Solid-core current transformer
Enclosed current transformers, or TE-type enclosed current transformers, are slipped around the conductor before…
Closed current transformers, referenced type TE, are an electrical measurement solution designed for new installations where the live conductor can be threaded through the magnetic core before final commissioning. These closed CTs offer accuracy classes from 0.2s to 1 depending on the reference, with various transformation ratios suited to the rated current of each circuit. The closed ferromagnetic-core construction and removable terminal covers provide optimum protection for connections in electrical panels.
Operating principle and accuracy classes of closed CTs
Secondary winding and transformation ratio
Our installation experience confirms that choosing the transformation ratio is the first parameter to check when sizing a closed CT. The primary current flowing through the central conductor induces a proportional current in the secondary winding, reduced according to the chosen ratio. In the field, we find that the ranges commonly cover standard market ratios, allowing each closed CT to be precisely matched to the downstream ammeter or energy meter.
Accuracy classes 0.2s, 0.5 and 1 by application
Class 0.2s meets the requirements of billed energy metering or precise industrial balance measurements. Class 0.5 suits most measurement units for panel supervision, while class 1 is sufficient for general ammeter indications. Feedback from installers confirms that over-specifying the accuracy class is a common mistake: a class 0.2s closed CT on a simple panel ammeter brings no metrological benefit.
Fixing and connection of closed current transformers
Screwed metal brackets or clip-on DIN rail
Two mounting methods coexist on the closed CTs we regularly observe. Drilled metal brackets allow rigid anchoring to the back of an enclosure by screws, the preferred solution in industrial environments subject to vibration. The clip-on DIN-rail alternative speeds up mounting in modular panels and makes later changes easier. Our standard practice favours DIN rail for tertiary panels and screwed brackets for industrial IP enclosures.
Screw or fast-on lug connections depending on the model
The choice of secondary connection type directly affects wiring speed and long-term reliability. Screw terminals offer universal compatibility with flexible or rigid conductors of varying cross-section. Fast-on lugs allow quick plug-in connection, particularly appreciated on series of identical closed CTs mounted in a row. In both cases, the removable terminal cover protects the connection points from accidental contact and conductive contamination.
Construction standards and materials of closed CTs
Compliance with the IEC 61869-2 standard
Closed CTs meet the IEC 61869-2 standard, which specifies the requirements applicable to AC measuring current transformers. This standard defines accuracy classes, overcurrent-withstand tests, insulation levels and mandatory markings. Our installation feedback shows that checking compliance with this standard is a systematic point for design offices in their bills of quantities.
Closed ferromagnetic core and terminal protection
The closed ferromagnetic core is the key structural difference from split-core or clip-on toroidal CTs. This closed geometry guarantees optimum magnetic coupling between primary and secondary, the source of high accuracy and long-term stability. The removable terminal cover, made of insulating material, completes this architecture by maintaining isolation distances and making connection checks easier without full dismantling.
Typical applications and choice of transformation ratio
Instrumentation of new circuits in panels and enclosures
Closed CTs naturally apply to any new circuit where the conductor is not yet connected. Multi-channel measurement units, panel energy meters and analogue or digital ammeters are the most common receiving devices. We recommend planning for the closed CT to be threaded during the wiring stage, as any later modification requires a supply interruption incompatible with continuity of service.
Sizing the transformation ratio for each circuit
The wide choice of transformation ratios allows the closed CT to be precisely matched to the rated current of the circuit breaker or protected outgoing circuit. Our observation shows that too high a ratio degrades resolution at the low end of the range, while an insufficient ratio risks saturation under overload conditions. We recommend selecting a rated primary current slightly above the expected maximum value, with a margin of 20 to 30% relative to the upstream circuit breaker's setting.
Frequently asked questions
Can a closed CT be installed on a circuit already live?
No, the very design of the type TE closed CT requires the primary conductor to be threaded through the core before final connection. For an existing circuit, split-core or clip-on toroidal CTs should be used instead, as they can be installed without a shutdown.
Which accuracy class should be chosen for a billing energy meter?
Class 0.2s is required for any metering intended for billing or regulatory certification. Class 0.5 suits internal supervision measurements, while class 1 remains limited to simple indications with no financial stakes.
"