The single-line diagram is the blueprint for electrical system analysis. It is the first step in preparing a critical response plan, allowing you to become thoroughly familiar with the electrical distribution system layout and design in your facility.

Whether you have a new or existing facility, the single-line diagram is the vital roadmap for all future testing, service and maintenance activities. As such, the single-line diagram is like a balance sheet for your facility and provides a snapshot of your facility at a moment in time. It needs to change as your facility changes to ensure that your systems are adequately protected.

An effective single-line diagram will clearly show how the main components of the electrical system are connected, including redundant equipment and available spares. It shows a correct power distribution path from the incoming power source to each downstream load – including the ratings and sizes of each piece of electrical equipment, their circuit conductors, and their protective devices.

In many process facilities, loads are continually added or removed in small increments. The net effect is not always seen until some part of the system becomes overloaded or exhibits other problems. Many times circuits are added without appropriate modifications of the standard settings on the associated upstream circuit breakers. The protective devices you use must be coordinated with their time/current curves and with each other. The single-line diagram provides the roadmap to enable proper design of equipment, redundancy, and protection.


An up-to-date single-line diagram is vital for a variety of service activities including:

  • Short circuit calculations
  • Coordination studies
  • Load flow studies
  • Safety evaluation studies
  • All other engineering studies
  • Electrical safety procedures
  • Efficient maintenance




  • Help identify fault locations and simplifies troubleshooting
  • Identify potential sources of electric energy during LOTO procedure
  • Ensure safety of personnel
  • Stay compliant with NFPA 70E requirements
  • Ensure safe, reliable operation of facility




To give you an accurate picture of your electrical system, the single-line diagram information normally includes:

  • Incoming lines (voltage and size)
  • Incoming main fuses, potheads, cutouts, switches and main/tie breakers
  • Power transformers (rating, winding connection and grounding means)
  • Feeder breakers and fused switches
  • Relays (function, use and type)
  • Current/potential transformers (size, type and ratio)
  • Control transformers
  • All main cable and wire runs with their associated isolating switches and potheads (size and length of run)
  • All substations, including integral relays and main panels and the exact nature of the load in each feeder and on each substation
  • Critical equipment voltage and size (UPS, battery, generator, power distribution, transfer switch, computer room air conditioning)

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