Designing a Distribution System (Part - 1)

System One-Line

It is important for the power system designer to ensure the System One-Line and other design documents contain as much information as possible, to assure that bidding contractors include all the correct requirements in their pricing. Errors and/or omissions on the construction contract documents can lead to expensive contractor change orders and project cost overruns after the contract is awarded. 

During the various stages of a project design, changes are made often to reflect the client’s preferences and budget. As the design process continues coordination between the MEP (Mechanical, Electrical and Plumbing) design disciplines become more critical.

If the design professionals are not synchronized on these changes, a previously unanticipated piece of equipment may be chosen or added to the project. As an example, where an engineer had previously allocated a 250A circuit breaker to feed the anticipated load, as a result of an equipment change, a 400A breaker must now be provided. The impact of this change can result in a contractor bid that does not include both the correct breaker AND the correct cable sizes to feed the larger load. 

Oftentimes, requirements such as electronic trip units or their protective functions such as Long, Short, Instantaneous and Ground (LSIG) or Ground Alarm (LSIA) are not indicated on the One-Line. This can lead to equipment being supplied with standard thermal magnetic trip units that may lack the setting capabilities to achieve the proper selective coordination required. 

Other requirements such as: Zone Selective Interlocking of breakers, 100% rated breakers, drawout or electrically operated breakers and key interlock schemes can be overlooked if they are not documented on a One-Line and coordinated in the specifications.

Finally, electrical equipment is subject to environmental issues such as wet areas and may require specific enclosure types to be provided. Nomenclature on the One Line, such as 3R or 4X, adjacent to these items can clarify what enclosure type is to be provided.

The proper use of notes on the One-Line can further define the requirements. As an example, a note can be added clarifying that all NEMA 4X rated enclosures are to be of 316 stainless steel versus the less expensive 304 Grade. The difference between these two grades is critical as 316 Stainless is far more resistant to saltwater, sulfuric acid and chlorides, and is preferred in several applications including pharmaceutical manufacturing and wastewater treatment plants.

 

Figure 1.1-1. Example of Notes on One-Line

The System One-Line is the common map that all the other project documents must reference and be checked against. To ensure consistency and avoid conflicts after a project is awarded to a contractor, distribution panel board schedules and specifications also need to include the correct information about details such as the enclosure type required. 

For these reasons, it is critical that the engineer be vigilant and takes a proactive role in identifying changes and updating the System One-Line and associated design documents appropriately and consistently. 

The One-Line diagram on the following pages is an example developed for illustrative purposes only and was developed to show a wide range of product applications. This diagram will be referenced throughout the remainder of this section. 

The references to external drawings are for illustration only and not referencing actual documents within this section or elsewhere.

Figure 1.1-2. Power System One-Line