Protective Current Transformer Selection Guide: Ratio, Protection Class, ALF, and Burden

To select a protective current transformer (CT), first match its primary ratio and secondary output to the circuit and connected relay, then verify protection class, accuracy limit factor (ALF), and rated burden. These specifications work together: a CT must deliver a usable current signal to the protection relay without exceeding the specified error or saturating too early under fault conditions. A notation such as 100/5 A, 5P10, 15 VA is an illustrative combination—not a universal recommendation—and each value must be checked against the project design and applicable standard. At ANNEZEE, we help buyers organize these requirements before comparing products or requesting a quotation.

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Who This Guide Is For

This guide is for electrical engineers, panel builders, EPC contractors, distributors, and procurement teams sourcing CTs for protective relays. It is especially useful when a drawing or specification lists a ratio, protection class, ALF, and burden but does not explain how to assess their compatibility. The guidance supports initial selection and supplier discussions; final ratings should be confirmed against the system study, relay documentation, installation conditions, and applicable standards. Where a project has unusual fault levels or long secondary wiring, engineering review is important.

Protective CT Basics

What a Protective Current Transformer Does

A protective CT reproduces primary current as a lower secondary current that can be measured by a relay or other protection device. During normal operation, it provides a signal within the required accuracy range; during a fault, it is expected to support dependable relay operation over the specified current range. Its core and electrical design are selected for protection performance, which is different from the priorities of a metering CT designed primarily for measurement accuracy over normal operating conditions. The exact performance depends on the specified CT, connected circuit, and installation.

Understanding the Key Terms

• Ratio: The rated primary current divided by the rated secondary current, such as 100/5 A or 100/1 A.
• Protection class: A designation describing accuracy under protection-related conditions. For example, “5P” is used in IEC protection-class notation; confirm the standard and complete marking specified for the project.
• ALF: The accuracy limit factor, indicating the multiple of rated primary current up to which the CT meets its specified protection accuracy at the stated conditions.
• Rated burden: The apparent power, in VA, that the CT is designed to supply at its rated secondary current while meeting its specified performance.

These values should be read together, not as independent checkboxes. A protection class and ALF are meaningful only with the applicable standard and rated burden conditions understood. Secondary wiring and connected devices also contribute to the actual burden seen by the CT.

Match the CT to the Application

Start with the Protection Scheme

Identify the relay function and the currents it must detect, including relevant fault conditions. Feeder overcurrent protection, transformer differential protection, and busbar protection can impose different requirements on CT performance and configuration. Confirm the relay’s required secondary input and whether the scheme specifies a particular class, ALF, knee-point voltage, or other characteristic. Do not assume that one CT specification suits every protection function.

Choose a Compatible Ratio and Secondary Current

Select a primary rating that suits the circuit’s expected operating current and system design, while preserving the relay’s ability to detect the required conditions. Common secondary ratings include 1 A and 5 A, but the correct choice depends on the relay, existing equipment, and project specification. For example, a 100/5 A CT produces a rated secondary current of 5 A when its primary current reaches 100 A; that example does not establish suitability for a particular installation. Confirm the selected ratio with the designer and relay documentation.

Check Construction and Installation Constraints

CTs may be specified in different constructions, such as window, bar, or wound-primary designs, depending on the application and installation arrangement. Core material and construction can influence performance, but buyers should assess the declared characteristics and test documentation rather than infer performance from material descriptions alone. Verify mounting method, aperture or conductor dimensions, insulation requirements, indoor or outdoor use, and available space. For multiple protection functions, determine whether separate cores or CTs are required by the design.

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A Practical Selection Framework

1. Collect system information. Record nominal voltage, primary operating current, prospective fault current, frequency, installation environment, and the protection scheme.
2. Confirm relay requirements. Check the relay’s secondary-current input, burden, accuracy needs, and any specified CT characteristics.
3. Set the ratio. Match the rated primary and secondary currents to the circuit and connected equipment. Ask the project engineer to verify the choice against operating range and protection settings.
4. Specify protection class and ALF. Use the exact class, factor, and standard required by the design. A marking such as 5P10 is commonly used to express a class and ALF combination under IEC conventions; confirm the complete requirements for the project.
5. Calculate total burden. Include relay inputs, leads, terminals, and other connected devices. Compare the resulting burden with the CT’s rated burden using the applicable calculation method and design standard.
6. Review the complete specification. Check construction, insulation, dimensions, frequency, terminals, environmental conditions, drawings, and required inspection or documentation before placing an order.

How ALF and Burden Work Together

ALF is specified at a stated burden, so a CT marked 5P10 should not be treated as having the same accuracy limit under every connected-load condition. The “10” indicates an accuracy limit factor of 10 under the specified conditions; it does not mean the CT is guaranteed to perform identically with any burden or fault waveform. If the actual secondary burden differs from the rated value, the effective performance may change. Ask the supplier or design engineer to assess the actual circuit rather than relying on the marking alone.

Burden is expressed in volt-amperes at the rated secondary current. A 15 VA rating is one example of a possible specification, not a default for every project. Longer or smaller-gauge secondary leads can add burden, so include their resistance and the connected devices in the calculation. Keep secondary wiring within the project’s design requirements and follow safe practices; a CT secondary must not be left open-circuit while primary current is flowing.

Common Selection Mistakes to Avoid

• Choosing a ratio based only on nominal load current while overlooking protection settings or fault-current requirements.
• Specifying an ALF without stating the associated protection class, rated burden, and applicable standard.
• Ignoring lead resistance or relay input burden when checking the CT’s total connected load.
• Assuming a metering CT and a protection CT are interchangeable because their ratios are the same.
• Ordering before confirming mechanical dimensions, mounting, terminal arrangement, and environmental conditions.

These omissions can create avoidable design revisions, procurement delays, or performance mismatches. If a requirement is not available, mark it as an open technical item rather than filling the gap with an unsupported assumption. A clear specification helps both the buyer and supplier identify what must be confirmed before production.

Pricing, MOQ, and Lead-Time Considerations

CT pricing and availability depend on the specified ratio, class, ALF, burden, construction, insulation level, dimensions, quantity, and documentation requirements. Custom configurations or project-specific drawings may require additional technical review, so buyers should not assume that a standard item will match every project. Minimum order quantities and lead times vary by product configuration and order details; ANNEZEE can confirm them after reviewing the requirement. For accurate comparison, request quotations using the same technical specification and delivery terms.

Supplier Evaluation Checklist

Before selecting a supplier, ask for a datasheet that states the full ratio, secondary rating, protection class, ALF, rated burden, and applicable standard. Confirm that drawings or dimensional information match the intended installation and that requested inspection or test documents can be provided for the offered configuration. Ask how the supplier handles technical clarification, specification changes, and order documentation. Compare offers on confirmed technical scope as well as price and delivery, rather than on headline cost alone.

Key Takeaways and Next Steps

Protective CT selection begins with the circuit and relay requirements, then connects ratio, protection class, ALF, and burden into one verified specification. The ALF must be understood at its stated burden, and the actual secondary circuit—including leads and connected devices—must be considered. Before ordering, confirm the standard, construction, dimensions, and documentation needed for the application. This answers the central selection question: choose a CT only when its complete declared performance matches the protection scheme and installation.

To move forward, prepare the system current and fault information, relay model or input requirements, desired ratio, class, ALF, burden, installation dimensions, quantity, and destination. Send these details to ANNEZEE for a technical review and quotation discussion. If some values are still under design, identify them clearly so we can help clarify the information needed before product evaluation.