Current Transformers

A current transformer (CT), or intensity transformer, is a measuring device that delivers a secondary current proportional to the primary current flowing through the conductor. Socomec's models cover closed, split-core and flexible Rogowski-coil ranges, with accuracy classes from 0.2s to 10P depending on the application. Admissible continuous overload reaches 1.2 In and insulation class E allows an operating temperature of 120°C.

Ranges and technologies of CT and intensity transformers

Closed and split-core CTs for installation on an existing conductor

Closed current transformers provide optimum accuracy thanks to a continuous ferromagnetic core surrounding the conductor. Our installation feedback confirms their reliability for new electrical panels. Split-core CTs allow fitting without dismantling the conductor, considerably reducing intervention time on existing installations. Both configurations feature removable terminal covers and double terminals to make short-circuiting the secondary easier.

Flexible Rogowski coils for confined spaces

Rogowski coils are a ferromagnetic-core-free alternative for reduced-space areas or multiple busbar sets. Our standard practice favours this technology when a conventional CT cannot be fitted. The signal delivered requires an electronic integrator to reproduce a proportional image of the primary current.

Choosing the secondary rating and accuracy classes

5A CTs for compact installations and 1A CTs for long distances

A 5 A secondary current suits wiring runs under 10 metres, which covers most standard electrical panel installations. A 1 A current is recommended for longer distances, as it limits the voltage drop in measurement circuits and improves the sensitivity of connected instruments. Feedback from installers confirms that this technical choice directly determines the accuracy of readings on long cable runs.

Accuracy classes suited to each measurement function

Class 0.2s or 0.5 is required for tariff metering and energy billing. Class 1 is sufficient for internal measurement and consumption monitoring. Classes 5P and 10P are reserved for relay protection, where faithful reproduction of fault currents matters more than absolute accuracy. In the field, we find that choosing an unsuitable class leads either to unnecessary extra cost or to regulatory non-compliance.

Construction standards and operating safety

IEC 61869-2 compliance and dielectric tests

Socomec current transformers comply with the IEC 61869-2 standard, which defines the additional requirements specific to measuring and protective current transformers. Dielectric quality is validated by a test at 3 kV/50 Hz for 1 minute between primary and secondary. This overvoltage withstand guarantees operator safety and protects downstream measurement equipment.

Materials and mounting for industrial environments

The ferromagnetic core is paired with fixing by metal brackets or DIN rail depending on the model. The removable terminal cover protects the connections while allowing access for maintenance operations. Our installation experience confirms that the double short-circuit terminal is a major operational safety feature when replacing a live measuring device.

Typical applications and system integration

CT and intensity transformers mainly supply DIRIS measurement units, COUNTIS meters and analogue or digital ammeters. They also equip protective relays for monitoring overcurrents and insulation faults. We recommend checking the compatibility of the transformation ratio and accuracy class with the receiving device from the wiring-diagram design stage.

Frequently asked questions

What is the difference between a 1A CT and a 5A CT on an installation?

The 5A CT suits short wiring runs under 10 metres, while the 1A CT limits line losses over long distances and improves measurement sensitivity. The choice depends on the cable length between the transformer and the measuring device.

Why short-circuit the secondary of a current transformer before intervention?

An open secondary under primary current generates dangerous overvoltages through the transformer effect. The double terminals of Socomec models allow this short-circuit without special tools, protecting the operator and preserving the CT's integrity.

Which accuracy class should be chosen for tariff metering?

Class 0.2s or 0.5 is mandatory for energy billing, as it guarantees a measurement error under 0.2% or 0.5% under reference conditions. A lower class would invalidate the installation's metrological compliance.

Can Rogowski coils replace a conventional CT?

Rogowski coils are a good alternative to traditional CTs in confined spaces, but they require an electronic integrator to produce a usable signal. Their accuracy depends on the quality of this integrator and how the coil is fitted around the conductor.