Read a single-line power path
Click a supply, breakers, transformer, busbar and load to trace a simplified three-phase single-line diagram.
Educational reference only. Examples and calculator results are not approved installation designs, equipment selections or operating instructions.
Learning goal: Trace the source-to-load route, identify a transformer voltage boundary and distinguish a common busbar from its outgoing feeder.
Explore the diagram
A single-line diagram (SLD) compresses a power circuit into one line rather than drawing each phase conductor. Read connectivity, not physical distance or cable routing. Start at the source, follow equipment tags, and consult the drawing legend before interpreting unfamiliar symbols.
Click or tap equipment to learn its purpose. Keyboard: Tab then Enter or Space. On small screens, scroll the drawing sideways or use the equipment selector.
- Solid: primary power connection
Read it in order
- Read the legend: one solid line represents the three-phase power circuit. Tags S1, Q1, T1, B1, Q2 and L1 are local labels, not standardized protection device numbers.
- Trace S1 → Q1 → T1. Pause at T1: the primary and secondary voltage levels may differ; no numerical voltage is given here.
- Continue to B1, then follow the connected feeder through Q2 to L1. Other feeders, neutral and protective conductors are omitted, not declared unnecessary.
All equipment, explained
Read every explanation here without using the interactive controls.
S1 — Incoming supply
The source feeding this conceptual distribution path.
How to read it here
Follow the solid line from S1 to Q1 and T1. Here it represents a three-phase circuit, not one physical wire.
What you cannot infer
A real supply may have multiple sources or backfeed. This drawing cannot establish isolation or a safe work condition.
Supporting reference: Schneider Electric — MV supply requirements and typical architectures.
Q1 — Incoming circuit breaker
A switching device that can interrupt current, including fault current within its specified capability.
How to read it here
Q1 is upstream of T1. Its protection or trip unit is not drawn in this introductory view.
What you cannot infer
The teaching glyph does not report open, closed or safe-to-touch status. An open breaker is not proof of safe isolation.
Supporting reference: Schneider Electric — Circuit-breaker functions.
T1 — Power transformer
Transfers AC power between windings through magnetic coupling and changes the voltage level according to its design.
How to read it here
The overlapping circles mark a voltage boundary. A real drawing would specify rated voltages, kVA/MVA and winding connections.
What you cannot infer
No voltage ratio or vector group is implied. Do not infer a neutral or earthing arrangement from these circles.
Supporting reference: Schneider Electric — Transformer characteristic parameters and winding configurations.
B1 — Distribution busbar
A common conducting connection from which outgoing circuits can be supplied.
How to read it here
The thick horizontal line is B1. The filled dot at the feeder junction means the branch is connected to the bus.
What you cannot infer
A busbar is not a protective device. Actual current and short-circuit withstand ratings must come from equipment documentation.
Supporting reference: Schneider Electric — Distribution switchboards.
Q2 — Feeder circuit breaker
Provides a current-interruption point for the outgoing circuit to L1.
How to read it here
Trace B1 → Q2 → L1. Separate outgoing circuits can have their own protective devices.
What you cannot infer
This drawing does not demonstrate selectivity or fault-clearing times; those require a protection study.
Supporting reference: Schneider Electric — Protection of transformer and circuits.
L1 — Electrical load
Represents equipment receiving electrical power; its internal circuits are outside this lesson.
How to read it here
L1 is the end of the illustrated feeder, not a claim about the type or size of a real installation.
What you cannot infer
A generic load symbol cannot determine operating current, power factor or starting duty.
Supporting reference: Schneider Electric — Distribution switchboards.
Check your understanding
Think through each question, then reveal its explanation. This is practice, not a competence assessment.
Which item marks the voltage-level boundary?
T1, the power transformer. B1 distributes power at its bus voltage; it does not transform that voltage.
Does one drawn line mean one conductor, or prove the plant is operating?
Neither. This SLD represents a three-phase circuit and contains no live operating data.
Textbook references
Suggested technical background for this topic. These books are further reading, not proof that every explanation was checked against every edition. See the full bibliography and review scope.
- Electric Power Distribution HandbookDistribution equipment, transformers and power-system arrangements.
- Electric Power SystemsHow generators, transmission and distribution fit together.