What is a critical step to prevent injury when testing isolated equipment?

The act of attempting to start the equipment when it is isolated is not advisable, particularly when it comes to ensuring safety during testing. The emphasis in testing isolated equipment is primarily on verifying that it is indeed safe to work on to prevent the risk of electric shock or equipment malfunction.

The critical step involves ensuring that the equipment is not just isolated but also verified to be de-energized and safe before starting any work. This includes confirming that the power source is not connected, checking that any stored energy in components like capacitors is adequately discharged, and that there are no live circuits connected. The goal is to prevent any unexpected energization which can potentially lead to injuries.

Testing the power source, opening switches gradually, and discharging capacitors are indeed important steps in the overall procedure for safely dealing with isolated equipment, but they don't directly relate to the act of attempting to start the equipment, which should not occur until absolute certainty of safety is achieved.