Voltage Transformer (PT) Symbol
Definition: The Voltage Transformer (PT) symbol represents an instrument potential transformer used in electrical schematics to denote a precision two-winding transformer that steps down high AC voltage to a standardised low-voltage signal — typically 110 V or 120 V secondary — for safe connection to voltmeters, protective relays, and metering equipment, as defined in IEC 60617-06 and IEC 60044-2 (instrument transformers); the designator is VT or PT.
Also known as: potential transformer, PT, voltage transformer, VT, instrument voltage transformer, metering transformer, capacitor voltage transformer, CVT.
What the Voltage Transformer (PT) symbol means
The Voltage Transformer (PT) symbol denotes a precision step-down transformer whose primary winding connects directly to a high-voltage circuit (typically 1 kV to 765 kV) and whose secondary winding produces a standardised 110 V (IEC) or 120 V (ANSI) output with a tightly controlled turns ratio accuracy, typically 0.1–3% (accuracy class). The two-terminal primary (H1–H2) and two-terminal secondary (X1–X2) arrangement is shown in the schematic symbol, reflecting the physical bushing designations on the instrument transformer.
In a power system single-line diagram, the Voltage Transformer symbol communicates that the high-voltage bus potential is being safely reproduced at low voltage for measurement, protection, and metering instruments. Engineers reading the diagram understand that the secondary terminals must never be short-circuited (which would damage the transformer and possibly cause injury) and that the PT's accuracy class determines whether the secondary signal is suitable for revenue-grade metering or only for indication and protection.
How to identify the Voltage Transformer (PT) symbol
The Voltage Transformer (PT) symbol is drawn as a standard two-winding transformer icon — two sets of arc-shaped coils facing each other across a magnetic core — labelled 'VT' or 'PT'. The primary side shows two high-voltage terminals (H1 and H2) on the left; the secondary side shows two low-voltage terminals (X1 and X2) on the right. The symbol is physically smaller than a power transformer on the diagram, reflecting the PT's instrumentation role. Some IEC single-line diagrams show the VT as a circle (representing the transformer) with one primary line entering and one secondary line exiting, particularly in simplified one-line formats.
Function and usage boundary
The Voltage Transformer steps down high-voltage AC from a power system bus to a safe, standardised low voltage that can be measured by panel instruments, fed to electronic protection relays, or supplied to revenue meters. By reproducing the primary voltage waveform at a known, stable ratio (e.g., 11,000 V / 110 V = 100:1), the PT allows accurate voltage measurement and phase-angle determination across the full primary voltage range without exposing instruments or personnel to high voltage.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-06 defines the voltage transformer symbol as a standard two-winding transformer with labelled HV and LV terminals. IEC 60044-2 (now IEC 61869-3) specifies voltage transformer accuracy classes (0.1, 0.2, 0.5, 1, 3), standard secondary voltage (110 V line-to-line), burden ratings, and insulation levels. Primary terminals are designated H1/H2; secondary terminals X1/X2. |
|---|---|
| ANSI/IEEE 315 | ANSI C57.13 / IEEE C57.13-2016 defines the accuracy classes and ratings for instrument voltage transformers in North America. The schematic symbol under ANSI Y32.2/IEEE 315 is the same two-winding transformer rectangle or coil pair, with primary designated H1/H2 and secondary X1/X2. Standard secondary voltage is 120 V line-to-line. |
| Key difference | IEC and ANSI symbols are structurally identical. The main differences are in secondary voltage standard (110 V IEC vs 120 V ANSI), accuracy class designations (IEC 0.2S vs ANSI 0.3), and burden units (IEC uses volt-amperes at rated burden; ANSI defines standard burdens W, X, M, Y, ZZ). |
The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.
Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| h1 | H1 |
| h2 | H2 |
| x1 | X1 |
| x2 | X2 |
Where the Voltage Transformer (PT) symbol is used
- High-voltage switchgear and substations for bus voltage metering and protection relay inputs
- Revenue metering panels where the PT secondary feeds kWh and kVAr meters with accurate voltage signals
- Protection systems supplying voltage to undervoltage, overvoltage, and distance relays
- Power quality monitoring instruments sampling the true HV bus waveform at safe low voltage
- Synchronising panels comparing bus and generator voltages before closing a circuit breaker
- Capacitor bank and reactor protection circuits requiring precise voltage measurement for switching decisions
- Industrial medium-voltage (3.3 kV–33 kV) motor control centres measuring feeder bus voltages
Key facts
- The Voltage Transformer (PT) symbol is a two-winding transformer icon with HV terminals H1/H2 (primary) and LV terminals X1/X2 (secondary), defined in IEC 60617-06 and IEC 61869-3 (formerly IEC 60044-2).
- The designator for a voltage transformer in a schematic is VT or PT, per IEEE C57.13 and ANSI Y32.2/IEEE 315-1975.
- Standard secondary voltages are 110 V line-to-line (IEC) and 120 V line-to-line (ANSI), regardless of the primary voltage.
- Accuracy classes defined in IEC 61869-3 range from 0.1 (highest accuracy, revenue metering) to 3 (indication only); class 0.2S is required for high-accuracy revenue metering.
- The PT secondary must NEVER be short-circuited; unlike a current transformer (CT) that must never be open-circuited, a short circuit across a PT secondary causes destructive current and possible arc flash.
- Capacitor Voltage Transformers (CVTs) are a special form used above 72.5 kV, using a capacitor divider to step the voltage down before the final electromagnetic transformer stage.
- A Voltage Transformer differs from a power transformer in that it is optimised for accuracy (tight turns-ratio tolerance) rather than power transfer capacity; its VA burden rating is much lower (10–500 VA vs kVA–MVA for power transformers).
- IEC 61869-3 replaced IEC 60044-2 as the governing standard for instrument voltage transformers; both specify the same circle-and-coil or coil-pair schematic symbol.
Frequently asked questions
What does the Voltage Transformer (PT) symbol mean in a circuit diagram?
The Voltage Transformer (PT) symbol represents a precision instrument transformer that steps down high-voltage AC to a standardised low-voltage signal (110 V or 120 V) for safe use by meters, relays, and instruments. The symbol shows two winding coils with H1/H2 high-voltage terminals and X1/X2 low-voltage terminals.
What does the voltage transformer symbol look like?
The voltage transformer symbol is a standard two-winding transformer: two sets of arc coils facing each other across a core bar, labelled 'VT' or 'PT'. The left terminals are H1 (and sometimes H2) for the primary high-voltage side; the right terminals are X1 (and X2) for the secondary low-voltage side.
What is the difference between a potential transformer (PT) and a current transformer (CT)?
A Potential Transformer (PT/VT) is connected in parallel with the high-voltage circuit to step down voltage; it has high primary impedance and low secondary impedance. A Current Transformer (CT) is connected in series and steps down current; its secondary must never be open-circuited. The schematic symbols differ: the PT uses a standard two-winding transformer icon, while the CT uses a toroidal or bar-through-core symbol.
What is the standard secondary voltage of a voltage transformer?
The standard secondary voltage of an instrument voltage transformer is 110 V line-to-line under IEC 61869-3 (IEC standard) or 120 V line-to-line under ANSI C57.13 (North American standard). These values are fixed regardless of the primary voltage, which can range from a few kilovolts to hundreds of kilovolts.
What is the designator letter for a voltage transformer?
The voltage transformer is designated VT or PT in schematics, per ANSI Y32.2/IEEE 315-1975 and IEEE C57.13. 'VT' is the modern IEC-preferred term; 'PT' (potential transformer) is the older ANSI/NEMA term still widely used.
What standard defines the voltage transformer symbol and ratings?
IEC 60617-06 defines the schematic symbol; IEC 61869-3 (which replaced IEC 60044-2) specifies accuracy classes, burden ratings, and secondary voltages for instrument voltage transformers. In North America, ANSI C57.13 / IEEE C57.13-2016 governs equivalent specifications.
Can I short-circuit the secondary of a voltage transformer?
No. Short-circuiting the secondary of a voltage transformer is dangerous and will destroy the transformer. Because the secondary is the output of a voltage source with low internal impedance, a short circuit drives very high fault current through the secondary winding. This is the opposite of a current transformer, whose secondary must never be open-circuited.
Related symbols
- 3-Phase Supply symbol
- Split-Core CT symbol
- Current Transformer (CT) symbol
- Ground / Earth symbol
- Neutral Rail symbol
- Power Rail symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
Place the Voltage Transformer (PT) symbol directly on a wiring diagram or schematic in the free online editor — no download required.