Lets ignore unusual situations that may arise in certainly sectors. There is no reason that can be justified for working on a live electrical circuit, that simple.
If you wish to test a live electrical circuit then you should be operating under a "Sanction To Test" Permit under which you confirm the circuit is safe/unsafe and if you are then going to work on it you confirm the circuit has been isolated under a LOTO system and then you should be given a separate permit to undertake the works.
The sanction to test should remain open until the work has been completed, the Lockouts removed and you have tested and proved the circuit safe and it has been re-energised.
Complicated way to do things, but that is because the world is full of people who take chances and get other people injured or killed. It is a pain in the bum to do, but it is there to save your life.
Remember, the MHSAW requires your employer to undertake RAMS before commencing works, thus you should identify the risks and then take necessary action to moderate or remove those risks, failure to do so adequately leaves your employer and possibly you liable to prosecution by the HSE, and in extreme cases, the Police for Corporate Manslaughter if they can be shown to have been willfully negligent in respect to safe operating procedures.
Understanding TrueNAS JBOD Servers: A Comprehensive Guide