To select a BH 0.66 protective current transformer for relay protection, first confirm that its primary current, secondary rating, protection accuracy, burden, and installation dimensions match your system and relay. The “0.66” designation commonly refers to a 0.66 kV voltage level, but buyers should verify the exact meaning and rated insulation voltage on the product documentation rather than infer it from the model name alone. We recommend treating the relay and CT as one measurement and protection circuit: a suitable CT must deliver the current information the relay needs under both normal and fault conditions. Before ordering, compare the complete CT nameplate and technical drawing with your project requirements.
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Start by identifying what the relay is expected to protect, such as a feeder, motor, transformer, or busbar circuit. The protection scheme determines how the relay uses current measurements and which CT characteristics matter most. A CT selected only for normal load measurement may not be suitable for protection, particularly when fault current could cause the core to saturate before the relay receives a usable signal. We advise confirming the relay model, protection function, and system design with the responsible electrical engineer.
Record the system voltage, rated load current, maximum expected current, frequency, and available installation space. For a system identified as 0.66 kV, confirm the voltage category and insulation requirements from the switchgear or project specification; the model label alone is not a substitute for that check. Common power-system frequencies include 50 Hz and 60 Hz, but the CT’s rated frequency must match the actual installation. Also establish whether the primary conductor is a busbar or cable and how it will pass through or connect to the transformer.
Select a primary-current rating that is appropriate for the protected circuit and the relay’s measurement range. A rating that is too low can limit normal operation, while an unnecessarily high ratio may reduce useful resolution for some relay applications. Check the ratio printed on the CT nameplate, such as a specified primary-to-secondary relationship, and confirm it against the relay configuration. Do not choose a ratio based only on nominal load if the protection study or project documents specify another value.
Confirm whether the relay input is designed for a 1 A or 5 A secondary. Both are used in protection systems, but they are not interchangeable without checking the relay input and the CT specification. The CT secondary rating must match the connected device and wiring design. If the secondary current is not stated in the project documents, resolve it with the relay supplier or protection engineer before placing an order.
Protection CTs are selected not only for their accuracy at ordinary operating current but also for their behavior during fault conditions. Review the specified protection class and accuracy limit factor, or the equivalent performance parameters stated in the applicable project specification. For example, a designation such as 5P10 is a possible protection-class marking in relevant specifications, but it is only an example—not a claim about the rating of any particular BH 0.66 model. Ask the supplier to confirm the actual marked class and supporting technical data for the exact configuration.
The relay, CT, and secondary wiring form a connected burden that can affect CT performance. Add the relay input burden and the burden from leads and connections, then compare the total with the CT’s rated burden, expressed in volt-amperes (VA). For example, a 5 VA entry on a datasheet is a rating to verify against the real circuit, not a universal requirement for every application. If the total circuit burden is unclear, provide the relay input data and approximate lead length to the protection engineer or supplier for review.
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For applications with high prospective fault current, do not rely on a general statement that a CT is “for protection.” Ask for the relevant saturation or accuracy-limit information and check that it aligns with the protection study. The required performance depends on the fault level, relay operation, and system design. Where differential or other demanding protection schemes are involved, use the engineer’s specified CT requirements rather than selecting by a generic rule of thumb.
Check the overall dimensions, mounting method, aperture or busbar opening, terminal position, and conductor arrangement against the equipment layout. A transformer with suitable electrical ratings can still be unsuitable if it cannot be installed safely or if the primary conductor does not fit as intended. Request a dimensioned drawing for the specific model and compare it with the available space before approving the order. Also confirm the enclosure or insulation requirements for the intended location from project documentation.
Verify the marked primary and secondary polarity terminals, commonly identified with markings such as P1/P2 and S1/S2 where applicable. Correct polarity is important for directional, differential, and other protection functions that depend on current direction or phase relationships. Follow the project wiring diagram and the CT manufacturer’s instructions during installation. A CT secondary should not be left open-circuited while primary current flows; arrange safe shorting and isolation procedures according to site rules and qualified personnel.
| Selection point | What to verify | Why it matters |
|---|---|---|
| System and insulation | 0.66 kV application, rated voltage, and insulation requirements | Ensures the CT is specified for the installation conditions |
| Current ratio | Primary rating and 1 A or 5 A secondary compatibility | Matches the circuit current range to the relay input |
| Protection performance | Protection class, accuracy limit factor, and fault requirements | Helps the relay receive suitable current information during faults |
| Burden | Rated VA compared with relay and wiring burden | Supports performance within the specified circuit conditions |
| Installation | Dimensions, conductor fit, terminal layout, and polarity | Reduces fit and wiring issues during assembly |
Prepare a concise specification package before requesting a quotation. Include the system voltage and frequency, primary and secondary current, protection class and fault-performance requirement, rated burden, installation dimensions, quantity, and any project-specific documentation needs. Attach the relay details or relevant circuit information where available. This gives suppliers a clear basis to identify a suitable configuration and flag missing requirements.
Ask the supplier to identify the exact model and configuration offered, provide a datasheet and dimensioned drawing, and state any assumptions made in the quotation. Compare offers against the same requirement list rather than comparing model names or unit prices alone. If a specification is not available, request confirmation before treating it as compliant. Keep the approved data and drawings with the project records so purchasing, engineering, and installation teams work from the same revision.
At ANNEZEE, we support B2B buyers evaluating BH 0.66 protective current transformers by reviewing the application information they provide and aligning the inquiry with the required electrical and mechanical specifications. To help us assess your request, share the system voltage, primary current, relay secondary input, protection class or fault-performance requirement, burden, frequency, and mounting dimensions. We can use those details to clarify the requested configuration and identify which product documentation should be confirmed before an order. Final selection should remain consistent with your protection study, project specification, and applicable installation requirements.
The right BH 0.66 protective current transformer is the one whose verified ratio, secondary rating, protection performance, burden, insulation, and dimensions match the relay circuit and installation. Begin with the protection function, confirm the system and relay data, then check the specific product documentation and drawing before procurement. If any required rating is missing, request clarification rather than assuming it from the BH 0.66 designation. Send ANNEZEE your application requirements to discuss a suitable configuration and the information needed for a confident purchasing decision.
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