-energized 4160 volt bus. He had checked it with an electronic voltage tester that was not rated for that voltage. He failed to test the tester before and after checking the bus, and his test of the bus showed (incorrectly) that the bus was dead. He was blown back 15 feet.
The point of the pre- and post-test is to validate the test of the bus (or panel or whatever other thing you are about to touch with your hands). Suppose, for example, and this is absolutely no stretch of my imagination, that an electrician tests the tester before going out to the field, and one of the test leads comes loose while in transit, or the battery goes dead, or some other failure takes place. The thing that is important is for the electrician to be able to verify that the tester is working properly, before staking his or her life on the information that it gives, the information (trur or false information) that the bus is not energized.
