Split-Core CT Symbol
Definition: The Split-Core CT symbol represents a split-core current transformer — drawn as a circle or toroid with a gap indicating the openable core, secondary winding terminals (S1 and S2), and a primary conductor passing through the core aperture — used in electrical measurement and metering diagrams to denote a clamp-on current transformer that measures AC line current without disconnecting the primary circuit, as specified under IEC 61869-2 (instrument transformers) and ANSI/IEEE C57.13.
Also known as: split core CT symbol, clamp-on CT symbol, split-core current transformer, clip-on CT, CT metering symbol, current transformer symbol.
What the Split-Core CT symbol means
The split-core CT symbol denotes a current transformer whose magnetic core is manufactured in two hinged halves that can be opened to allow the primary conductor (the cable being measured) to be passed through the core window without disconnecting it. Once closed around the conductor, the split-core CT senses the magnetic field produced by the primary current and induces a proportional secondary current in its wound secondary winding (S1, S2), scaled by the transformation ratio (e.g. 100:5 A, meaning 100 A primary produces 5 A secondary).
Split-core CTs appear in electrical metering diagrams for energy monitors, power quality analysers, and building energy management systems (BMS) where retrofitting the measuring device to an existing live circuit without shutdown is required. The symbol marks the measurement pickup point on the primary conductor, and the secondary terminals S1/S2 connect to the measuring instrument or energy meter.
How to identify the Split-Core CT symbol
The split-core CT symbol is drawn as a circle or oval (representing the toroidal magnetic core) with a visible gap or hinge line on one side indicating the split-core opening mechanism. A line passing through the centre of the circle represents the primary conductor being measured. The secondary winding is shown as a small coil symbol on the outside of the circle, with two terminals labelled S1 (primary, P1, or polarity terminal) and S2 (secondary return). Some drawings use the standard IEC current transformer symbol (a circle with an arrow and two terminals) and add a gap marker to indicate the split-core variant.
Function in a circuit
The split-core CT operates on Faraday's law of electromagnetic induction: the AC primary current in the conductor passing through the core creates an alternating magnetic flux in the toroidal core, which induces a proportional secondary EMF in the secondary winding. The secondary current (or voltage across a burden resistor) is proportional to the primary current, scaled by the turns ratio N2/N1. The secondary is always connected to a low-impedance burden (the meter input, typically < 1 Ω) to maintain accuracy; an open-circuited secondary can develop dangerously high voltages. Most modern split-core CTs used with energy monitors include an internal burden resistor and output a 0–1 V or 0–333 mV signal instead of a current.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61869-2 (instrument transformers — current transformers) defines accuracy classes (0.1, 0.2, 0.5, 1, 3, 5), burden ratings (VA), and overcurrent limits (ALF — accuracy limit factor) for current transformers including split-core types. IEC 60617 provides the schematic symbol for current transformers. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE C57.13 (Requirements for Instrument Transformers) defines accuracy classes (0.3, 0.6, 1.2, 3.0) and burden designations (B-0.1 through B-8.0) for current transformers. IEEE 315-1975 specifies the schematic symbol. |
| Key difference | IEC 61869-2 and ANSI C57.13 use essentially the same circular CT schematic symbol; the difference is in the accuracy class designation system (IEC uses decimal accuracy classes like 0.5S; ANSI uses alphanumeric burden codes like 0.3B-0.1) and the polarity marking convention (IEC uses P1/P2 for primary and S1/S2 for secondary; ANSI uses H1/H2 and X1/X2 for primary/secondary terminals). |
Terminals / pins
| Pin | Name |
|---|---|
| s1 | S1 |
| s2 | S2 |
Typical values
Current ratios: 10:1, 50:1, 100:5, 200:5, 400:5, 1000:5 A (typical split-core CT ratios for metering). Accuracy classes: IEC 0.5, IEC 1.0, ANSI 0.6 (metering grade); IEC 5P, IEC 10P, ANSI C (protection grade). Burden: 1 VA to 30 VA (IEC metering CTs). Output voltage (with internal burden): 0–333 mV or 0–1 V (solid-state energy monitor CTs). Aperture sizes: 13 mm, 16 mm, 24 mm, 36 mm, 60 mm diameter for different cable sizes. Frequency: 50 Hz or 60 Hz (rated for specific frequency).
Where the Split-Core CT symbol is used
- Retrofit energy metering — split-core CTs are clamped onto live LV cables in existing distribution boards to add energy monitoring (kWh metering) without shutting down the circuit or disconnecting conductors.
- Building energy management systems (BMS) — multiple split-core CTs on branch circuits feed current data to a BMS or sub-meter, enabling per-circuit energy consumption tracking for tenant billing and efficiency analysis.
- Power quality monitoring — power quality analysers use split-core CTs (and VTs) to measure harmonic distortion, power factor, voltage sags, and interruptions on distribution feeders without interrupting supply.
- Industrial energy auditing — portable power analysers with clip-on split-core CTs measure motor, HVAC, and production line energy consumption for energy audits without requiring process shutdown.
- Solar PV system monitoring — a split-core CT on the grid import/export connection provides real-time current data to the solar inverter or energy storage controller for feed-in tariff monitoring and demand management.
- Smart home energy monitors — consumer energy monitors (e.g. EnergyHive, Sense, Emporia Vue) use small split-core CTs (typically 100 A rating) clamped onto the main incoming supply cables to measure whole-home electricity consumption.
Example
In a commercial building sub-metering diagram, three split-core CTs (CT1, CT2, CT3, rated 200:5 A, accuracy class 0.5) are shown clamped onto each of the three-phase cables feeding a tenant distribution board. The S1 and S2 secondary terminals of each CT connect to the corresponding current input terminals of a DIN-rail energy meter. A shorting link is shown across each CT secondary to protect against accidental open circuit. The CT symbols, with their core-gap marker, indicate the clamp-on installation method that allows metering without outage.
Key facts
- The Split-Core CT symbol shows a circular or toroidal core with a visible gap (indicating the hinged-open installation method), a primary conductor line through the centre, and secondary terminals S1 and S2 for connection to the measuring instrument.
- The split-core design allows the CT to be installed on a live conductor without disconnecting it — the core is opened, placed around the cable, and snapped shut, making it the preferred type for retrofit metering and energy auditing.
- Pins on this symbol: S1 (x=0 y=10) and S2 (x=0 y=20) — the secondary winding terminals that carry the measurement output current (or voltage) to the connected meter or instrument.
- The secondary of a CT must never be left open-circuited while the primary carries current; an open secondary produces a dangerous high voltage (hundreds of volts) because all primary ampere-turns drive the core into saturation with no secondary MMF to oppose them.
- Accuracy class 0.5 (IEC 61869-2) means the ratio error does not exceed ±0.5% at rated current and specified burden — suitable for revenue-grade energy metering. Class 1 is adequate for general monitoring; protection-class CTs (5P, 10P) are rated for higher overcurrents but lower metering accuracy.
- Most modern split-core CTs used with electronic energy monitors include a fixed internal burden resistor and deliver a 0–333 mV or 0–1 V output signal proportional to primary current, eliminating the risk of open-circuit secondary voltage.
- IEC 61869-2 defines the primary terminal polarity markers: current entering P1 (H1) induces current flowing out of S1 (X1). Correct polarity connection is essential for accurate active and reactive power measurement.
- Split-core CT aperture sizes range from 13 mm (small branch circuits, < 25 mm cable) to 150 mm (large main feeders, multi-core busbars); the aperture must be selected to accommodate the conductor or busbar being measured.
Frequently asked questions
What does the split-core CT symbol look like in a diagram?
The split-core CT symbol is a circle or oval representing the toroidal magnetic core, with a visible gap or hinge marker on one side showing the split. A line passes through the centre of the circle representing the primary conductor being measured. The secondary winding is shown with two terminals labelled S1 and S2 connected to the measuring instrument.
What does a split-core CT measure?
A split-core CT (current transformer) measures the AC current flowing through the primary conductor that passes through its core aperture. The secondary output current (or voltage, for solid-state CTs) is proportional to the primary current, scaled by the CT's turns ratio (e.g. 100:5 A means 100 A primary produces 5 A secondary). The secondary output is connected to a meter or analyser that displays or logs the primary current value.
Why is a split-core CT called 'split core'?
A split-core CT is called split-core because its magnetic core is manufactured in two separable halves that can be opened like a clamp or clamshell. This allows the CT to be installed around an existing conductor without disconnecting or cutting the cable. Once the conductor is inside the aperture, the core halves are closed and latched, completing the magnetic circuit.
What happens if a CT secondary is left open-circuit?
If a CT's secondary terminals S1–S2 are left open-circuit while the primary carries current, the unopposed primary ampere-turns drive the core into deep magnetic saturation, generating a very high secondary EMF — potentially hundreds or thousands of volts — which can destroy the CT insulation, cause electric shock, or damage connected equipment. CT secondaries must always be connected to a burden (meter input or shorting link) before the primary circuit is energised.
What is the difference between a metering CT and a protection CT?
A metering CT (accuracy class 0.2, 0.5, or 1 per IEC 61869-2) provides accurate ratio reproduction at normal load currents (5%–120% of rated current) but is designed to saturate at high fault currents to protect the meter. A protection CT (class 5P, 10P, or ANSI C) is designed to remain linear at high fault currents (up to 20–40 times rated current) to allow protective relays to correctly sense and clear faults.
What standard defines split-core CT accuracy and ratings?
IEC 61869-2 (Instrument transformers — Additional requirements for current transformers) defines accuracy classes (0.1, 0.2, 0.2S, 0.5, 0.5S, 1, 3, 5 for metering; 5P, 10P for protection), burden ratings, and test requirements for all current transformers including split-core types. The North American equivalent is ANSI/IEEE C57.13 (Requirements for Instrument Transformers).
What is the polarity of S1 and S2 on a split-core CT?
Per IEC 61869-2, current entering the P1 primary terminal induces secondary current flowing out of S1. S1 is the polarity terminal (dot-marked or P1-aligned) and S2 is the return terminal. Correct polarity connection is essential for power factor and active power measurement — reversing S1 and S2 gives a 180° phase error that causes active power to read negative (import appears as export).
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