The Single-Line Diagram: Important for Safety
- Engineers perform power systems analysis studies including the calculation of fault currents, determination of selective coordination, and calculation of incident energy.
- Suppliers bid projects based on these documents.
- Inspectors determine NEC compliance.
- Electrical workers determine lockout/tagout to create electrically safe work conditions.
- Designers plan power system expansion projects.
- The designer must lay out the single-line showing the power distribution system.
- Assumptions are used to estimate fault-current calculations to conduct equipment evaluation and make
selective coordination decisions.
The contractor performs the installation marking up the diagrams as changes are made. The contractor will add lengths for each of conductors and accounts for transformer and motor/generator nameplate data, including impedances, as power generation additions can increase fault current within a system.
The as-built drawings must then take shape (the original design drawings revised to reflect hanges made in the field), complete with updated fault-current calculations.
The engineer must then again review the power systems analysis studies based upon these final documents. Fault studies will be updated, coordination studies reviewed, and incident energy values updated.
Important documents for reference include the following:
- NFPA 70, The National Electrical Code
- NFPA 70B, Recommended Practice for Electrical Equipment Maintenance
- NFPA 70E, Standard for Electrical Safety in the Workplace
These references work together over the life of the system to ensure the power distribution system performs as expected over the life of the system. These three documents are a must for the book shelf library of every facility engineer or maintenance team.
NFPA 70E defines the term "Single-Line Diagram" as "A diagram that shows, by means of single-lines and graphic symbols, the course of an electric circuit or system of circuits and the component devices or parts used in the circuit or system." NFPA 70E in many instances references up-to-date drawings and diagrams which include the single-line diagram. Some key section references include:
- 120.2 Lockout/Tagout Principles
- 120.5, Process for Establishing and Verifying and Electrically Safe Work Condition
- 205.2, Single-Line Diagram
- 340.5, Specific Measures for Personnel Safety
- E.3, Typical Electrical Safety Program Procedures
- 130.5 (G) specifically states, "The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed five years."
The single-line diagram plays an important role in safety with regard to the electrical worker and NFPA 70E.
NFPA 70B places a lot of weight on the single-line diagram and its accuracy. Section 6.2.2, Diagrams and Data, states, "The availability of up-to-date, accurate, and complete diagrams is the foundation of a successful EPM program." Note that EPM stands for Electrical Preventative Maintenance. That's a pretty strong statement. Section 18.104.22.168 recommends that SLDs show all electrical equipment in the power system and give all pertinent ratings for voltage, frequency, transformers impedance, available short-circuit current and overcurrent protective devices, among others.
Plan for the Single-Line Diagram
In my opinion, a budget line item specifically calling out the documentation relating to the power distribution system including the single-line diagram must be visible and executed for every project. It's not the size of the project that is the determining factor. Every project conducted where the power system is modified should be reflected on the diagrams. Documentation should not have dates greater than 5 years old. The electricl professional relies on accurate diagrams for safety. So do your due diligence by preparing and maintaining this documentation. Make it a part of your ongoing relationship with your customers. If you are the contractor on a project making infrastructure changes to the electrical system, make sure you include time and money for updating the drawings for the owner. It's your value as you know what is being changed. These changes could spark additional study opportunities as fault current, selective corrdination, and incident energy reference materials could be impacted. Remember that per 70E we need to review these diagrams every five years as a minimum, earlier if you've made changes.
As always, keep safety at the top of your list and ensure you and those around you live to see another day.