Current Transformer (CT) Symbol
Definition: The Current Transformer (CT) symbol represents an instrument transformer that produces a secondary current proportional to the primary current flowing through its core, used for metering and protection in AC power systems; it is standardised in IEC 60044-1 / IEC 61869-2 and represented in schematic diagrams by two coupled coil symbols sharing a common magnetic core, with terminals labelled P1, P2 (primary) and S1, S2 (secondary).
Also known as: CT, current transformer, instrument transformer, measurement CT, protection CT, split-core CT.
What the Current Transformer (CT) symbol means
The Current Transformer symbol denotes a device that steps down a large primary AC current to a standardised, safe secondary current—typically 1 A or 5 A—for input to ammeters, energy meters, protective relays, and power quality analysers. The CT core links the magnetic flux created by the primary conductor to the secondary winding, inducing a secondary current in a fixed ratio (the turns ratio) to the primary. Because the primary is usually just a single conductor pass-through (one turn), the device does not alter the primary circuit voltage or current.
In power system diagrams the CT symbol signals a current measurement or protection injection point. The polarity marks (P1/S1 are the dotted or starred terminals) indicate the phase relationship between primary and secondary currents, which is critical for correct connection of protection relays and energy meters.
How to identify the Current Transformer (CT) symbol
The Current Transformer symbol consists of two adjacent semicircular coil loops (representing the primary and secondary windings) sharing a vertical core line between them. The primary side (P1, P2) is often drawn as a single straight line through the core, representing the heavy bus bar or conductor that carries the measured current. The secondary side (S1, S2) is drawn as a wound coil on the right. A polarity dot marks P1 and S1 to indicate that when current enters P1 it exits S1 in the standardised direction.
Function in a circuit
A current transformer isolates the metering/protection system from the high-voltage primary circuit and steps the primary current down to a safe level. The secondary winding drives a low-impedance burden (ammeter, relay coil, or energy meter current input). If the secondary circuit is open-circuited while the primary carries current, the core flux is unrestricted and the secondary can develop a dangerously high voltage; CTs must never be open-circuited under load.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61869-2 (superseding IEC 60044-1) defines current transformer specifications, accuracy classes (0.1, 0.2, 0.5, 1, 3, 5 for metering; 5P, 10P for protection), and schematic symbol conventions. IEC 60617 represents the CT with coupled winding symbols and a shared core. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE C57.13 / IEEE 315 defines CTs for the North American market. The symbol follows IEEE 315: two coupled inductors on a common core. American metering CTs use secondary ratings of 5 A (vs. the IEC preference for 1 A or 5 A). |
| Key difference | IEC and ANSI symbols are visually identical (coupled coils on a core). The primary difference lies in accuracy-class labelling conventions: IEC uses numeric accuracy classes (e.g. 0.5S), while ANSI/IEEE uses accuracy classes expressed as a C or T designation (e.g. C100, C200). |
Terminals / pins
| Pin | Name |
|---|---|
| p1 | P1 |
| p2 | P2 |
| s1 | S1 |
| s2 | S2 |
Typical values
Primary current rating: 5 A to 10,000 A. Standard secondary current: 1 A or 5 A (IEC), 5 A (ANSI). Burden: 2.5–30 VA. Accuracy classes (metering): IEC 0.1, 0.2S, 0.5, 1; ANSI C200, C400. Protection accuracy classes: IEC 5P, 10P; ANSI C or T class. Transformation ratio examples: 100/5 A, 400/1 A, 2000/5 A.
Where the Current Transformer (CT) symbol is used
- MV/HV switchgear panels where direct connection of ammeters to the bus bar is impractical due to high current.
- Energy meters (kWh meters) in utility substations and industrial plants that require accurate billing-grade current measurement.
- Overcurrent and earth-fault protection relays that trip circuit breakers when measured secondary current exceeds a set threshold.
- Power quality analysers measuring harmonic distortion on distribution feeders.
- Differential protection schemes for power transformers and generators, where CT secondary currents are compared to detect internal faults.
- Split-core CTs installed on live conductors during retrofitting of energy monitoring systems without disconnecting the load.
Example
In a 415 V three-phase motor protection panel, three 200/5 A current transformers are connected one per phase of the motor supply. Each CT secondary feeds a 5 A input on a motor protection relay. The relay monitors current balance, overload, and locked-rotor conditions, tripping the contactor if secondary current (proportional to motor current at 40:1 ratio) exceeds programmed limits.
Key facts
- A Current Transformer (CT) steps down primary AC current to a standard secondary level (1 A or 5 A IEC, 5 A ANSI/IEEE) using magnetic induction, allowing safe connection of metering and protection equipment.
- The four terminals are P1/P2 (primary, high-current) and S1/S2 (secondary, low-current); P1 and S1 carry the polarity dot, indicating that current entering P1 exits S1.
- IEC 61869-2 defines CT accuracy classes: 0.1, 0.2, 0.5, 1, 3, 5 for metering and 5P, 10P for protection applications.
- ANSI/IEEE C57.13 uses a different accuracy classification system (C or T class, e.g. C200) and favours a 5 A secondary rating.
- A CT secondary must never be open-circuited while primary current is flowing—the resulting uncontrolled core flux can generate kilovolt-level voltages on the secondary terminals.
- Split-core CTs (clamp-on type) can be installed on live conductors without interrupting the circuit, making them common for retrofit energy monitoring.
- The CT transformation ratio is expressed as primary-to-secondary current ratio (e.g. 400/5 A = 80:1 ratio); the reciprocal (5/400) is the turns ratio N_s/N_p.
Frequently asked questions
What does the current transformer symbol look like in a circuit diagram?
The current transformer (CT) symbol shows two coupled coil symbols (semicircular loops) on a shared magnetic core line. The primary side (P1, P2) is typically drawn as a straight conductor pass-through, and the secondary side (S1, S2) is shown as a wound coil. Polarity dots on P1 and S1 indicate the phase relationship of the secondary current.
What does the CT symbol mean in a wiring diagram?
The CT symbol marks a point where the primary circuit current is measured indirectly by transformer action. The device reduces the large primary current to a safe, standardised 1 A or 5 A secondary output that drives ammeters, energy meters, or protective relays without direct connection to the high-current line.
What is the difference between IEC and ANSI current transformer symbols?
The schematic glyph is identical in IEC 60617 and IEEE 315—both show coupled coils on a core. The standards differ in accuracy class notation (IEC: 0.5, 5P10; ANSI/IEEE: C200, C400) and preferred secondary current (IEC allows 1 A or 5 A; ANSI favours 5 A).
Why can't a current transformer secondary be left open-circuit?
When primary current flows, the CT core must have a low-impedance secondary burden to limit flux. An open secondary removes the demagnetising ampere-turns of the secondary current, allowing the core to saturate and produce high peak voltages (potentially several kilovolts) at the secondary terminals—creating a lethal shock hazard and potentially destroying the CT insulation.
What are the pins on a current transformer?
A current transformer has four terminals: P1 and P2 on the primary winding (carrying the full line current) and S1 and S2 on the secondary winding (carrying the scaled-down metering current). P1 and S1 are the polarity (dot) terminals, indicating that current entering P1 exits S1.
What standard defines the current transformer?
Current transformers for AC power systems are defined by IEC 61869-2 (instrument transformers, current transformers) in Europe and internationally, and by ANSI/IEEE C57.13 in North America. The schematic symbol follows IEC 60617 and IEEE 315 conventions.
What accuracy class should I use for energy metering versus protection?
For revenue-grade energy metering, IEC 61869-2 accuracy classes 0.2S or 0.5 are required. For feeder or motor protection applications, classes 5P or 10P are used; these are optimised for accurate operation during fault conditions rather than normal load measurement.
Place the Current Transformer (CT) symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.