The Single-Line Diagram: Important for Safety

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