The Single-Line Diagram and Safety
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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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.