A current, accurate one-line (single-line) diagram is the source-of-truth for your electrical system, from utility or generator all the way to critical loads. It underpins safe work practices, engineered studies, maintenance planning, and emergency response. When kept current and tied to labels and procedures, it reduces risk, speeds restoration, and strengthens audit readiness.
A one-line diagram is a simplified schematic that represents the electrical topology from source to load on a single line. It shows how power flows through major equipment and where protection and switching occur—enabling safe isolation, analysis, and operation.
- Sources: utility service, generators, uninterruptible power supplies (UPS), photovoltaic (PV).
- Major gear: main switchgear/switchboards, automatic transfer switches (ATS), motor control centers (MCCs), panelboards.
- Transformers & feeders: kVA, voltages, impedance (when available), conductor sizes, feeder IDs.
- Loads: distribution panels and critical end-use equipment by tag.
- Protective devices: circuit breakers, fuses, protective relays, current transformers (CTs).
- Settings & ratings: interrupting ratings, short-circuit current rating (SCCR), relay setpoints, trip units, grounding references.
- Operating schemes: ties, bypasses, and transfer paths that affect isolation and backfeed risk.
- Consistent IDs that match physical labels and the Computerized Maintenance Management System (CMMS).
- Controlled revision history (as-found vs. as-left).
- Accessible outputs: master DWG under document control; easy-to-find PDFs near equipment (QR codes on gear).
Keep a QR-linked PDF at each major lineup so operators and responders can access the latest diagram in seconds.
- Lockout/Tagout (LOTO; 29 CFR 1910.147): One-lines identify all isolation points and cross-ties so procedures prevent unexpected energization or backfeed.
- De-energized work (29 CFR 1910.333): Diagrams guide the sequence to de-energize and verify absence of voltage across utility, generator, UPS, and PV sources.
- Equipment within ratings (29 CFR 1910.303): Proper device application and SCCR checks rely on accurate system documentation.
A current one-line supports job planning, shock and arc-flash risk assessments, and establishing an Electrically Safe Work Condition (ESWC). Arc-flash labels, approach boundaries, and Personal Protective Equipment (PPE) selections all depend on the accuracy of the one-line–based model.
NFPA 70B (2023) elevates documentation as part of a formal Electrical Maintenance Program (EMP). A current one-line is foundational for setting intervals, defining maintenance scope, and recording changes that drive future inspections and tests.
Teams can quickly locate the correct upstream device, understand alternate sources, and avoid closing into faults—shrinking outage windows and preventing mis-operations.
A single, authoritative diagram aligns operations, maintenance, safety, and first responders—improving communication and execution of switching orders.
Accurate one-lines speed safe, stepwise restoration and help incident reviewers trace fault paths, device operations, and coordination performance.
- Desktop review: collect existing drawings, labels, panel schedules, and studies.
- Site capture: verify feeder paths, nameplates, device settings (where safe), and tie/bypass schemes.
- CAD update: draft revisions and reconcile with schedules and the analysis model.
- Stakeholder review: operations/safety sign-off.
- Issue for use: publish controlled PDFs/DWGs; distribute QR-linked updates; log revision.
- Gear and device IDs, bus/feeder ratings, interrupting/SCCR data.
- Relay and trip settings, CT ratios, transformer data (kVA, %Z).
- Conductor sizes, grounding, and any temporary or seasonal sources (rental gensets).
- Complete & current: all sources/feeds/major loads represented.
- Accurate device data: ratings and settings sufficient for studies and LOTO.
- Traceable IDs: match labels and CMMS.
- Controlled revisions: “as-found” and “as-left” states preserved; approvals recorded.
- Master DWG in a controlled repository; PDFs posted at point-of-use via QR.
- Link the one-line to CMMS assets and to the engineering model (ETAP/SKM/easyPower).
Engineered studies are built on the one-line. The accuracy of arc-flash labels and time-current coordination depends on the fidelity of the diagram.
Use the hierarchy to define breaker testing scope, infrared (IR) thermography routes, and torque/cleaning priorities by bus/feeder—then capture as-found/as-left results.
Any project (new transformer, VFD, generator, PV tie-in) must be reflected on the one-line before re-labeling and reissuing procedures.
Remediate with a targeted field-walk of high-criticality gear first; expand to lower tiers on a scheduled cadence.
Standardize tag conventions; relabel equipment to match drawings and CMMS, then lock future projects to the convention.
Capture trip settings, CT ratios, and temporary source connections; add a change-control step to prevent drift.
- Quick wins: correct obvious omissions, publish interim PDFs, add QR access.
- Deeper redraw: rebuild complex areas (e.g., plants with multiple ATS/UPS) and synchronize with the studies model.
- % of equipment represented on the one-line
- Time since last update (target: review at least annually and after any system change)
- Variance between field labels and diagram IDs
- LOTO procedures validated against current one-line paths
Annual documentation review under the EMP, plus immediate updates after projects or temporary source connections.
If it isn’t on the one-line, it isn’t safely operable: review after every project and reissue the PDF at point-of-use.
Get a One-Line Diagram Readiness Assessment
Gaps in your one-line create blind spots in safety, maintenance, and emergency response. Close them now—before they become incidents.
References
- National Fire Protection Association. (2023). NFPA 70B: Standard for Electrical Equipment Maintenance (2023 ed.). NFPA.
- National Fire Protection Association. (2024). NFPA 70E: Standard for Electrical Safety in the Workplace (2024 ed.). NFPA.
- Occupational Safety and Health Administration. (n.d.). The control of hazardous energy (lockout/tagout), 29 CFR 1910.147. U.S. Department of Labor.
- Occupational Safety and Health Administration. (n.d.). Selection and use of work practices, 29 CFR 1910.333. U.S. Department of Labor.
- Occupational Safety and Health Administration. (n.d.). General requirements, 29 CFR 1910.303. U.S. Department of Labor.
- IEEE. (2010). IEEE 3007.2™-2010: Recommended Practice for the Maintenance of Industrial and Commercial Power Systems. IEEE.
(Abbreviations defined on first use: Electrical Maintenance Program (EMP), Electrically Safe Work Condition (ESWC), Lockout/Tagout (LOTO), Personal Protective Equipment (PPE), Computerized Maintenance Management System (CMMS), short-circuit current rating (SCCR), infrared (IR).)x