Current Transformer for Circuit Breaker Trip Units

Product Specifications

Technical details and manufacturing context for Current Transformer

Definition

Within a Circuit Breaker Trip Unit, the current transformer is a critical sensing component that isolates and scales down the high current flowing through the main circuit to a safe, standardized level suitable for the trip unit's electronic monitoring and decision-making circuits. This component is essential for accurate current measurement and overcurrent protection in electrical distribution systems. It operates on the principle of electromagnetic induction, where the primary conductor carrying the load current acts as a single-turn primary winding, creating a magnetic flux in the core that induces a proportional, reduced current in the secondary winding. The secondary current is then used by the trip unit to monitor the circuit and initiate protective actions when necessary. The current transformer is designed to meet various electrical and environmental specifications, including rated primary current from 5 to 5000 A, rated secondary current from 1 to 5 A, accuracy classes from 0.2 to 5P20, rated burden from 2.5 to 30 VA, rated voltage from 0.66 to 35 kV, frequency of 50 or 60 Hz, thermal short-circuit current from 10 to 50 kA, dynamic short-circuit current from 25 to 125 kA, insulation class from E to H, operating temperature from -40 to 85 °C, degree of protection from IP54 to IP65, and weight from 5 to 200 kg. These parameters are specified in accordance with IEC 61869-2, IEC 60085, and IEC 60529 standards. The materials used include silicon steel laminations for the core, copper wire for the windings, and insulating resin or epoxy for encapsulation. The current transformer is a passive component that does not require external power for its operation. It is typically installed in the main circuit of a circuit breaker and is designed to withstand fault currents for a short duration. The selection of a current transformer depends on the maximum load current, the required accuracy, the burden of the connected devices, and the system voltage. It is important to verify the specific model's parameters and compliance with the relevant standards with the legal manufacturer or supplier before procurement.

Working Principle

The current transformer operates on the principle of electromagnetic induction. The primary conductor, which carries the load current, passes through the core and acts as a single-turn primary winding. This current creates a magnetic flux in the core, which induces a proportional, reduced current in the many-turn secondary winding. The secondary winding is connected to the trip unit's input, providing a scaled-down current that is safe for electronic monitoring. The transformation ratio is determined by the number of turns in the secondary winding relative to the primary. The core is made of silicon steel laminations to minimize losses and improve accuracy. The secondary current is typically in the range of 1 to 5 A, which is standard for meters and relays. The accuracy class indicates the permissible error in the transformation, with classes such as 0.2 for metering and 5P20 for protection. The burden, expressed in VA, must match the connected load to ensure accurate operation. The current transformer is designed to operate within specified voltage, frequency, and temperature ranges, and to withstand short-circuit currents for a limited time.

Common Materials

Silicon Steel Laminations, Copper Wire, Insulating Resin/Epoxy

Technical Parameters

Parameter	Typical range	Notes & selection driver
Rated Primary Current	5–5000 A	Select based on max load currentIEC 61869-2
Rated Secondary Current	1–5 A	Standard output for meters and relaysIEC 61869-2
Accuracy Class	0.2–5P20	Metering vs protection classIEC 61869-2
Rated Burden	2.5–30 VA	Must match connected loadIEC 61869-2
Rated Voltage	0.66–35 kV	System insulation levelIEC 61869-2
Frequency	50–60 Hz	Matching grid frequencyIEC 61869-2
Thermal Short-Circuit Current	10–50 kA	Withstands fault current for 1sIEC 61869-2
Dynamic Short-Circuit Current	25–125 kA	Peak withstand capabilityIEC 61869-2
Insulation Class	E–H	Thermal endurance of insulationIEC 60085
Operating Temperature	-40–85 °C	Ambient range for continuous operationIEC 61869-2
Degree of Protection	IP54–IP65	Dust and water ingress protectionIEC 60529
Weight	5–200 kg	Depends on voltage and current rating

Ranges are indicative industry figures for RFQ preparation, not a supplier commitment. Confirm every value and standard with the legal manufacturer before ordering.

Components / BOM

Core Part
Provides a low-reluctance path for the magnetic flux generated by the primary current.
Primary Winding/Conductor Part
Carries the load current to be measured; often a single bus bar or turn passing through the core window.
Secondary Winding Part
Multiple turns of insulated wire wound around the core, producing the scaled-down output current.
Insulation Part
Electrically isolates the windings from each other and the core, and provides mechanical protection.
Terminal Block
Provides secure connection points for the secondary circuit wires to the trip unit.

Application & selection

Application Fit & Sizing Matrix

Operational Limits

pressure:	Atmospheric to 1.5 bar (typical), up to 3 bar (special)
other spec:	Frequency: 50/60 Hz (standard), 16-400 Hz (special); Accuracy: Class 0.1 to Class 3; Insulation: 0.66 kV to 36 kV
temperature:	-40°C to +85°C (standard), up to +120°C (special)

Media Compatibility

✓ Air (dry/non-corrosive) ✓ Transformer oil (mineral/synthetic) ✓ SF6 gas (in switchgear)

Unsuitable: Conductive/abrasive slurries or corrosive chemical environments

Sizing Data Required

Primary current rating (A)
Secondary current rating (A)
Burden/load impedance (VA/Ω)

Risk, maintenance & compliance

Reliability & Engineering Risk Analysis

Failure Mode & Root Cause

Insulation Breakdown

Cause: Thermal aging, moisture ingress, or contamination leading to dielectric failure and short circuits.

Core Saturation

Cause: Excessive primary current or DC offset causing magnetic saturation, resulting in inaccurate measurements and overheating.

Maintenance Indicators

Audible humming or buzzing indicating loose windings or magnetic core issues
Visible discoloration, bulging, or oil leaks (in oil-filled units) signaling overheating or insulation failure

Engineering Tips

Implement regular insulation resistance testing and thermographic inspections to detect early degradation
Ensure proper burden matching and install protective devices like surge arrestors to prevent overcurrent and voltage transients

Indicative industry ranges for design and RFQ preparation. Confirm the exact figures and applicable standard with the manufacturer before specifying.

Compliance & Manufacturing Standards

Applicable Standards

IEC 61869-2:2012 (Instrument transformers - Part 2: Additional requirements for current transformers) ANSI C57.13-2016 (IEEE Standard Requirements for Instrument Transformers) DIN EN 60044-1:2000 (Instrument transformers - Part 1: Current transformers)

Quoted from the published standard.

Manufacturing Precision

Ratio error: +/-0.1% at rated current
Phase displacement: +/-5 minutes at rated current

Quality Inspection

Accuracy class verification test (including ratio and phase angle measurements)
Dielectric withstand test (high-voltage insulation test)

Procurement Evaluation Criteria

A practical evidence checklist for RFQ preparation and supplier evaluation.

Technical documentation

Request current drawings, revision history, and a signed specification sheet.

Manufacturing capability

Verify equipment lists, process limits, capacity, and representative production evidence.

Inspection readiness

Confirm test methods, calibrated equipment, sampling plans, and traceable reports.

Supplier transparency

Check the legal entity, factory address, ownership, certifications, and direct contacts.

Compare the published product scope and source details, then include your specifications and quantity in an enquiry.

Manufacturers of Current Transformer

13 companies list this product among what they make. Company figures are quoted from each company's own website; every card states where the relationship came from.

CNC Electric

Wenzhou, Zhejiang, CN

The official website lists a current transformer range. Select by primary current, secondary output, accuracy class, rated burden and window size, and separate metering from protection use.

Listed on the company's own website · profile compiled by CNFX from public sources

View source page ↗ cncele.com · recorded 2026-09-30

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Acrel Co., Ltd.

Shanghai, CN

CE IEC VDE

Also makes: Remote Terminal Unit, Active Power Filter, Power Distribution and 5 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ acrel-electric.com · recorded 2026-08-22

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Andeli Group Co., Ltd.

Shanghai, CN

Founded 19853000 + staff235,000 m²

Also makes: Automatic Transfer Switch, Temperature Controller, AC-DC Converter and 9 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ andeligroup.com · recorded 2026-08-22

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Huayi Electric Co., Ltd.

Wenzhou, Zhejiang, CN

Also makes: Power Transformer, Circuit Breaker, PLC Control System and 9 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Zero Sequence Current Transformer”
View source page ↗ heag.cn · recorded 2026-08-22

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Lugao Power Co.,Ltd

Xiangyang, Hubei, CN

Founded 2016nearly 60 employees staff3000 square meter

ISO9001 SGS CCC CE

Also makes: Current Transformer (CT), Power Transmission, Power Transformer and 6 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ liugaopower.com · recorded 2026-08-22

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MEWOI Electronics Co., Ltd

Guangzhou, Guangdong, CN

Also makes: High Voltage Generator, Surge Protection Device, Current Sensor (CT) and 6 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “MEWOI-Current Transformers”
View source page ↗ mewoi.com · recorded 2026-08-22

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Xi'an Xigao Electricenergy Group Co., Ltd.

Xian, Shaanxi, CN

Founded 1993320000m2

Also makes: Power Distribution, Power Transformer, Circuit Breaker and 3 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “CT Current Transformer”
View source page ↗ loadbreak-switch.com · recorded 2026-08-22

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ZHEJIANG VOLCANO ELECTRICAL TECHNOLOGY CO.,LTD.

Zhejiang, CN

over 10000 m²

ISO 9001 3C CQC

Also makes: Voltage Sensor (Potential Transformer), Medium Voltage Vacuum Interrupter Module, Surge Protection Device and 1 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ volcano-electrical.com · recorded 2026-08-22

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Beijing GFUVE Electronics Co., Ltd.

Beijing, CN

Also makes: Voltage Sensor (Potential Transformer), Reference Standard, Power Analyzer and 1 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ gfuve.com · recorded 2026-09-15

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Blue Jay

Chongqing, CN

Also makes: Protection Relay, Dehumidifier, Alarm System and 6 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformer”
View source page ↗ cqbluejay.com · recorded 2026-08-27

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Changan Group

Zhejiang, CN

Also makes: Branch Circuit Breakers, Surge Protection Device (SPD), Surge Protection Device and 7 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Indoor Current Transformer”
View source page ↗ changanelectric.com · recorded 2026-09-06

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Changan Group Co.,Ltd

Zhejiang, CN

Also makes: Voltage Sensor (Potential Transformer), Circuit Breaker, Surge Protection Device (SPD) and 1 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Indoor Current Transformer”
View source page ↗ changangroup.com.cn · recorded 2026-09-15

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Cheemi Technology Co. Ltd.

Nanjing, Jiangsu, CN

Also makes: Photo-eye Sensors, Photoelectric Sensor, Pressure Gauge/Sensor and 2 more

Listed on the company's own website · profile compiled by CNFX from public sources

Listed there as: “Current Transformers”
View source page ↗ cheemi-tech.com · recorded 2026-08-30

View profile Send enquiry

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Frequently Asked Questions

What is the function of a current transformer in a circuit breaker trip unit?

The current transformer senses the high current flowing through the main circuit and transforms it into a lower, proportional secondary current. This secondary current is used by the trip unit's electronic circuits to monitor the current level and initiate protective actions, such as tripping the breaker, in case of overcurrent or fault conditions.

What are the typical accuracy classes for current transformers?

Typical accuracy classes include 0.2, 0.5, 1, 3, and 5P20, as per IEC 61869-2. Class 0.2 is used for metering applications where high accuracy is required, while 5P20 is a protection class that ensures the transformer maintains accuracy up to 20 times the rated current. The specific class depends on the application.

How do I select the correct rated primary current for a current transformer?

The rated primary current should be selected based on the maximum load current expected in the circuit. It is recommended to choose a rating that is slightly higher than the maximum continuous load to avoid saturation, but also consider the short-circuit current withstand capability. Refer to the manufacturer's guidelines and the system design.

What standards apply to current transformers?

The primary standard is IEC 61869-2 for current transformers. Additionally, IEC 60085 defines insulation classes, and IEC 60529 specifies degrees of protection (IP ratings). These standards provide requirements for performance, safety, and environmental protection. Always verify compliance with the specific model.

Current Transformer

Product Specifications

Application & selection

Application Fit & Sizing Matrix

Risk, maintenance & compliance

Reliability & Engineering Risk Analysis

Compliance & Manufacturing Standards

Procurement Evaluation Criteria

Manufacturers of Current Transformer

Request manufacturing insight for Current Transformer

Frequently Asked Questions

What is the function of a current transformer in a circuit breaker trip unit?

What are the typical accuracy classes for current transformers?

How do I select the correct rated primary current for a current transformer?

What standards apply to current transformers?

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