Chapter Corner

The Single-Line Diagram and Safety

Posted in: Safety Corner, July 2017

The single-line diagram to the electrical professional is comparable to what a map is to a person driving across the country. If this was “back in the day,” I’d say that before I go on a journey somewhere in the car I grab my Atlas and appropriate maps. Today, I use my smartphone and my “Maps” app.

A map is essential and provides key information, including road markers; types of roads (highway or two-lane or other); cities, towns, and states; and key locations, such as airports and other important venues. The map to the traveler is essential. Similar could be said about the single-line diagram for the electrical professional. Another similar parallel between the map and the single-line diagram pertains to the accuracy of the information. An outdated map can get you lost and an outdated single-line can do worse. The single-line diagram is your road map if you are an engineer, electrical contractor, electrical inspector, maintenance professional, or similar. Accurate and updated singleline diagrams could save a life.
The single-line diagram is sometimes called the one-line diagram. Regardless of how you refer to this document, it has some unique basic qualities that set it apart from other electrical diagrams. Many electrical diagrams can be quite confusing, like the schematics we find on the inside doors of industrial control panels and similar. The single-line diagram focused on here is that which addresses the overall power distribution system. These diagrams are meant to be simple and easy to read, providing important, accurate information to the electrical professional.
The following items should be shown on the single-line diagram:

1. Power sources including utility, generator, and alternative energy solutions. When working on electrical distribution systems, we must understand from where power is coming and how much power can be expected. This could pertain to how much fault current is available based upon the system configuration or just that a presence of voltage should be expected on a specific terminal. This is not only important to the design engineer or installer, it is important for those who work on the power distribution system well after the first installation.

    • Utility: The information shown is obtained directly from the utility and should include the voltage of the source connection, the available short-circuit current and the X/R ratio. The available short-circuit current can be represented as Short-Circuit kVA/MVA or Short-Circuit Amperes. Note who the utility is as well.

    • Generators: The generator plays an important role for the reliability of the power distribution system. Information should include the voltage rating, VA rating either in kVA or MVA and sometimes could be represented instead as Watts. Sub-transient reactance and X/R ratio should also be shown at a minimum. The nameplates of generators will have all of this information and more, as required by the UL listing of the product.

    • Alternative energy solutions: These solutions will be connected to the system via an inverter. The key parameters of voltage ratings and output current ratings are those that should be included on the single-line diagram.
2. Significant power distribution equipment that contain switching devices as well as overcurrent protective devices should be shown. This includes panelboards, switchboards, transfer switches, safety disconnects, and similar equipment.

    • Panelboards, switchboards and similar equipment: Assigned identifiers, voltage rating, rated equipment ampacity, available short-circuit current, shortcircuit current rating (SCCR).

    • Transfer switch: Normal and emergency line side designations, amp rating, SCCR or withstand close-on rating should also be identified, available short-circuit current.

    • Disconnects: Disconnect switches should include the ampere ratings and SCCR. The available shortcircuit current at the equipment should be shown as well.
3. Significant distribution system components that add impedance into the system, including transformers, conductors, reactors, and busway.
    • Conductors: Size, insulation type, material (CU/AL), ampacity, length, and number of conductors in parallel. Impedance information is obtained from other industry references based on the information available on the single-line.

    • Busway: Rated amps, type, material (CU/AL), and voltage rating. Impedance information is obtained from other industry references based on the information available on the single-line.

    • Transformers: Assigned designation, kVA rating, primary voltage, secondary voltage, primary winding configuration, secondary winding configuration, and percent impedance. If the transformer has fans, the kVA ratings with fans should also be included.
4. Identification of loads.
    • Motors: Voltage rating of the motor, motor type induction or synchronous, and sub-transient reactance. Quite often, only motor HP ratings will be provided on the single-line diagram. This is acceptable for smaller motors as the information formulated for systems analysis studies will be based on IEEE standard assumptions. Larger motors though should be accompanied with the additional information noted here.

    • Lighting, heaters and other similar loads: The level of granularity on the single-line diagram varies when we get down into the system looking at the loads at the end of branch feeders. A lot of this information is assembled on what we call panel schedules, with the single-line only having the panel name and possibly the largest OCPD installed inside. The information included in the panel schedule should include the OCPD ampere rating, connected conductor size, and the information necessary for load calculations.

5. Overcurrent protective devices.
    • Fuses: Fuse class, voltage rating, ampere rating, and interrupting rating.

    • Circuit breakers: Voltage rating, ampere rating (sometimes referred to as “amp trip”), and frame rating (sometimes referred to as “amp frame”). Circuit breakers with electronic trip units should include the various possible pickup and time delay settings. The model number of the circuit breaker and electronic trip unit type are also critical information as trip curves will be based upon this information.

    • Relays: Type of relays and relay settings for both phase and ground faults
6. Zones of protection.
    • Differential relaying: Differential relaying information should include the CTs with the CT ratios, showing the connection points to the relay and the trip signal from the relay to the specific OCPD or switch that will open upon actuation.

    • Zone selective interlocking: Zone Selective Interlocking (ZSI) is a technology that works between circuit breaker devices only. The circuit breakers connected together in the zone protection should be shown on the single-line diagram.
7. Type and location of surge arresters and capacitors can be shown on the single-line, but
more often this information is included in the panel schedules.
The actual single-line/single-line drawing should be kept as simple as possible. It is a diagram and need not show geographical relationships. Duplication should also be avoided.
The single-line diagram tells the electrical professional how the system is connected and where power comes from. It can be used to identify what switches must be opened to work deenergized. It also is a key reference to ensure electrical equipment is applied correctly. Make sure it is updated after every change in the system and make sure the latest version is your reference. 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.