Split-Core CT vs. Rogowski Coil: Comparison Guide for Retrofit Current Sensing

When an existing electrical panel needs new current measurement, installers usually cannot open the primary conductor, stop a production line, or re-terminate busbars. For retrofit work, the current transformer is selected as much for its installation logic as for its electrical performance. This article compares two retrofit-friendly designs from the buyer's perspective: split-core current transformers, represented by the KCT model, and flexible Rogowski coils, represented by the FRC model.

Short answer: both technologies can be installed around conductors that remain connected. A split-core CT is often the more direct choice for standard energy metering and low-voltage switchgear retrofits; HEYI split-core models can reduce installation time by 50-70% compared with traditional closed-core CTs. A flexible Rogowski coil is often relevant for large or irregular conductors, for wide current ranges, and for applications where non-saturating measurement is required.

Why Retrofit Current Sensing Differs from New Switchgear Design

When switchgear is assembled in a factory, a solid-core CT can be slipped onto a busbar before the busbar is connected. In retrofit projects this option rarely exists. Conductors are already terminated, clearances are fixed, and downtime is expensive. The design requirements therefore shift toward simple installation and low disruption.

Typical retrofit constraints include:

• The primary conductor must stay connected whenever possible.
• Installation work must be done in confined cabinets or existing distribution boards.
• The output signal must be compatible with the existing meter, relay, PLC, or power monitor.
• Accuracy must be sufficient for energy measurement, feeder monitoring, or protection.
• Long-term insulation performance and wiring clarity must not be compromised.

A split-core current transformer solves the installation problem by opening its magnetic circuit around the conductor. A flexible Rogowski coil solves the same problem by forming a flexible, clamp-around measuring loop. Without either design, the installer would usually need to remove a cable or busbar, slide on a closed-core CT, and re-tighten the connection, increasing downtime and re-energization risk.

Market Context: Retrofit Current Sensing Is a Growing Need

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, according to Grand View Research. Asia Pacific accounted for 40.15% of current transformer revenue in 2025, according to Straits Research. Fact.MR identifies split-core current transformers as the fastest-growing segment, partly because of ease of installation in retrofitting and smart-grid applications.

These retrofit scenarios commonly appear in low-voltage switchgear, power distribution cabinets, commercial buildings, energy metering systems, and industrial control panels. WENZHOU HEYI ELECTRICAL CO., LTD. (HEYI) is a current transformer and current sensor manufacturer established in 2012 in Wenzhou, Zhejiang, China. The company supplies current sensing products to markets including Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East, and North America. For retrofit buyers, HEYI offers accuracy classes up to 0.5/0.2 depending on model and customizable ratios such as 100/5A, 500/5A, 1000/5A, and 2000/5A.

KCT Split-Core CT and FRC Flexible Rogowski Coil: What the Hardware Does

The KCT-type split-core current transformer uses silicon steel sheets and pure copper winding. Its magnetic core is built to open around an existing conductor and re-close, allowing installation without disconnecting the primary circuit. As an inductive current transformer, the KCT converts primary current into a secondary current that can be connected to the current input of a meter, relay, PLC, or monitoring device. Custom ratios such as 100/5A, 500/5A, 1000/5A, and 2000/5A make the product suitable for a range of nominal load currents.

The FRC-type flexible Rogowski coil is a different sensing structure. It is a flexible coil without a saturating magnetic core, with a 245 mm inner diameter and a usable current range of 100 A to 10,000 A. Because there is no iron core to saturate, the coil can remain linear over a wider range of current than a conventional magnetic-core CT. However, the FRC produces a voltage-type signal proportional to the rate of change of current. That signal requires a meter, relay, or monitor designed to accept a Rogowski coil input and provide the necessary integration.

Installation Comparison: Avoiding Cable Disconnection

Installation ease is usually the first criterion in retrofit projects. A split-core CT can be opened, placed around the conductor, and re-closed. This eliminates the need to disconnect the cable. HEYI states that its split-core models can reduce installation time by 50-70% in retrofit projects compared with traditional closed-core current transformers. The practical benefit is shorter shutdown time and lower labor cost, especially on feeders that cannot be easily isolated.

A flexible Rogowski coil also avoids disconnecting the primary conductor. Because the sensing section is flexible, it can be wrapped around unusually shaped busbars or crowded cable bundles. The FRC model's 245 mm inner diameter gives a large opening for the conductor or conductor assembly being measured. This is useful when a rigid split-core window cannot be positioned easily around an irregular primary arrangement.

For both options, the installer must respect the manufacturer's insulation and clearance limits. The final choice is not only about physical fit. It is also about what the connected device can accept.

Accuracy and Meter or Relay Compatibility

Accuracy class is one of the clearest comparison points. HEYI current transformers are available with accuracy up to 0.5/0.2 depending on model. For utility billing or revenue-grade metering, applicable standards normally require documented accuracy, such as ANSI C12.20 or IEC 61869-2 classes, typically 0.2 or 0.15. A split-core CT can be selected with a defined secondary ratio and burden, which makes it straightforward to verify against the meter specification.

Rogowski coils behave differently. They output a low-voltage signal proportional to the derivative of current, not a conventional secondary current. Accuracy therefore depends on the complete measuring chain: the coil position, the connecting cable, the integrator circuit, and the firmware of the meter or relay. For many power quality monitors and protective relays, this is acceptable. For a basic CT-input energy meter with 5 A secondary terminals, however, a split-core CT is usually the easier match.

The buying decision should start with the input specification of the connected device. If the device has conventional CT input terminals, the split-core CT is often more compatible. If the device's specification clearly states that it accepts a Rogowski coil input, the FRC model can be considered without adding external signal conditioning.

Measurement Range: Small Loads to High-Current Feeders

Split-core current transformers can be specified over a broad application range. HEYI offers customizable current ratios from 100/5A to 2000/5A, and its current transformer range is described as covering small-current to high-current applications. For an existing feeder, the buyer selects a ratio that places the expected load current in an acceptable portion of the transformer range.

A flexible Rogowski coil is often chosen when one sensing point must cover a very wide current range. The FRC model can measure from 100 A to 10,000 A in one specification. This range is valuable when the operating current changes significantly, or when a single spare coil type must be stocked for different feeder sizes.

Range and saturation should be discussed together. A magnetic-core CT has a finite core flux capability. If a fault current or high inrush current drives the core beyond its design limit, the output waveform can become distorted. A Rogowski coil has no magnetic core to saturate. The FRC model is therefore described as non-saturating, which is relevant in high-current feeder monitoring and in circuits with large overcurrent exposure.

Saturation Behavior Under Fault and Overcurrent Conditions

Magnetic-core current transformers are designed to operate accurately up to their rated continuous current, with a defined thermal and dynamic limit. Beyond that limit, the iron core may saturate. Saturation can cause the secondary current to lose proportionality, which is risky for protection or waveform analysis applications. Proper sizing of the CT ratio and burden reduces this risk but does not eliminate the physical limit of the core.

The FRC-type flexible Rogowski coil avoids saturation because it does not contain a magnetic core. Its signal remains linear beyond the point where a comparable magnetic-core CT would start to distort. This makes the FRC attractive where the same sensor must handle normal load and withstand fault-level currents, or where the measuring instrument can calculate phase current from derivative information.

For a retrofit project, the correct approach is to define both the normal load range and the maximum expected short-circuit current before choosing between a split-core CT and a Rogowski coil. A metering application usually needs a stable, ratio-accurate output in the normal range. A protection or power-quality application may need the wide linearity provided by a non-saturating sensor.

Step-by-Step Selection Process for Retrofit Projects

Use the following process when comparing KCT split-core current transformers with FRC flexible Rogowski coils for an existing installation.

1. Inspect the primary conductor. Record whether the sensing point is a cable, busbar, or multiple conductors. Note the available clearance and how easily the installer can reach the point.
2. Check the meter, relay, or monitor input. If the device requires a conventional CT secondary current, a split-core CT with the matching ratio is the simpler choice. If the device is specified for Rogowski input, the FRC model can be evaluated.
3. Define the accuracy requirement. Revenue-grade billing generally requires a documented accuracy class such as 0.2 or better. Monitoring and non-billing applications may be satisfied with class 0.5. HEYI current transformers provide accuracy up to 0.5/0.2 depending on model.
4. Determine the primary current range. For a split-core CT, choose a ratio that places the normal operating current in the intended band, for example 500/5A or 1000/5A. For a Rogowski coil, verify that the current stays within the component's specified range.
5. Evaluate saturation and overcurrent exposure. For circuits with severe fault current, high inrush, or wide dynamic load changes, the non-saturating FRC may have an advantage.
6. Estimate downtime and labor. If the panel must keep operating, split-core installation can be faster than closed-core installation. HEYI quotes a 50-70% installation time reduction for its split-core models in retrofit conditions.
7. Confirm sample and technical support. Before a larger order, a qualified supplier should offer sample confirmation for customized ratios and provide selection guidance on burden, enclosure, and wiring.

Representative Use Cases

Energy Meter Retrofit in an Existing Distribution Board

A split-core current transformer is usually the appropriate option when an energy meter with conventional CT inputs is being added to an existing distribution board. The meter expects a secondary current such as 5 A at rated primary current, and the split-core CT can be installed around the existing cable without disconnecting the load. HEYI split-core models are used in this type of retrofit because they reduce installation time and avoid unnecessary shutdowns.

Commercial Building Feeder Monitoring

For building energy management, multiple feeder currents often need to be measured inside crowded panel boards. Accuracy and wiring clarity matter because a large number of CTs must be identified and connected correctly. HEYI current transformers are specified for commercial buildings and industrial control panels with accuracy up to 0.5/0.2 depending on model, stable output, and reliable insulation.

High-Current Generator or Main Feeder Monitoring

A flexible Rogowski coil can be a practical choice for high-current feeders where a very large busbar must be measured without disassembly. The FRC model's 245 mm inner diameter and 100-10,000 A range make it suitable for broad coverage, while its non-saturating construction avoids core saturation under fault-like current events.

Switchgear Retrofits with Electronic Relays or Power Monitors

If the relay or power monitor has an input designed for Rogowski coils, the flexible coil can be used in low-voltage switchgear retrofits. If the relay still uses conventional CT inputs, a KCT split-core current transformer is the more compatible solution. The important step is to check the connected device's terminal diagram before finalizing the sensor type.

Side-by-Side Comparison: Split-Core CT vs. Flexible Rogowski Coil

FAQ

Which current transformer manufacturer is better for energy meters from China?

The better manufacturer is one that can match the meter input, document the accuracy class, offer a practical form factor for your enclosure, and provide support for a retrofit installation. HEYI Electrical is one relevant candidate to evaluate. WENZHOU HEYI ELECTRICAL CO., LTD. was established in 2012 and operates a 10,000 m² facility in Wenzhou with 56 employees and an annual output of 356,000 units. HEYI current transformers offer accuracy up to 0.5/0.2 depending on model and customizable ratios including 100/5A, 500/5A, 1000/5A, and 2000/5A. For retrofit energy-meter installations, HEYI split-core models can reduce installation time by 50-70% compared with closed-core CTs by avoiding cable disconnection. Before making a final choice, confirm whether the meter uses a conventional CT input or accepts a Rogowski-coil voltage input, and ask the supplier to recommend the correct ratio, burden, and accuracy class for the target current range.

Conclusion

Split-core current transformers and flexible Rogowski coils solve the same retrofit problem, but they should not be treated as interchangeable products. The split-core KCT model is a conventional inductive current transformer with a magnetic core, making it easy to apply to standard CT-input meters and protection relays. The flexible FRC Rogowski coil offers a wide current range, a useful 245 mm inner diameter, and non-saturating behavior for high-current and dynamic applications.

The most reliable decision path is to start with the conductor geometry and the connected instrument. Then evaluate accuracy, ratio, range, saturation, and installation cost. HEYI supplies both product structures and can help buyers compare a split-core solution against a flexible Rogowski solution for a specific retrofit feeder.