4.6 One-Lines, Three-Lines, Schematics & Electrical Symbols
Key Takeaways
- A one-line diagram collapses all three phases into a single line to show power flow, equipment, ratings, and protection; it is the map you use to plan isolation and testing.
- A three-line diagram shows each phase separately and is the only drawing that reveals CT and PT connections, polarity marks, and phase-by-phase relay inputs.
- An elementary or schematic diagram shows control logic in functional order with no regard for physical layout, while a connection or wiring diagram shows terminal-by-terminal physical wiring.
- Device function numbers are ANSI/IEEE C37.2 numbers and are combined with suffixes and prefixes - 50/51 is a combined instantaneous and time overcurrent element, 51N is its neutral-residual counterpart.
- Reading the drawing set in the right order - one-line for scope, three-line for instrument transformers, schematic for logic, connection diagram for terminals - is what turns a trip investigation from guesswork into a procedure.
Four Drawings, Four Jobs
Every NETA job runs on a drawing set, and the Level 2 exam expects you to know which drawing answers which question. Reaching for the wrong one wastes time on the exam and on the job.
| Drawing | Shows | The question it answers |
|---|---|---|
| One-line (single-line) | Three phases collapsed to one line; sources, transformers, buses, breakers, feeders, ratings, protective devices | "What is upstream of this equipment, and what do I have to open to isolate it?" |
| Three-line | Each phase drawn separately, with CTs, PTs, polarity marks, and per-phase relay inputs | "Which CT feeds which relay input, and which way does polarity point?" |
| Elementary / schematic (ladder) | Control logic drawn in functional order, ignoring physical layout | "Why did this breaker trip, and what has to be true for it to close?" |
| Connection / wiring / interconnection | Physical terminal blocks, wire numbers, cable routing between panels | "Which screw do I land this jumper on?" |
The One-Line Diagram
The one-line is the plan-view of the power system. It shows one line where three conductors exist, because in a balanced three-phase system all three phases carry the same information. A usable one-line carries: utility source and available fault current, transformer kVA, primary and secondary voltages, impedance percentage and winding configuration, main and feeder breaker frame and trip ratings, CT and PT ratios, relay device numbers, bus ratings, and the grounding arrangement.
On a test job the one-line is what you mark up for the switching plan. Before you touch anything, you trace every source that can energize the equipment — including backfeed paths through tie breakers, generators, and UPS outputs. The most dangerous one-line error is an undocumented second source, which is why the field verification of the one-line is itself part of commissioning.
What a one-line will not tell you: which CT polarity mark faces the bus, how a control circuit sequences, or which terminal a wire lands on. Those need the other three drawings.
The Three-Line Diagram
The three-line (sometimes "three-phase diagram") expands each phase. It is the only drawing that shows:
- Individual CTs on each phase, their ratios, taps in use, and polarity dot or H1 marking
- Which CT secondaries are paralleled, and where the residual or neutral connection is formed
- PT connections — wye, delta, open-delta, and the grounding of the secondary
- Which phase feeds which relay input, and the phase rotation assumed by the scheme
For differential (87) and directional (67) work, the three-line is mandatory. A polarity reversal on one CT is invisible on a one-line and obvious on a three-line.
Elementary (Schematic) Diagrams
An elementary diagram — often drawn as a ladder with two vertical rails and horizontal rungs — shows the control circuit as logic. Devices are drawn in their de-energized, de-activated state by convention, so a normally open contact is drawn open even if it happens to be closed right now with the equipment in service.
The conventions that matter:
- Contacts are drawn shown normal: NO contacts open, NC contacts closed, with the device de-energized and, for a breaker auxiliary, the breaker open.
- 52a contacts follow the breaker (closed when the breaker is closed); 52b contacts are opposite (closed when the breaker is open). The 52a in series with the trip coil is what prevents the trip coil from burning up after the breaker has opened.
- DC control circuits are usually drawn positive rail on the left, negative on the right; the coil sits at the right end of the rung so that a ground fault on the wire between the contacts and the coil cannot energize the coil.
Read a trip circuit right to left: which contacts had to close for that coil to energize? That reading is exactly how you troubleshoot an unexplained trip.
Connection and Interconnection Diagrams
The connection diagram (panel wiring diagram) shows the physical reality: terminal block TB1 point 7, wire number 52TC, going to the cubicle door. An interconnection diagram shows the cables between enclosures. These are the drawings you use when landing test leads or restoring wiring you lifted, and the discipline is simple: photograph and record before you lift, restore against the drawing, then verify by test rather than by memory.
Symbols You Must Recognize
Symbols vary between drafting standards, but the recurring set on power one-lines is small:
| Symbol | Device |
|---|---|
| Square or rectangle in the line, sometimes with an X | Drawout circuit breaker |
| Two interlocking circles / coils | Two-winding transformer, with winding configuration marked (delta triangle, wye Y) |
| Circle with a diagonal blade | Disconnect switch (open blade shows the isolating gap) |
| Circle with a number inside | Protective relay, numbered per ANSI device number |
| Circle on the line with a bar | Current transformer, with the polarity dot indicating H1 |
| Small rectangle with a squiggle | Fuse |
| Arrow or arrowhead to ground | Grounding connection or grounding electrode |
| Circle with a wavy line | Generator or motor, lettered G or M |
Device Function Numbers in Context
ANSI/IEEE C37.2 device numbers appear inside the relay circles. Section 4.1 covers the numbers themselves; what the drawing set adds is how they are qualified:
- A suffix letter narrows the element: 51N is a time overcurrent element on the neutral or residual circuit, 51G on a dedicated ground CT, 87T a transformer differential, 87B a bus differential.
- A combined number shows one device performing two functions: 50/51 is one relay providing both instantaneous and time overcurrent.
- A prefix number identifies the circuit or equipment: 52-1 and 52-2 are two different breakers, not two functions.
Using the Set Together: A Worked Investigation
A 480 V feeder breaker has tripped with a 51N target. The order of drawings is not optional:
- One-line — identify the feeder, its CT ratio, the relay, and the upstream device you must coordinate with. Confirm the isolation points and whether any alternate source can backfeed the feeder.
- Three-line — confirm how the residual connection that feeds the 51N element is formed. If the three phase CTs are residually connected, an open or shorted CT secondary on one phase produces a false residual current and a genuine-looking 51N trip with no actual ground fault.
- Schematic — trace the trip path: 51N output contact, through the 52a, into the trip coil. Confirm nothing else in that rung could have initiated the trip.
- Connection diagram — verify the physical CT secondary landings and shorting screws before you inject.
Only then do you test. Skipping straight to the megger is how technicians spend an afternoon proving a healthy cable is healthy.
Which drawing is required to verify that a current transformer's polarity mark faces the bus on a differential scheme?
On an elementary diagram, a 52b contact is shown closed. What does that indicate?
A relay circle on a one-line is labeled 51N. What does the N designate?