In the first few weeks of a new project the engineer will need to roughly draft a key single-line diagram and a set of subsidiary single-line diagrams. The key single-line diagram should show the sources of power e.g. generators, utility intakes, the main switchboard and the interconnections to the subsidiary or secondary switchboards. It should also show important equipment such as power transformers, busbars, busbar section circuit breakers, incoming and interconnecting circuit breakers, large items of equipment such as high voltage induction motors, series reactors for fault current limitation, and connections to old or existing equipment if these are relevant and the main earthing arrangements. The key single-line diagram should show at least, the various voltage levels, system frequency, power or volt-ampere capacity of main items such as generators, motors and transformers, switchboard fault current levels, the vector group for each power transformer and the identification names and unique ‘tag’ numbers of the main equipment.
The set of single-line diagrams forms the basis of all the electrical work carried out in a particular project. They should be regularly reviewed and updated throughout the project and issuedin their final form at the completion of the project. They act as a diary and record the development of the work. Single-line diagrams are also called ‘one-line diagrams’.
At this stage the engineer can begin to prepare a load schedule for each subsidiary switchboard and motor control centre, and a master schedule for the main switchboard.
The master load schedule will give an early estimate of the total power consumption. From this can be decided the number of generators and utility intakes to install. The kW and kVA ratings of each generator or intake will be used to determine the highest voltage to use in the power system.