What do you get when you combine a barefoot 12-year-old kid, a basement and a plugged-in TV with the cover removed? Is the answer me, a near-death experience or an electrical engineer? The answer is all the above. I clearly remember the effects of electrical current through the human body. The painful shock with loss of muscular control is still a vivid memory 40 years later. I couldn't let go, but fortunately my arm muscles extended throwing the TV across the basement and bouncing me off the sofa.
As an engineer, technician or electrician, you may need to test the control circuit with the power on. However, the only thing that should be touching the live circuit is the probes of a suitable multimeter. Once the problem is found, suitable insulated tools should be used to correct the problem, but only after the power is turned off.
You may have met the electrician who brags about leaving the power on or getting shocked all the time. I've met several, and, even though they make fun of my tender, callus-free hand when they shake it, I'm not impressed when they work on an electrical circuit with the power on. Quite the opposite. I've seen the effects of electrical shock and felt it once, as a professional, on the job.