What is the critical first check to perform before tying two circuits together?

The critical first check before tying two circuits together is to ensure that the load is disconnected. This is vital for safety, as connecting circuits while loads are energized can lead to short circuits, system overloads, or hazards such as electric shock or arc flash. Disconnecting the load ensures that no current is flowing through the circuits you are working with, providing a safe environment to proceed with the work.

Although checking that all phases are functioning is important for the overall operation of the electrical system, it is secondary to ensuring that the circuits are safe to work on. Ensuring voltage levels are equal is also crucial in preventing damage to equipment or unsafe conditions, but without first disconnecting the load, you place yourself and others at risk of a dangerous situation. Likewise, accounting for tools is good practice but does not directly pertain to the immediate safety of connecting circuits. Therefore, the priority must be on safely managing any electrical loads before proceeding with any connections.