Ensuring that the power is turned off to electrical equipment before servicing is done by testing for absence of voltage. It could be dangerous to rely on the reading of a faulty voltage tester which is why it is critical to first verify the proper operation of the test meter on a known voltage source when dealing with circuits of 50 volts or more:

Step 1. Verify the proper operation of the test meter on a known voltage source (see image).

Step 2. Use the same test meter to verify the absence of voltage on the equipment about to be serviced.

Step 3. Re-verify the proper operation of the test meter on the known voltage source once again as a final guarantee that the tester is still operating correctly before servicing the electrical equipment mentioned in step 2.

Below is a sample of NFPA 70E. For the complete section, see the actual NFPA 70E text at NFPA.ORG. Once there, click on the free access link to NFPA 70E.

110.8(E) Operation Verification. When test instruments are used for testing the absence of voltage on conductors or circuit parts operating at voltages equal to or greater than 50 volts, the operation of the test instrument shall be verified on any known voltage source before and after an absence of voltage test is performed.