ในหน้านี้
A single-line diagram (SLD), also called a one-line diagram, is a simplified map of an electrical power system. It shows how sources, transformers, switchgear, buses, feeders and major loads are connected without drawing every phase conductor, neutral, control wire or terminal.
The most reliable way to read an SLD is to make two passes:
- Trace the topology: start at every source and follow the power path through voltage transformations, incomers, bus sections, protective devices, feeders and loads.
- Read the engineering data: return to each item and record its tag, voltage, current or power rating, interrupting or withstand rating, ratio, conductor callout, normal position and associated note.
Always begin with the title block, revision and legend. Symbols and naming conventions vary by standard, company and project. More importantly, an SLD is not proof that equipment is de-energized and is not a substitute for approved work procedures, field identification or voltage testing.
The Single-Line Diagram Reading Sequence
Use this sequence whenever you open an unfamiliar electrical SLD:
| สเต็ป | What to identify | Question the drawing should answer |
|---|---|---|
| 1. Establish context | Title, drawing number, revision, date, scope, legend and notes | Is this the current drawing for the equipment and operating state being reviewed? |
| 2. Find every source | Utility, generator, transformer secondary, UPS, inverter or tie supply | Where can energy enter the shown system? |
| 3. Mark voltage levels | Nominal voltage at each source, transformer and bus | Where does voltage change, and which equipment belongs to each voltage zone? |
| 4. Trace buses and ties | Main bus, split bus, section bus, bus coupler and bypass | Which sections can be energized together or separately? |
| 5. Read switching and protection | Breakers, fuses, disconnectors, contactors and protective relays | What can interrupt, isolate, transfer or command each circuit? |
| 6. Follow feeders to loads | Cables, busways, panels, motors, capacitor banks and other loads | What does each outgoing circuit supply? |
| 7. Verify operating state | Normally open/closed notes, interlocks, transfer logic and cross-references | What is the normal path, and what alternate paths are possible? |
What an Electrical Single-Line Diagram Shows
The U.S. Department of Energy describes an SLD as a simplified symbolic representation that gives a high-level view of an electrical system’s components and interconnections. That definition contains two important boundaries: simplified แล้ว high-level.
An SLD commonly shows:
- power sources and nominal system voltages;
- transformers and their principal ratings;
- switchgear, switchboards, panelboards and bus sections;
- circuit breakers, fuses, disconnectors and transfer devices;
- major metering and protective functions;
- feeders, major conductor or cable callouts and destination equipment;
- grounding or neutral arrangements when relevant to the system design;
- normal switch positions and alternate supply paths;
- equipment tags and references to schedules or other drawings.
One drawn line does ไม่ necessarily mean one physical conductor. It can represent a complete three-phase circuit, a multi-core cable or another grouped connection defined by the drawing convention. The phase count, neutral, protective conductor, cable construction and parallel runs must come from labels, notes, schedules or supporting drawings.
What an SLD does not prove
Even a detailed SLD normally cannot prove:
- the physical terminal-to-terminal wiring;
- conductor routing, bending, gland entry or panel layout;
- the actual present position of a field device;
- that a circuit is isolated or safe to touch;
- final breaker or relay settings unless they are explicitly recorded;
- protection selectivity, incident energy or available fault current without the supporting study assumptions;
- installation compliance, workmanship or equipment condition;
- whether undocumented modifications have been made in the field.
Treat the SLD as the system map, not as the entire evidence package.
Pass 1: Trace the System Topology
The first pass answers one question: How can electrical power move through this system? Ignore most detailed ratings until the path is clear.
1. Check the title block, revision and legend
Before tracing a line, confirm that the document belongs to the facility, assembly or project in front of you. Check:
- drawing title and number;
- issue or revision identifier;
- revision description and date;
- drawing status, such as preliminary, as-built or superseded;
- sheet references and continuation markers;
- symbol legend, abbreviation list and project notes;
- stated normal positions or operating assumptions.
A perfectly read obsolete drawing still gives the wrong system model. If the field equipment tags do not match the drawing, stop and resolve the discrepancy rather than guessing.
2. Identify all possible sources
Do not assume the utility incomer is the only source. Scan the whole sheet and related sheets for:
- utility or service entrance;
- standby or emergency generator;
- transformer secondary fed from another drawing;
- uninterruptible power supply (UPS) output;
- photovoltaic inverter or battery energy storage system;
- bus tie or interconnector from another switchboard;
- backfeed-capable equipment where the design permits it.
Mark each source and record its nominal voltage, frequency, phase arrangement and grounding information when shown. A transfer switch, bus coupler or normally open tie can create a second energization route that is easy to miss when reading only left to right.
3. Follow each voltage transformation
Transformers divide the diagram into voltage zones. Read the connection from the primary side to the secondary side, then note any tertiary winding or separately derived neutral shown.
Typical transformer annotations may include:
- apparent-power rating in kVA or MVA;
- primary and secondary voltages;
- phase and frequency;
- winding connection or vector group;
- ค่าอิมพีแดนซ์เป็นเปอร์เซ็นต์;
- grounding arrangement;
- tap range or selected tap, if documented.
These values do different jobs. The voltage ratio identifies the system transition. The kVA rating indicates rated capacity under stated conditions. Percentage impedance is an input to voltage-drop and fault-current calculations; it is not a breaker rating and does not by itself give the final fault current at a downstream bus.
4. Map buses, sections and ties
A heavy horizontal or vertical line often represents a bus, but follow the project legend rather than relying on line weight alone. Give each bus section a temporary identifier if the drawing does not already provide one.
มองหา:
- main bus and section buses;
- bus voltage and continuous-current rating;
- short-time withstand or short-circuit rating where stated;
- normally open or normally closed bus-tie device;
- interlocks that prevent unintended source paralleling;
- continuation references to other sheets.
Do not treat a drawn connection as evidence that two sources are permitted to operate in parallel. That requires the documented operating philosophy, equipment capability, interlocks and protection scheme.
5. Separate switching, isolation and protection functions
Two devices that both appear in series can have very different jobs:
- เป็ วงจร breaker can make, carry and interrupt current within its ratings and provides or works with protective functions.
- เป็ ฟิวส์ interrupts overcurrent by melting its fuse element and must be replaced after operation.
- เป็ disconnector or isolator establishes isolation within its rated and approved use; it may not provide overcurrent protection.
- เป็ สวิตช์ตัดตอน combines switching and isolation functions within its declared ratings.
- เป็ contactor performs frequent electrically controlled switching but normally relies on separate short-circuit and overload protection.
- เป็ สวิตช์ถ่ายโอน selects between sources; its protection and neutral-switching arrangement depend on the system design.
If a device symbol is unfamiliar, use the drawing legend first. Then consult an appropriate symbol reference. VIOX maintains a broader IEC and ANSI electrical symbols chart plus focused references for circuit breaker symbols แล้ว fuse symbols.
6. Trace every feeder to a named destination
Follow each outgoing protective device to its destination. The endpoint may be another panel, a motor control center, a distribution board, a motor, a capacitor bank, a UPS, a rectifier or a cross-reference to another sheet.
Record the feeder tag and destination together. This avoids a common reading error: interpreting a breaker only from its rating while overlooking what it actually supplies.
Feeder annotations may state conductor material, number of conductors, size, insulation designation, number of parallel runs, raceway or cable type, and protective-conductor details. Because notation varies widely, do not infer a complete cable assembly from one number or abbreviation.
7. Reconstruct normal and alternate power paths
After tracing each branch, describe the system in plain language:
In the normal state, source A supplies bus section A and source B supplies bus section B. The bus tie is normally open. If one incomer is unavailable, the approved transfer sequence may open that incomer and close the tie, subject to interlocks and the remaining source capacity.
This statement is only valid if the drawing notes and operating documents support it. For systems with transfer equipment, the VIOX ATS wiring diagram guide explains the difference between power paths, sensing circuits, start contacts and neutral treatment without turning the SLD into terminal-level wiring.
Pass 2: Read Tags, Ratings and Annotations
Once the topology is clear, make a second pass. Read the text around each symbol as carefully as the symbol itself.
Use a three-column interpretation rule
For every important annotation, ask:
- What is stated? Record the exact text and unit.
- What does it allow me to conclude? Keep the conclusion within the rating’s scope.
- What still requires verification? Identify the datasheet, setting record, schedule, study or field check needed next.
| Drawing annotation | What it can indicate | สิ่งที่ไม่สามารถพิสูจน์ได้ด้วยตัวเอง |
|---|---|---|
Bus 400 V, 1600 A |
Nominal bus voltage and stated continuous-current rating | Available fault current, short-time withstand, present loading or temperature rise |
หม้อแปลงไฟฟ้า 1000 kVA, 11/0.4 kV, Z = 6% |
Rated capacity, voltage ratio and stated impedance input | Exact secondary fault current without source, tolerance and conductor data |
Breaker 800 AF / 630 AT |
Typically a stated frame size and trip or sensor rating under the drawing convention | Actual long-time, short-time, instantaneous or ground-fault settings |
Breaker Icu 36 kA or another interrupting value |
A declared breaking rating under a named product standard and voltage | Suitability unless the prospective fault current, standard basis and all conditions are verified |
ซีที 800/5 A |
Nominal primary-to-secondary current-transformer ratio | Meter or relay accuracy, burden, polarity and wiring correctness |
Feeder 4C × 185 mm² Cu |
A project-specific conductor or cable callout | Installation method, parallel-run arrangement or ampacity unless the notation and schedule define them |
มอเตอร์ 90 kW |
Stated mechanical or electrical equipment power according to the project convention | Starting current, efficiency, power factor or required protective settings |
N.O. หรือ N.C. |
Defined normal state under the drawing notes | Actual field position at the time of work |
Abbreviations such as AF, AT, N.O. and N.C. are common but not universal. Use the legend and equipment documentation to confirm their project-specific meaning.
Device tags connect the SLD to the real system
A tag such as QF-101, 52-MAIN, T1 หรือ MDB-1 is more than a label. It links the symbol to schedules, nameplates, protection settings, test records, cable lists and the physical asset.
If the SLD uses ANSI/IEEE device function numbers, read them as function identifiers rather than equipment ratings. IEEE C37.2-2022 is the current IEEE standard for electrical power-system device function numbers, acronyms and contact designations. Common examples encountered on power-system drawings include 50 for instantaneous overcurrent, 51 for time overcurrent, 52 for an AC circuit breaker, 27 for undervoltage and 59 for overvoltage. Prefixes and suffixes can change or refine the meaning, so the project legend and protection documentation remain controlling.
Read protective functions separately from breaker hardware
A breaker symbol tells you that a switching and interrupting device exists. It does not necessarily show which protective functions are active, how they are set or whether an external relay issues the trip command.
For each protected circuit, look for:
- trip-unit type or relay tag;
- overcurrent function references;
- current-transformer ratios;
- ground-fault or residual-current function;
- shunt-trip, undervoltage-release or intertrip notes;
- setting-sheet or coordination-study reference;
- upstream and downstream protective devices.
Never infer selectivity because two breakers have different ampere ratings. Coordination depends on the actual device characteristics, settings, fault levels and applicable study.
Worked Example: Reading a Simplified Low-Voltage SLD
Consider this illustrative system. It is deliberately simplified and is not a construction drawing:
11 kV utility
|
MV protective device
|
T1 — 1000 kVA, 11/0.4 kV, Z = 6%
|
Main LV breaker QF-1
|
400 V main bus MDB-1
|--------------------|--------------------|
Feeder QF-11 Feeder QF-12 Tie QF-T (N.O.)
| | |
MCC-1 DB-LIGHTING MDB-2First pass: establish the topology
The utility is the shown upstream source. T1 changes the voltage from 11 kV to 400 V. QF-1 is the main low-voltage switching/protective device feeding bus MDB-1. Two outgoing feeders supply MCC-1 and DB-LIGHTING. A normally open tie can connect MDB-1 to MDB-2, but the diagram alone does not prove when closing the tie is permitted.
Second pass: interrogate the labels
1000 kVAdefines the stated transformer capacity, not the actual load.11/0.4 kVdefines the nominal transformation ratio.Z = 6%is relevant to system calculations, but source impedance and downstream conductor impedance are still needed for a defensible fault-current result.N.O.states the tie’s documented normal position; it does not confirm the field position now.- The absence of visible feeder ratings in this simplified view means the cable schedule, breaker schedule or another sheet is required before evaluating conductor or protective-device suitability.
Write the unresolved questions
A competent review does not force the drawing to answer what it does not contain. For this example, the next questions are:
- What is the available short-circuit level at the utility connection?
- What are QF-1, QF-11 and QF-12’s exact standards, voltage ratings, breaking capacities and settings?
- What interlocks and operating procedure control QF-T?
- Can T1 carry the combined load if MDB-2 is supplied through the tie?
- Where are the neutral and protective-earth arrangements defined?
- Is this revision consistent with field labels and the latest equipment schedule?
That list is a successful outcome of reading the SLD. It separates known topology from missing engineering evidence.
Essential Symbols: Learn the Function, Then Check the Legend
การ ฐานข้อมูล IEC 60617 contains the IEC graphical symbols used in electrotechnical diagrams. The IEC’s Technical Committee 3 identifies that database as the IEC standard source for diagram symbols. However, real drawings may use IEC, ANSI/IEEE, national, company-specific or legacy conventions.
Instead of memorizing a drawing shape without context, classify each symbol by function:
| Function group | Typical items | Reading question |
|---|---|---|
| Source and conversion | Utility, generator, transformer, UPS, inverter | What produces or changes the electrical supply? |
| Conducting path | Bus, feeder, cable, busway, connection node | Which equipment is electrically connected at the system level? |
| การสับเปลี่ยนและการตัดตอนวงจร | Breaker, switch, disconnector, contactor, transfer switch | What can open, close, isolate or transfer the path? |
| การป้องกัน | Breaker trip unit, fuse, relay function, surge protective device | What abnormal condition is detected or interrupted? |
| การวัด | CT, VT/PT, meter, transducer | What quantity is measured, and through what ratio? |
| โหลด | Motor, panelboard, heater, capacitor bank, converter | Where is power delivered or converted? |
| Grounding and neutral | Ground/earth, neutral point, bonding connection | What grounding or neutral relationship is explicitly shown? |
This functional method is more robust than assuming every rectangle is a breaker or every open contact is an isolator.
Single-Line Diagram vs Wiring Diagram, Schematic and Layout
These documents can describe the same installation at different levels. They are not interchangeable.
| เอกสาร | วัตถุประสงค์หลัก | Usually shows | Usually does not show |
|---|---|---|---|
| แผนภาพเส้นเดียว | System topology and principal ratings | Sources, voltage levels, buses, major devices, feeders and loads | Every conductor, terminal and physical route |
| Schematic or elementary diagram | Functional circuit logic | Contacts, coils, relay logic, control relationships and operating sequence | Physical placement or exact cable route |
| Wiring or connection diagram | Physical electrical connections | Terminals, conductor identifiers, cores and connection points | Complete system-level power architecture |
| Panel layout or general arrangement | Physical construction and location | Device placement, dimensions, clearances and enclosure arrangement | Full electrical logic or every connection |
| Cable schedule | Cable definition and routing record | Cable tag, origin, destination, cores, size and route data | Complete switching and protection logic |
Use the SLD to decide which supporting document you need next. For example, an SLD may identify an automatic transfer switch between two sources, while the wiring diagram defines sensing, generator-start and status connections.
A Practical SLD Review Worksheet
Complete this worksheet before using the drawing for planning, maintenance preparation, procurement review or engineering discussion.
Document control
โทโพโลยี
Engineering data
Verification and stop conditions
Safety Boundary: A Diagram Is Not an Isolation Procedure
An SLD can help identify sources and disconnecting devices, but it cannot establish an electrically safe work condition. In U.S. workplaces within its scope, มาตรฐาน OSHA 29 CFR 1910.333 requires de-energizing, lockout/tagout and verification practices; equipment that has been switched off but not properly locked/tagged and verified must not simply be assumed safe.
For any field task, follow the applicable law, site electrical-safety program, approved switching procedure and equipment instructions. Only qualified persons should perform work that exposes them to energized electrical parts. If the SLD is outdated, ambiguous or inconsistent with the installation, stop and correct the document-control problem before relying on it.
แหล่งอ้างอิงและมาตรฐาน
- U.S. Department of Energy, Industrial Electrification Assessment Framework, Appendix D: Reviewing Single-Line Wiring Diagram
- IEC 60617:2026 DB, Graphical Symbols for Diagrams
- IEC TC 3, IEC 60617 Standard as Database
- IEEE C37.2-2022, Device Function Numbers, Acronyms, and Contact Designations
- OSHA 29 CFR 1910.333, Selection and Use of Work Practices
