Why do I need electrical diagrams to perform a work task?

If workers do not have an accurate map (i.e. one-line diagram) of the electrical system, they can be exposed to potential back feeds from alternate sources, energized capacitors, undocumented switching conditions, and unknown voltages, in addition to the problem of not being able to accurately perform lockout/tagout procedures.

Source: "Compliance Takes More Than a Label", Facility Safety Management, January 2008, Chet Davis