Electric power systems utilize a range of measurement and protection devices to assist engineers and technicians in operating correctly and securely. Most crucial of all probably is the potential transformer, or voltage transformer.
Electric equipment on schematic drawings is all indicated by a symbol instead of an image. The potential transformer symbol allows for recognition and comprehension of measurement and control of voltage in high-voltage systems.
This article reveals what the symbol of a possible transformer is, its appearance, operation, and application in electric circuits and power grids.
What Is a Potential Transformer
A potential transformer or PT is a device transformer used to measure high voltage because of safety reasons. It reduces power cable high voltages to a lower voltage that is safe to be measured by protection relays or ordinary meters.
In easy words, a potential transformer is a voltage divider between the high voltage circuit and measuring instruments.
Example:
An 11,000-volt power cable is stepped down to 110 volts through a potential transformer and it becomes very easy to measure.
Symbol Meaning of Potential Transformer
Symbol for potential transformer is a diagram that is employed in circuit diagrams to show the existence of a PT in a circuit.
The conventional symbol is also drawn as two spirals (primary and secondary windings) close together, occasionally with parallel lines and the notation like “P1, P2” for the primary side and “S1, S2” for the secondary side.
In some cases, the symbol is also marked with the letter “PT” or “VT” beside it to ensure ease of recognition.
This symbol warns the reader of the electrical diagram that voltage is reduced or is being measured at the location in the circuit.
Importance of Potential Transformer Symbol
Electrical circuits carry thousands of volts through lines. It is neither practical nor safe to provide such voltages directly to measurement tools.
The potential transformer symbol is significant because:
- It shows where voltage measurement takes place.
- It offers protection in circuit design.
- It provides the recognition of the relationship between the low voltage and high voltage equipment.
- It ensures one understands power system arrangements.
Without it, circuit diagrams would be meaningless and unsafe to study.
Potential Transformer Operation
A potential transformer operates via electromagnetic induction. It has two windings:
| Winding | Connection |
| Primary winding | In the high-voltage circuit |
| Secondary winding | On measurement instruments such as voltmeters or protection relays |
When the primary winding is supplied with alternating current, it creates a magnetic field. The field induces a proportionally smaller voltage in the secondary winding depending on the turns ratio.
The turns ratio also illustrates the amount of voltage that gets dropped.
Example:
If the ratio is 100:1, then an input of 10,000 volts will drop to an output of 100 volts.
Structure of a Potential Transformer Symbol
A typical potential transformer symbol is made up of:
- Two vertically wound lines (primary and secondary)
- One dotted-line magnet core line in between
- High-voltage side and low-voltage side labels
- Ground connection mark or terminals occasionally
The symbol differs slightly depending on the drawing convention (e.g., IEC, ANSI, or IEEE), but the concept is similar.
Standard Symbol for Potential Transformer
Different global standards use slightly different versions of the symbol.
| Standard Organization | Symbol Description / Use |
| IEC (International Electrotechnical Commission) | Two coils with a dashed line in the center for the core and the letters “VT” |
| ANSI (American National Standards Institute) | Circles of coils connected with lines and markings “PT” |
| BIS (Bureau of Indian Standards) | Similar to IEC pattern with minimal visible terminal markings |
Regardless of the pattern, the role of the symbol remains the same to show that voltage transformation is going on safely.
Types of Potential Transformer
There are really three types of potential transformer, each with a unique form of symbol in line diagrams.
| Type | Description | Symbol Representation |
| Electromagnetic Type | Most commonly used type, copper-wound and iron core | Shows two coupled coils |
| Capacitive Voltage Transformer (CVT) | Used on very high-voltage applications | Combines transformer coils and capacitor symbols |
| Electronic Type | Contains sensors and electronic circuitry | Includes rectangular boxes or signal arrows |
Role of a Potential Transformer
The main role of a potential transformer is to step down high voltage to a measurement-safe level, control, and protection.
Other main functions:
- Safe, convenient, and precise measurement of high voltage
- Supply insulation between high voltage and measuring instruments
- Provide accurate measurement of voltage for instruments and meters
- Provide reference voltage to power protection system relays
- Provide accuracy and safety to control circuits
In short, a potential transformer renders measurement of high voltage safe, simple, and precise.
Difference Between Potential Transformer and Current Transformer
Both are instrument transformers but used for different applications.
| Feature | Potential Transformer (PT) | Current Transformer (CT) |
| Measurement Type | Voltage | Current |
| Connection | Parallel with the circuit | Series with the circuit |
| Function | Steps down voltage | Steps down current |
| Use | Provides safe voltage for meters and relays | Provides safe current for relays and meters |
Both have special symbols on circuit diagrams in order to avoid ambiguity.
How to Read a Circuit with a Potential Transformer Symbol
When reading an electrical diagram, the PT symbol shows where the circuit voltage is being taken.
If you find a PT symbol between voltmeter or relay and high voltage bus, it shows the PT reduces the voltage to be measured by the relay or voltmeter in a secure way.
These indications are used by engineers to study voltage levels, connections, and safety isolations inside the system.
Uses of Potential Transformer
Potential transformers in electrical systems serve applications for multiple purposes.
| Application | Function |
| Measurement | To measure the voltage using standard metering devices |
| Protection | For protection relay voltage supply |
| Control | For voltage stabilizer and controller control |
| Metering | For metering energy at power stations and substations |
| Signal Transmission | For transmitting proportional voltage data to monitoring systems |
PTs are used by substations, factory plants, or large commercial facilities that contain electrical systems in some form or another.
Standard Ratings and Specifications
There are different voltage and accuracy ratings available in potential transformers.
| Voltage Rating | Secondary Voltage | Common Use |
| 6.6 kV | 110 V | Distribution systems |
| 11 kV | 110 V | Substations |
| 33 kV | 110 V | Transmission systems |
| 132 kV | 110 V | High-voltage networks |
Accuracy classes are 0.1, 0.2, 0.5, or 1.0 depending on the requirement. Accuracy class shows how close the output voltage is to the true value.
Practical Example of PT Symbol Use
Assume that there is a substation delivering power of 11,000 volts. Direct connection between such a voltage type and the measuring instruments is risky. A potential transformer is utilized across the line in such a scenario. The PT transforms the voltage to 110 volts, and this voltage can be safely read by the voltmeter.
In the circuit diagram, this is shown as:
Power line → PT symbol → measuring device
This simple-to-interpret symbol helps students and engineers visualize how high voltage is measured and stepped down.
Safety Implication of Potential Transformer Symbol
The symbol reminds engineers that the device provides electrical protection between low-voltage and high-voltage circuits.
This protection prevents dangerous voltages from traveling to measuring devices or human bodies. It provides protection in control rooms, labs, and power plants.
Also Read: Electric Flux Symbol Definition: Symbol, Formula, and Easy Understanding
Educational Implication
Physics or electrical engineering students are typically familiar with the symbol for a potential transformer on the circuit diagram. Their familiarity helps in learning voltage measurement, safe power system design, and electromagnetic induction.
It is also helpful in proper reading of substation plans, industrial prints, and control system schematics.
Tips for Memorization of the Symbol
| Tip | Explanation |
| Two coils in parallel | Show two voltage levels |
| Coil with more turns | Indicates primary (high-voltage) side |
| PT or VT label | Used for easy identification |
| Secondary side | Connected with meters or relays, not power lines |
These points help easily recognize PT symbols in any diagram.
Benefits of Using a Potential Transformer
| Benefit | Explanation |
| Provides safety | Protects while measuring high voltages |
| Ensures isolation | Electrically separates systems |
| Increases accuracy | Displays exact voltage readings |
| Aids automation | Helps design protection and control systems |
| Eases maintenance | Makes power systems easier to test and maintain |
Potential transformer symbol is a simple yet effective element of electric circuit diagrams. It is used as a symbol of a transformer that is used to transform high voltage down to the level at which it is safe for the protection devices and instruments.
Knowing this PT symbol, you can already grasp how electricity is metered and controlled in real systems. Reading a substation diagram or constructing a new control system becomes simpler when you know this symbol as it makes it safe, precise, and dependable.
The prospective transformer can be small on drawings, but in practice it is a vitally important device that stabilizes the high-voltage network and makes it measurable.