The central design trade-off
A CT ratio must be low enough for protection sensitivity and measurement resolution, yet high enough to carry maximum load and reproduce severe fault current without unacceptable saturation. Accuracy class, burden and knee point cannot be selected independently: the connected secondary circuit determines the voltage the CT must develop.
Modern relays tolerate some saturation better than older devices, but algorithm capability does not excuse an unverified CT. Differential, REF, high-impedance and directional elements can have very different CT requirements even on the same bay.
1. Choose the ratio from four currents
- Maximum continuous/emergency primary current.
- Minimum fault current that must be detected.
- Maximum through-fault current.
- Relay nominal and continuous input ratings.
A ratio chosen only from switchgear rated current can destroy sensitivity. Conversely, selecting a ratio almost equal to normal load may overload the secondary and reduce transient performance. Check the relay’s usable measuring range and the protection pickup in secondary amperes. If multiple cores are available, metering and protection should normally use cores designed for their different duties.
2. Calculate the actual burden
For copper, use the project’s design resistivity at the relevant temperature; do not assume a room-temperature value for a hot switchgear cable route. With a 1 A CT, two 30 m leads of 2.5 mm² copper give approximately 0.42 Ω at 20°C before terminals. Add relay input and connection resistance. At rated current this is only about 0.5 VA, one reason 1 A secondaries are advantageous on long runs. A 5 A CT would impose 25 times the VA for the same resistance.
Never compare a relay burden in VA quoted at 5 A directly with one quoted at 1 A. Convert to impedance.
3. Interpret IEC protection classes
| Marking concept | Meaning | Application note |
|---|---|---|
| 5P or 10P | Composite error limit at the stated accuracy-limit condition. | General overcurrent protection when burden and ALF are correctly applied. |
| ALF, e.g. 20 | Accuracy limit factor relative to rated current at rated burden. | Actual ALF changes when connected burden differs from rated burden. |
| PX | Special requirements based on excitation characteristic, knee-point voltage, winding resistance and turns ratio. | Common for high-impedance differential/REF and schemes requiring explicit excitation data. |
| PR/PXR and transient classes | Address remanence or transient performance as defined by the applicable standard. | Use when DC offset, long time constants and demanding differential duty require it. |
“5P20 15 VA” does not mean the CT is accurate to 20 times rated current with any burden. It states performance at the rated burden and specified conditions. Obtain winding resistance and excitation data when a scheme calculation requires them.
4. Why CTs saturate
The secondary voltage needed is approximately secondary current multiplied by total secondary-loop impedance. Primary DC offset, high X/R ratio, remanent flux, long fault duration and asymmetric current increase core flux beyond the steady-state symmetrical case. When the core saturates, the secondary waveform is clipped and delayed. Residual current can be created by unequal phase-CT saturation even for a fault with no zero-sequence current.
Effects include delayed 50/51 pickup, false directional quantities, reduced differential restraint or false operate current, incorrect fault location and distorted COMTRADE. Examine both dependability for internal faults and security for external through faults.
5. Knee-point checks for high-impedance schemes
High-impedance differential and REF schemes force stability by using a stabilizing resistance so spill current from a saturated CT does not operate the relay. The required knee-point voltage is calculated from maximum external fault current, CT ratio, CT winding resistance and the maximum lead resistance to the CT. The exact equation and safety factors depend on the scheme and relay manual.
6. Application checks by protection function
| Application | Dominant CT concern | Recommended evidence |
|---|---|---|
| Feeder 50/51 | Pickup sensitivity and delay during asymmetric high fault current. | Ratio, class/ALF, actual burden and waveform test or validated calculation. |
| Sensitive earth fault | Phase-CT spill or CBCT low-current accuracy. | Standing residual measurement and primary injection where practicable. |
| Transformer differential | Ratio mismatch, through-fault saturation, inrush and vector compensation. | CT model/excitation, maximum through fault and relay stability assessment. |
| Low-impedance bus differential | Unequal saturation across many bays and dynamic zone selection. | Maximum external fault, CT classes/ratios, relay algorithm application guide. |
| High-impedance differential/REF | Knee point, CT resistance and lead resistance. | Individual excitation curves, matched ratios, Vk and complete stability calculation. |
7. Example review
A 600/1 A feeder CT supplies a numerical relay through 30 m of 2.5 mm² copper each way. Assume total secondary resistance of 0.50 Ω after temperature and terminals. At 12 kA primary, ideal secondary symmetrical current is 20 A and the resistive voltage demand is about 10 V. This alone does not prove adequacy: add CT winding impedance, relay model, DC offset/X-R requirement, remanence and the protection’s acceptable error. But it immediately shows why actual loop resistance must be measured and why a blanket “10 VA CT is enough” statement is incomplete.
If the minimum earth fault is only 60 A, the phase CT secondary current is 0.10 A; phase-CT mismatch may be significant. A dedicated 100/1 CBCT may provide better earth-fault sensitivity even though the phase CT is adequate for phase protection.
8. Specification and commissioning checklist
- Ratio, secondary current and multi-ratio connection.
- Core purpose, IEC/IEEE class, rated burden and ALF or excitation requirements.
- Rated short-time thermal and dynamic current.
- Winding resistance, knee point/excitation curve and remanence class where required.
- Maximum secondary lead length, conductor size and terminal resistance.
- One-point secondary earthing and safe shorting/test facilities.
- Polarity and physical zone boundary shown on drawings.
- Individual CT certificates matched to serial numbers.
- Primary ratio/polarity test, secondary loop resistance and excitation test where justified.
- As-left phase and residual current under load.
Open-circuiting an energized CT secondary is dangerous. Testing and isolation require approved CT shorting terminals and procedures.
Related protection guides
Engineering limitation
This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.
References and further reading
- IEC 61869-1:2023 — general requirements for instrument transformers
- IEC 61869-2:2012 — additional requirements for inductive CTs
- IEEE C37.110-2023 — CT application for protective relaying
- IEEE 3004.1-2013 — instrument-transformer application in industrial/commercial systems
- SEL — CT Sizing for Generator and Transformer Protective Relays — comparison of IEC/IEEE approaches and CT models
Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.