The standard schematic symbol for potential transformer (PT) circuits consists of two overlapping or adjacent circles representing the primary and secondary magnetic windings. In North American ANSI/IEEE schematics, it is drawn with parallel primary connections and labeled with H1/H2 (primary) and X1/X2 (secondary) terminals. In international IEC schematics, the circles are often separated or drawn with specific winding geometry, using A-N and a-n terminal designations. Also known as a Voltage Transformer (VT), the PT steps down high line voltage to a safe, standardized level (typically 120V or 110V) for metering and protective relaying.
Potential Transformer Symbol & Terminal Reference Table
The table below maps the schematic symbols you will encounter on single-line diagrams (SLDs) to their physical terminal markings and practical applications. Use this as your bench and jobsite reference.
| Symbol / Configuration | Standard | Terminal Markings | Practical Meaning & Application |
|---|---|---|---|
| Standard Magnetic PT (Two overlapping circles) |
ANSI/IEEE | H1, H2 (Primary) X1, X2 (Secondary) |
Steps down line voltage for standard metering. H1/H2 connected in parallel with the line. Polarity dot on H1 and X1 indicates subtractive polarity. |
| Standard Magnetic VT (Two adjacent circles, distinct windings) |
IEC 61869 | A, N (Primary) a, n (Secondary) |
Functionally identical to ANSI PT. 'N' and 'n' denote the grounded neutral terminals. Used extensively in European and global SLDs. |
| Capacitive Voltage Transformer (CVT) (Capacitor stack above transformer circles) |
ANSI / IEC | H1, H2 / A, N X1, X2 / a, n |
Uses a capacitive voltage divider for EHV (Extra High Voltage) lines >115kV. The symbol adds a capacitor stack above the magnetic transformer symbol. |
| Grounding PT (Wye-Broken Delta) (Three PTs, secondary in open delta) |
ANSI/IEEE | H1, H2 (x3) X1, X2, X3 |
Used for ground fault detection. The secondary windings are wired in a broken-delta configuration to produce zero-sequence voltage during a fault. |
Regional Standard Variants: ANSI vs. IEC vs. Old UK
When interpreting legacy prints or working on international projects, you must know which standard governs the drawing. Misinterpreting a terminal designation can lead to shorted secondaries or miswired protective relays.
ANSI/IEEE (North America)
Governed by IEEE C37.2 and NEMA C57.13. The defining feature is the H and X nomenclature. H1 is always the primary line terminal, H2 is the primary neutral/ground. X1 is the secondary line, X2 is the secondary neutral. ANSI PTs are almost universally wound for subtractive polarity, meaning H1 and X1 are physically adjacent on the nameplate and share the same instantaneous polarity.
IEC (International / Modern Europe)
Governed by IEC 61869-3. The IEC prefers the term Voltage Transformer (VT) over PT. Terminals are marked with uppercase letters for primary (A, B, C, N) and lowercase for secondary (a, b, c, n). The schematic symbol often explicitly draws the core and winding geometry rather than just using overlapping circles, making it easier to identify multi-ratio or tertiary windings on complex SLDs.
Old UK (Legacy BS Standards)
Before harmonization with IEC, older British installations followed BS 3937. You may encounter legacy prints where primary terminals were simply marked with phase colors (Red, Yellow, Blue) or used non-standard alphanumeric sequences. If you are retrofitting a legacy UK substation, do not trust the physical terminal block layout to match modern IEC A-N/a-n standards; trace the windings with a meter.
The 'Rows People Get Wrong' & Faded Nameplate Identification
Rows People Get Wrong on Schematics
- Confusing PT with CT Symbols: A Current Transformer (CT) symbol typically features a single circle with a straight line passing through it (representing the primary bar conductor). A PT always shows two distinct winding circles because both primary and secondary are wound coils connected in parallel.
- Missing the Polarity Dot: In relay protection (especially directional overcurrent or differential relays), the polarity dot on the PT symbol is critical. If the dot is on H1 and X1, they are in phase. Reversing the secondary wiring on a directional relay will cause it to trip for faults in the opposite direction.
- Ignoring the Tertiary Winding: If a PT symbol shows a third circle or a third set of terminals (e.g., Y1, Y2), it is a three-winding PT. The tertiary winding is almost always used for open-delta ground fault detection, not for standard metering. Do not parallel it with the main secondary.
Safe Interpretation When Markings are Faded or Missing
On aging jobsites, UV exposure and weathering can obliterate PT nameplates. If you cannot read the H1/X1 designations, use a digital multimeter (DMM) to identify the windings safely while the circuit is de-energized and locked out.
- Identify Primary vs. Secondary: Measure DC resistance across the terminals. The primary winding (H1-H2) consists of thousands of turns of fine wire and will show a high resistance (often hundreds or thousands of ohms, or read 'OL' if an internal fuse is blown). The secondary winding (X1-X2) uses thick wire with very few turns and will read near 0 ohms (typically < 1 ohm).
- Verify Core Integrity: Check for continuity between the windings and the core/ground. There must be infinite resistance (OL) between any winding terminal and the grounded steel core. Any reading below infinite indicates a dielectric breakdown; the PT is condemned and must be replaced.
- Determine Polarity: If you must verify subtractive polarity for a relay, use a low-voltage DC source (like a 9V battery) momentarily tapped across the primary while monitoring the secondary with an analog DC voltmeter. A positive 'kick' on the meter when the battery positive is applied to H1 confirms X1 is the corresponding secondary terminal.
Frequently Asked Questions
What is the difference between a PT and VT symbol?
There is no electrical difference; they represent the exact same device. 'Potential Transformer' (PT) is the legacy North American ANSI/NEMA term, while 'Voltage Transformer' (VT) is the modern IEC and IEEE preferred terminology. On older US single-line diagrams, you will see 'PT' written next to the overlapping circle symbol. On modern international prints, 'VT' is used. The schematic symbol itself remains identical.
How do I read the polarity dot on a potential transformer symbol?
The polarity dot indicates terminals that share the same instantaneous voltage polarity. If current enters the dotted primary terminal (H1), it exits the dotted secondary terminal (X1) in phase. In 99% of North American metering applications, PTs are wound for subtractive polarity. This means if you look at the physical nameplate, H1 and X1 are located on the same side (usually the left). When wiring wattmeters or directional relays, maintaining this dot-to-dot phase relationship is mandatory to prevent reverse-power tripping.
Why does my PT symbol have a zig-zag or broken-delta secondary?
If the schematic shows three PTs with their secondary windings wired in series but left open at the ends (a broken-delta or open-delta configuration), this is a zero-sequence voltage filter. Under normal balanced three-phase conditions, the vector sum of the secondary voltages is zero, and no voltage appears across the open ends. During a single-line-to-ground fault, the system becomes unbalanced, and a residual voltage (typically 3V0) appears across the open delta, triggering a 59N ground fault relay. You will see this symbol frequently on ungrounded or resistance-grounded medium voltage systems.
