Current transformers convert primary currents into measurable, safe secondary signals for energy monitoring, billing, and protection.

Not every monitoring or metering requirement needs the same current transformer. Choosing between a split-core CT, solid-core CT, mini PCB-mount CT, clamp CT, Rogowski coil, leakage current sensor, or three-phase integrated CT depends on the installation conditions, the accuracy required, and the application environment. This guide explains what each major CT family is used for, how it performs, and what limits buyers should keep in mind before specifying a component.

Why Current Transformer Selection Starts with the Application, Not the Specification Sheet

Current transformers are used in energy meters, EV chargers, solar inverters, power monitoring systems, switchgear panels, and smart grid infrastructure. They step down high primary currents to a standardized secondary signal — commonly 5 A, 1 A, 4-20 mA, or millivolt — so that meters, relays, transmitters, and data acquisition systems can measure current safely and accurately.

In practice, the first question is not “which CT is the most accurate?” but “where will this CT be installed and what does it need to do?” A revenue-grade billing CT used by a utility has different requirements from a retrofit energy monitoring CT in an existing distribution board, and both differ from a DC leakage detection sensor in an EV charging pile.

Main Current Transformer Families and Their Typical Uses

Split-Core Current Transformers

Split-core CTs, also called clamp-on current transformers, are designed to open around an existing conductor without disconnecting the circuit. This makes them the fastest-growing segment of the current transformer market, primarily because of their ease of installation in retrofit and smart grid projects.

HEYI’s KCT model is a split-core CT with a clamp-on design, an inner diameter of 0–50 mm, and a housing made of PA plastic. Its core combines silicon steel sheets and ferrites, with pure copper enameled wire for the winding. It is intended for solar and wind energy storage, smart grid, and EV charging infrastructure applications.

For larger conductors and busbar retrofits, the DP/HK busbar split-core CT offers a measurement range up to 8000 A and a larger window of 0–80×160 mm. Output options include 5 A, 1 A, mA, and mV, with accuracy classes 3.0, 1.0, and 0.5. Lead wire type, length, and connector are customizable.

Where an analog output is needed for PLC or building management integration, the KCT-L true RMS current transformer/transmitter provides a 4-20 mA DC output with a 0–1000 A measurement range. It is designed for remote power monitoring, preventive maintenance, IoT and smart city projects, and PLC/DCS system integration.

Solid-Core Current Transformers

Solid-core CTs have a fixed window and are typically installed during original panel build or when the primary conductor can be threaded through the opening. They generally offer more accuracy options than split-core designs at a given cost, but require the circuit to be de-energized during installation.

Several HEYI products fall into this category. The MSQ model is a plastic-cased, square-window, solid-core low-voltage CT and busbar-type measuring CT, with a measurement range up to 6000 A and accuracy classes from 0.2S to 1.0. The CP model is a compact DIN-rail mounted panel CT designed for smart grid, switchgear panels, HVAC, smart buildings, and data centers. The SDH model is a low-voltage window-type and busbar-type CT with a measurement range up to 10000 A, an inner diameter up to 220×80 mm, and accuracy options including 0.2 and 0.2S, making it suitable for large-scale industrial energy monitoring and relay protection.

Mini and PCB-Mount Current Transformers

Miniature and PCB-mount CTs are used inside energy meters, protection relays, and instrumentation where space is limited. They are typically specified for low secondary burdens and are available in compact housings or as open-frame components.

The DX/DM series is HEYI’s compact low-voltage CT family with 0.66 kV rated voltage, DIN-rail mounting, and an inner diameter of 125×35 mm. It features a plug-in terminal design and hinged terminal cover for quick connection, with accuracy class 1.0/0.5. It is designed for high-vibration environments, modular data centers, and EMU/high-speed rail applications.

Clamp Current Transformers and Current Transmitters

Clamp-type CTs are a convenient subset of split-core designs, allowing temporary or permanent installation around a cable without interrupting power. When connected to a transmitter, they become part of a current-to-standard-signal chain used by automation systems.

The KCD model is a split-core CT with RS485 digital output, also marketed as a Modbus RTU current and energy sensor. It measures 0–1000 A, outputs 4-20 mA, and has an inner diameter of 0–50 mm. It has been used in Beijing/Hangzhou metro line projects, elevator monitoring, smart building energy management, and smart factory retrofits.

Rogowski Coil Current Transformers

Rogowski coils are air-core, non-saturating current sensors shaped like a flexible rope. They are useful for measuring large or transient currents where a heavy iron-core CT is impractical. Because they output a millivolt signal proportional to the rate of change of current, they normally require an integrator to produce a usable current reading.

HEYI’s FRC model is a flexible Rogowski coil with an inner diameter of 245 mm, a measurement range of 100–10000 A, and millivolt output. A version with integrator (G1) is available for portable testing meters, data center power retrofits, heavy industry/smelting, and lightning or transient current measurement.

Leakage and Residual Current Sensors

Leakage current transformers, residual current sensors, and zero-phase CTs are used to detect insulation faults, ground faults, or residual currents. In EV charging and solar-plus-storage systems, DC leakage detection is a growing safety requirement.

HEYI’s HYCA sensor is classified as an RDC-PD (Residual Direct Current Protective Device) and a 6 mA DC leakage current sensor. It covers a differential current range of 0–300 mA with 0.2 mA resolution, supports load current up to 80 Arms single-phase or 3×32 Arms three-phase, and provides a switching output for 6 mA DC and 30 mA AC per IEC 62752. It includes integrated self-monitoring and test functions, and is intended for EV charging infrastructure and solar/wind/energy storage applications.

Three-Phase Current Transformers

Three-phase CTs integrate three measuring elements into one housing, saving space in multi-circuit monitoring applications. They are commonly used in distribution panels, UPS systems, motor control centers, and data center rack-level monitoring.

The DASN model is a compact three-phase CT available as a DIN-rail mounted 3-in-1 device. It measures 0–1000 A with accuracy class 1.0 and secondary output 5 A, and is intended for EPC solar/wind/storage projects, smart buildings, MCC, UPS, IDC and telecom, and main/branch monitoring.

Outdoor and Revenue-Grade Current Transformers

Outdoor installations require weatherproof, UV-resistant CTs. Revenue or utility billing applications require high-accuracy CTs that meet standards such as IEC 61869-2 or IEEE C57.13.

The OCT is an outdoor waterproof split-core CT with IP65/IP67 rating, designed for overhead line monitoring, outdoor solar and wind, railway trackside, and municipal infrastructure. The LMZW is an outdoor resin-cast solid-core CT rated IP65, suitable for ring main units (RMUs), marine and offshore, and chemical/metallurgy environments.

For revenue-grade metering, the RECT model is an extended range, anti-saturation, epoxy-resin-insulated CT with 0.15S high accuracy, IP65 protection, IEEE C57.13 compliance, and 5 A or 1 A secondary output. It is intended for utility and substation billing, utility-scale solar and wind plants, and data center or commercial/industrial revenue sub-billing.

Comparison: How the Main CT Types Differ

This table is a starting point. The right choice depends on whether you are retrofitting an existing panel, building new switchgear, embedding a sensor into a meter, or protecting an EV charger against residual DC current.

Accuracy, Standards, and the Procurement Impact

Accuracy class is one of the most important procurement criteria for current transformers. IEC 61869-2 defines accuracy classes such as 0.5 and 0.5S for inductive CTs. Revenue-grade CTs used in utility billing are typically required to meet ANSI C12.20 or IEC 61869-2 accuracy classes of 0.2 or 0.15. That is why the RECT model is classified as a 0.15S high-accuracy resin-insulated CT and IEEE C57.13 compliant revenue CT.

Buyers should match the accuracy class to the application. A 0.5 or 1.0 class CT is generally acceptable for energy monitoring, load management, and sub-metering. For utility billing or allocation of energy costs between tenants, a 0.2S class or better is often specified. For protection circuits, additional requirements such as saturation behavior and rated short-time withstand current become relevant.

Indoor CTs are typically specified for 0.66 kV low-voltage systems with ABS/PC or PA plastic housings. Outdoor installations require IP65 or higher protection and UV-resistant materials, as seen in the OCT, LMZW, and RECT models.

What to Ask a Current Transformer Manufacturer Before Placing an Order

Because CTs are often custom-specified, buyers should evaluate more than the headline measurement range. The following points are useful in supplier conversations:

• What is the required accuracy class at the actual burden of the connected meter or relay?
• What window size or inner diameter is needed to fit the cable or busbar?
• Is secondary output 5 A, 1 A, mA, mV, 4-20 mA, or a digital protocol such as RS485/Modbus RTU?
• Will the CT be installed indoors or outdoors, and what IP rating and UV protection are required?
• Is installation a retrofit (favoring split-core clamping) or new build (allowing solid-core)?
• What certifications apply in the target market, such as IEC 61869-2, IEEE C57.13, CE, or RoHS?
• Can the manufacturer customize lead wire type, lead wire length, connector, housing material, color, ratio, output, capacity, and OEM branding?
• What is the manufacturer’s production capacity and R&D support for custom designs?

Market Trends: Where Current Transformer Demand Is Moving

Market data points to several structural trends. The global current transformer market was estimated at USD 2.63 billion in 2024 and is projected to reach USD 3.90 billion by 2030. Asia Pacific accounted for 40.15% revenue share in 2025, reflecting the region’s scale in grid investment and manufacturing. Split-core current transformers are identified as the fastest-growing segment due to their ease of installation in retrofitting and smart grid applications.

EV charging infrastructure is an additional demand driver. The EV charging transformer market, which includes current monitoring components, is expected to grow at a CAGR of 23.6% from 2025 to 2033. Safety-related sensing, such as AC/DC leakage current detection per IEC 62752, is becoming part of the design conversation for EV chargers and solar-plus-storage systems.

These trends suggest that buyers will increasingly need a product family, not a single SKU: split-core CTs for retrofit monitoring, solid-core CTs for new panels, revenue-grade CTs for billing, and leakage sensors for charging and storage safety.

Traditional Solution Limitations: Honest Boundaries Every Buyer Should Know

Every current transformer type has practical limits. Split-core CTs are convenient for retrofit, but the split in the magnetic core can reduce accuracy and repeatability compared to a solid-core CT of the same materials. They are therefore less common in revenue-grade billing applications at high accuracy.

Solid-core CTs offer broader accuracy options, but they cannot be installed around an existing cable without opening the circuit. That means downtime, additional labor, and sometimes a redesign of the panel layout.

Rogowski coils are flexible and non-saturating, but they output a millivolt signal that needs an integrator. They are not drop-in replacements for a conventional CT with a 5 A or 1 A secondary output.

Miniature PCB-mount CTs save board space, but their smaller core cross-section limits the power that can be transferred to the secondary circuit. A meter with a heavy burden may require a larger CT than the compact housing allows.

Three-phase integrated CTs save space and installation time, but they are typically offered in a narrower accuracy range and current range. In this case the DASN model is specified at class 1.0, so it belongs more to sub-metering and monitoring than to utility-grade billing.

Outdoor CTs with IP65 or higher protection, such as the OCT, LMZW, and RECT, solve weather exposure but are a heavier design. Buyers should not specify an outdoor-rated resin-cast CT for an indoor monitoring application unless environmental conditions require it.

How HEYI Electrical Approaches CT Manufacturing and Customization

WENZHOU HEYI ELECTRICAL CO.,LTD., established in 2012, is a manufacturer specializing in current transformers and current sensors. Its headquarters and factory are located in Wenzhou, Zhejiang, China, with a facility of 10,000 m², about 56 employees, an R&D team of 5 engineers, and an annual production capacity of 356,000 units. Export business accounts for 95% of total sales, with major markets including Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East, and North America.

HEYI’s product range covers current transformers, current sensors, shunts, digital energy meters, current transmitters, and split-core current transformers. It also develops application-specific products such as outdoor split-core CTs for utility distribution transformer monitoring and AC/DC leakage current sensors for EV charging pile safety. The company states that its engineering is trusted by global technology and utility firms including Siemens, Schneider Electric, LG, Samsung, and SoftBank.

Across its product families, HEYI offers customization of housing material, housing color, accuracy, output, capacity, lead wire type, lead wire length, connector, inner diameter, and OEM branding. For a procurement team, this range of options matters because a CT is rarely a standalone catalog item; it must integrate with a specific meter, relay, busbar size, or enclosure.

Application Scenarios: Typical CT Requirements by Sector

Smart Grid and Power Distribution

For switchgear panels, distribution monitoring, and utility asset billing, common requirements include high accuracy, stable output, insulation safety, IEC 61869-2 compliance, customized ratio, and OEM logo printing. Depending on whether the project is a new substation or a retrofit of an aged distribution board, buyers choose between solid-core busbar CTs and split-core clamp-on CTs. In tropical high-humidity areas, outdoor resin-cast or waterproof CTs such as the LMZW or OCT are more relevant.

Solar, Wind, and Energy Storage

Renewable energy plants need CTs for inverter output monitoring, plant-level energy metering, and storage system management. CTs used in utility-scale plants may require high accuracy and outdoor durability, which is why the RECT is intended for utility-scale solar and wind plants. For distributed PV and home energy monitoring, compact split-core CTs with mA or mV outputs are often used alongside energy meters or home monitoring devices.

EV Charging Infrastructure

EV charging piles require residual current monitoring to detect both AC and DC leakage currents. The HYCA sensor is designed for this role, with IEC 62752 switching output for 6 mA DC and 30 mA AC, integrated self-monitoring, and load capacity up to 80 Arms single-phase or 3×32 Arms three-phase. Charging station manufacturers also use compact CTs to measure AC input current for billing and load management.

Data Centers and Smart Buildings

Data center power monitoring often happens at the rack level, branch level, or main distribution level. Three-phase integrated CTs such as the DASN are well suited to rack-level and branch-circuit monitoring because they save panel space. DIN-rail mounted CTs such as the DX/DM and CP simplify installation inside crowded enclosures. For building-level energy management and IoT integration, split-core CTs with RS485 or 4-20 mA outputs are common because they can be retrofitted without service interruption.

Industrial Facilities and Railway

Large industrial facilities need CTs for energy monitoring of motors, furnaces, compressors, and other heavy loads. CTs with wide measurement ranges and high short-circuit withstand capability, such as the SDH series up to 10000 A, are relevant here. For railway traction and high-vibration environments, the DX/DM series is designed with a secure plug-in terminal connection and a compact 0.66 kV construction.

Selection Framework: A Simple Decision Path for Buyers

A practical selection process can be reduced to five questions:

1. What is being measured? For energy monitoring, choose an inductive CT. For DC leakage in EV/ESS, choose an RDC-PD sensor. For large/transient currents, consider a Rogowski coil.
2. Can the circuit be opened? If yes, solid-core CTs give broader accuracy options. If no, split-core clamp-on CTs are the practical choice.
3. What output does the meter/PLC/gateway accept? 5 A and 1 A are traditional secondary outputs; mA and mV suit compact sensors; 4-20 mA is common for transmitters; RS485 is used for digital energy monitoring modules.
4. What accuracy and standards apply? For internal monitoring, class 1.0 or 0.5 is usually sufficient. For billing or cost allocation, choose 0.5S/0.2S or revenue-grade 0.15S/0.2 per IEC 61869-2 or ANSI C12.20.
5. What is the physical and environmental constraint? Consider window size, available panel space, indoor/outdoor location, IP rating, UV exposure, vibration, and cable or busbar dimensions.

Future Outlook: What Buyers Should Plan For

Current transformer selection is becoming more application-specific as the energy system becomes more distributed. Grid operators are retrofitting distribution transformers with monitoring CTs, charge point operators are integrating residual current sensors into chargers, data center operators are adding branch-circuit metering, and solar installers are embedding CTs into home energy monitors. The common thread is that CTs are shifting from generic catalog purchases to engineered components in a system design.

For buyers, the practical implication is to select suppliers with a wide enough product portfolio to cover multiple applications and with the ability to customize mechanical and electrical characteristics. Manufacturers that combine R&D, production capacity, and export experience, such as HEYI with its 356,000-unit annual capacity and 95% export ratio, are positioned to serve both OEM and project-based demand.

Buyers should also watch certification and standards requirements. IEC 61869-2 is the core international standard for inductive current transformers, and IEEE C57.13 matters for revenue metering in North American utility contexts. As EV charging and storage grow, IEC 62752 for 6 mA DC residual current detection is becoming a relevant specification in charger designs.

Frequently Asked Questions

For further product and company reference, the HEYI corporate brochure is available here: HEYI Electrical company brochure (PDF).