Protective Current Transformer Selection Guide: Accuracy Class, ALF, Burden, and Core Requirements

To select a protective current transformer (CT), start with the relay’s protection function and system fault-current requirements, then verify the CT’s protection accuracy class, accuracy limit factor (ALF), rated burden, secondary current, and core arrangement. These specifications work together: an appropriate class and ALF are meaningful only when the connected burden stays within the conditions stated for the CT. For differential or other demanding protection schemes, the required core characteristics may need a separate assessment rather than a standard protection-class rating alone.

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This guide is for electrical engineers, panel builders, EPC contractors, procurement teams, and distributors evaluating CTs for protection circuits. It explains what the principal ratings mean, how to match them to an application, and what information to prepare before requesting a quotation. Because system conditions and applicable standards vary, confirm the final selection against the relay documentation, project specification, and supplier’s technical data.

Key Takeaways

• Choose the protection class and ALF together: a designation such as 5P10 describes a protection-class CT with a specified accuracy limit factor under defined conditions.
• Calculate the connected burden: include relay input burden, secondary wiring, and any other devices connected to the same core.
• Match the core to the protection scheme: a standard protection core may suit overcurrent protection, while differential schemes can require additional core-performance data.
• Specify the complete application: primary and secondary ratings, fault current, burden, frequency, installation details, and applicable standard help suppliers evaluate the request.

How Protective CT Ratings Work

Protection accuracy class and ALF

A protective CT supplies a current signal to protective relays during normal operation and fault conditions. Its performance must be considered at the elevated currents that may occur during a fault, when magnetic saturation can distort the secondary waveform. Protection-class markings describe performance under specified standard conditions; they do not mean that the CT reproduces current perfectly at every possible fault level.

For example, IEC-style designations such as 5P10 combine a protection class with an ALF. The “5P” identifies a protection class associated with a specified composite-error limit, while “10” indicates an accuracy limit factor of 10 under the applicable rated conditions. In the IEC protection-class convention, 5P and 10P correspond to composite-error limits of 5% and 10%, respectively, at the accuracy limit current and specified burden conditions. Confirm the governing standard and full nameplate details for the particular product.

The ALF relates the accuracy limit current to the rated primary current. A 100/5 A CT with an ALF of 10 has a nominal accuracy-limit current of 1,000 A on the primary side, under its specified conditions. This is an illustrative calculation, not a statement that every CT will remain within its class at any burden or installation condition. If the actual connected burden exceeds the rated burden, performance may differ from the nameplate class and needs technical review.

Burden and secondary wiring

Burden is the load connected to the CT secondary, commonly expressed in volt-amperes (VA) at the rated secondary current. It includes the relay or meter input, lead impedance, terminal connections, and any other equipment sharing that core. Long or undersized secondary conductors can contribute significant impedance, so the relay’s stated input burden alone is not always enough for selection.

For a resistive burden, the basic relationship is VA = secondary current squared × resistance. Thus, the same secondary-loop resistance produces a higher VA burden on a 5 A circuit than on a 1 A circuit. When calculating burden, use the complete secondary-loop resistance and account for the project’s wiring arrangement; ask the relay or CT supplier to review the calculation if the circuit includes non-resistive loads or unusual connections.

Core requirements and protection function

A CT core’s magnetic characteristics affect how it responds as current rises. Overcurrent and earth-fault protection may use standard protection-class cores selected for the expected fault current and relay burden. Differential protection, restricted earth-fault protection, and other schemes sensitive to CT mismatch may need more detailed requirements, such as excitation characteristics, knee-point voltage, winding resistance, or remanence considerations, as defined by the protection design.

Where multiple functions are involved, separate cores can help keep protection and metering circuits appropriately specified. A metering core is generally selected for measurement performance under its relevant operating conditions, while a protection core is specified for fault-current performance. Do not assume a single core can meet every function’s accuracy and saturation requirements; confirm the arrangement with the protection engineer and relay documentation.

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Selection Framework: From Application to Specification

1. Define the system and relay requirements

Identify the equipment location, system frequency, primary current, required secondary current, and the protection functions served by the CT. Obtain the maximum prospective fault current at the CT location and the relay’s relevant input and CT requirements. Also note whether the installation is indoor or outdoor, the mounting arrangement, and any space or connection constraints.

2. Select a suitable protection class and ALF

Use the protection study and relay requirements to determine the accuracy class and ALF needed for the application. Compare the required fault-current range with the CT’s rated primary current, while considering the burden connected to the core. A higher ALF is not automatically the best choice: the correct rating depends on the actual fault duty, relay scheme, burden, and applicable specification.

3. Calculate total burden and check its basis

List every device connected to the secondary, then add the relay burden and secondary wiring burden using the appropriate method for the circuit. Compare the result with the CT’s rated burden and check that the ALF and accuracy-class declaration apply at the relevant burden. If the project specification gives burden in VA but the CT data gives another basis or condition, resolve the difference before ordering.

4. Confirm core count, ratings, and construction

Specify the number of cores and identify the purpose of each core, such as overcurrent protection, differential protection, or metering. Provide the required ratio, secondary rating, class, ALF or other core-performance requirements, rated burden, frequency, and applicable standard. For schemes that require excitation data or a particular knee-point voltage, state the project’s required values and tolerances rather than relying on a generic class label.

Application Matching and Common Selection Errors

A frequent mistake is selecting a CT by ratio alone. Another is comparing ALF values without checking the burden at which the rating applies. Buyers can also overlook secondary-lead resistance, specify a metering class for a protection core, or omit the relay scheme’s requirements for differential protection. Resolve these points during engineering review, before procurement fixes the ratio, core count, and interface dimensions.

Pricing, MOQ, Lead Time, and Supplier Evaluation

Price and lead time depend on the required ratio, number of cores, class and burden combination, construction, quantity, and project documentation. Minimum order quantities and production schedules are supplier-specific, so request these details for the exact configuration rather than assuming a standard value. Changes to the specification after order confirmation may affect cost or delivery timing; agree on the approved datasheet and drawing status before production.

Supplier checklist

• Can the supplier provide a datasheet that clearly states ratio, secondary rating, core count, class, ALF, burden, frequency, and applicable standard?
• Can the supplier review the relay and wiring burden against the requested performance conditions?
• For differential or other specialized schemes, can the supplier confirm which excitation or winding data can be provided?
• Are terminal, mounting, dimensional, marking, and documentation requirements defined in the quotation?
• Are MOQ, lead time, inspection documents, and any project-specific requirements stated clearly?

ANNEZEE supports B2B evaluation of protective current transformers by discussing the application requirements, configuration, and technical information needed for a quotation. To make the review more efficient, provide the single-line diagram or protection function, CT ratio, secondary rating, expected fault current, relay details, burden estimate, core requirements, and applicable standard. If some values are not yet confirmed, identify them as pending so the supplier can distinguish fixed requirements from items that still need engineering review.

Conclusion: What to Specify Before Ordering

The right protective CT is selected by matching the protection class and ALF to the fault-current duty, verifying that total burden stays within the stated conditions, and confirming that each core suits its assigned function. For a basic overcurrent application, begin with the relay requirements, fault level, ratio, secondary rating, and complete burden calculation. For differential or other sensitive schemes, obtain the scheme-specific core requirements from the protection design instead of relying on a general class marking.

Your next step is to prepare a complete technical inquiry and request a datasheet review before placing an order. Share the known electrical and mechanical requirements, ask the supplier to identify any missing inputs, and confirm price, MOQ, lead time, and documentation against the approved configuration. This process gives your engineering and purchasing teams a clearer basis for comparing protective CT options and moving toward a project-ready specification.

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