In a loaded power circuit, what is the most dangerous action to take?

Opening a loaded power circuit with a knife switch is particularly dangerous because it can create an arc. A knife switch is designed to be opened or closed manually, and when the circuit is under load, doing so can lead to the electricity jumping across the gap as the switch is opened. This can result in a hazardous situation, including electric shock or fire, due to the high current that may flow at the moment of disconnection.

In contrast, cutting the power supply completely (turning it off at the source) is a standard safety procedure to ensure the circuit is de-energized before any maintenance or adjustments are made. Disconnecting the ground wire removes an essential safety feature that prevents accidental electric shock, while checking voltage with a multimeter does not affect the circuit's state and can be done safely if the circuit is powered down appropriately first. Thus, among the actions listed, using a knife switch while the circuit is loaded poses the highest risk and is considered the most dangerous.