Split-core current transformer
Opening current transformers allow for the addition of measuring instrumentation to existing electrical…
Split-core CTs are type TR current transformers whose magnetic core opens mechanically for fitting around a conductor already in place. They create a proportional image of the primary current for measuring, metering or protective devices without interrupting the circuit. Common accuracy classes are 0.5 or 1 under the IEC 61869-2 standard, with fixing by metal brackets and removable terminal covers for access to the secondary connections.
Operating principle and structure of split-core CTs
Opening the magnetic circuit for installation without disconnection
Unlike closed current transformers, split-core CTs incorporate a hinged or separable ferromagnetic core that opens to fit around the live conductor. This design removes the need to cut or dismantle the primary cable, which we see as a decisive time saving on interventions in industrial environments. The core then closes with a controlled gap to keep magnetic reluctance within limits compatible with the declared accuracy class.
Isolation and protection of secondary connections
The secondary output terminals are protected by a removable terminal cover that ensures personal safety and preserves the integrity of the connections. Our installation experience confirms that this arrangement makes polarity checks and commissioning tests easier without fully dismantling the device.
Accuracy classes and metrological performance
Choosing between class 0.5 and class 1 depending on the application
Class 0.5 suits energy metering and billing measurements where the composite error must stay below 0.5% under reference conditions. Class 1 meets general indication, load monitoring or threshold-alarm needs where a wider tolerance is acceptable. Feedback from design offices shows that this choice is made according to the operating contract and the energy supplier's requirements.
Limitations and fitting precautions
The residual gap at the core's hinge introduces a parasitic reluctance that limits metrological performance compared with a closed CT of the same size. We recommend checking the tightening torque of the closing screws and inspecting the contact surfaces at every intervention to preserve measurement repeatability.
Typical applications and installation contexts
Retrofit instrumentation on existing panels and enclosures
Split-core CTs are mainly used to add consumption meters, network analysers or monitoring relays to electrical panels already in service. Our installation feedback shows that this approach avoids production-shutdown authorisations in continuous industrial environments.
Extending supervision on tertiary networks and infrastructure
Municipalities and infrastructure managers use these transformers to densify measurement points on public lighting, secondary distribution or electric heating networks without replacing existing cables. Fixing by metal brackets allows adaptation to different types of support and panel sizes.
Frequently asked questions
Does a split-core CT offer the same accuracy as a closed CT of the same rating?
No, the mechanical play inherent in opening the core slightly degrades metrological performance. For the same sizing, a closed CT generally reaches classes 0.5 or 0.2, while the split-core CT is limited to classes 0.5 or 1, depending on closure quality and the condition of the pole faces.
Can a split-core CT be installed on a live conductor?
Installation on a live conductor is technically possible but subject to work-safety rules in electrical environments. The conductor remains insulated and inaccessible; the operator works only on the split-core CT's housing and secondary connections, which are de-energised by design.
Which standard specifically governs the requirements for split-core CTs?
The IEC 61869-2 standard defines the specific requirements for measuring current transformers, including split-core variants. It specifies type tests, accuracy classes, error limits and test conditions that guarantee interchangeability and metrological traceability.
Is the removable terminal cover mandatory for compliance?
The terminal cover is a protective measure against direct contact with the secondary terminals. Its presence meets the general requirement for IP protection and personal safety depending on the installation context, although the IEC 61869-2 standard does not formally require it as a structural element.