Chapter Corner

The Single-Line Diagram: Important for Safety

Posted in: Safety Corner, May/June 2018

The single-line diagram (SLD) is an essential tool for the electrical professional that all too often gets overlooked at the end of the project. Budgets get spent on many things and it may be easy for someone, who doesn’t understand the importance of this document for electrical safety, to put off updating the single-line diagrams. This could be an opportunity for the electrical contractor to own and maintain this valuable resource. The challenge is conveying the value proposition to the right people to ensure the money is in the budget to fund the maintenance of this document. Let’s take time here to help build the safety case for supporting the single-line diagram.
The single-line diagram is important part of a facilities electrical infrastructure life and it begins even before ground is broken. This document is leveraged by many including the following:
  • 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 fact is that these and many other situations will arise throughout the entire life of the power system where the singleline diagram will be a valuable resource.
The single-line diagram must be maintained throughout the life of the structure. We need these documents whether the building is old or new. Consider the single-line your map of the electrical distribution system. It is as important as the map you use when you are driving in cities you’ve never been to before. 
We have to understand that when the power system is modified, studies must be updated to ensure adequate electrical equipment. If we add motors or change transformers, fault currents could be higher than expected. The system will function fine until it is called upon to operate in an emergency. If overcurrent protection devices (OCPD) are applied beyond their rating, they become the electrical hazard. Lives could be put at risk if we do not review the power system capabilities as it changes over time.
The success of projects depend upon the entire team working together. The same could be said for the creation of an accurate single-line diagram.

  • 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. 
Once a facility opens, all changes should be reflected on the single-line diagrams and handed over to the owner. Documentation reviews at a minimum should be conducted every five years to assure accuracy and clarity.
Codes and Standards

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 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.

Thomas Domitrovich, P.E. is VP of Technical Sales for Eaton’s Bussmann business within the Circuit Protection Division of Eaton Corporation. Thomas is based out of St. Louis, MO and has more than 25 years of experience as an Electrical Engineer. He is a LEED Accredited Professional and a licensed Professional Engineer in the state of Pennsylvania. Thomas is active in various trade organizations including the Independent Electrical Contractors (IEC), International Association of Electrical Inspectors (IAEI), Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA) and the National Fire Protection Association (NFPA). Thomas is Principle member on Code Making Panel 2 for the National Electrical Code (NFPA 70) and an Alternate member on NFPA 73 for electrical inspections of existing dwelling units both representing NEMA.