Question: 

Linemen working on high-voltage conductors do not simply rely on open disconnect switches to isolate sections of power lines from sources of electricity during maintenance. They also attach "grounding" cables from line to line, and then to earth ground like this:

Explain why this decreases the risk of electric shock for the linemen, based on what you know about electrically common points in a circuit.

Answer:

By connecting the three wires together, you make them electrically common to each other. This prevents any substantial voltage (potential difference) developing between them. Likewise, connecting the three wires to the earth makes them electrically common to the earth, preventing any substantial voltage from developing between any of the wires and ground.

Follow-up question: after the linemen are done with their work, they remove the grounding wires from the power lines before they close the disconnect switches. Explain why this is done, by describing the catastrophic consequences of closing the disconnect switches with the grounding wires still in place.