Remanence is the residual magnetic flux left in a current-transformer core after excitation returns to zero. Measure it by tracing the excitation and flux relationship under controlled, de-energized conditions, either with a voltage-current test plus digital integration or with dedicated CT test equipment. For a manufacturing station, a dedicated analyzer is usually easier to standardize than a manually configured oscilloscope and integrator.
Current, Thermal Load, and Timing
The number that matters first is the primary current at the moment the secondary circuit is opened. With primary current flowing, secondary ampere-turns normally oppose the primary magnetizing force. Opening the secondary removes that opposition, drives core flux upward, and can produce a hazardous secondary voltage.
The resulting excitation is also a thermal and insulation problem. High secondary voltage can injure personnel, damage connected instruments, stress CT winding insulation, and leave the core magnetized. This is heat and magnetic flux, not logic: changing software, relay settings, or data acquisition scaling cannot make an energized open-secondary test safe.
| Quantity or limit | Why it matters | Where to read or measure it |
|---|---|---|
| Primary current | Determines the primary ampere-turns present when the secondary circuit changes | De-energized test record or independently verified primary-current indication |
| Secondary excitation voltage | Drives core flux and can become hazardous if the secondary opens under load | CT test set or suitably rated measuring channel |
| Excitation current | Shows the current required to establish a given secondary voltage or flux state | Excitation-test current channel |
| Flux linkage | Provides the magnetic-state quantity derived from integrated winding voltage | Digital integration result or dedicated CT analyzer |
| Acceptance threshold | Defines whether the reported remanence passes the customer requirement | Customer test specification or approved manufacturing test procedure |
Symptom Interpretation
Abnormally high excitation current at a known point on the excitation curve is a practical indication of abnormal remanent magnetism. Compare the result at the same voltage, frequency, connection, ramp direction, and initial magnetic state. A different test point or ramp history can change the current even when the CT itself has not changed.
An excitation curve describes terminal voltage versus excitation current. A B-H curve describes flux density versus magnetic field strength. The two are related, but they are not interchangeable without CT design data. Converting an excitation test into absolute B and H values requires quantities such as turns, magnetic path length, core cross-sectional area, and winding resistance.
For incoming inspection or production screening, the excitation-current comparison may be sufficient if the approved test specification defines a reference point and tolerance. If the customer requires magnetic retentivity or a remanence ratio, the station must measure or calculate the specified magnetic quantity rather than report a visual difference between excitation curves.
Residual-Flux Mechanism
Core flux follows a hysteresis loop. After the applied magnetizing force rises and then returns to zero, the operating point may remain at nonzero flux. That residual state is remanence. A later protection or measurement event then begins from an offset magnetic condition, reducing the available flux excursion in one direction and changing when saturation starts.
For a winding, induced voltage represents the rate of change of flux linkage. A practical integrator derives flux linkage from:
lambda(t) = lambda(0) + integral[v(t) - i(t)R] dt
Here, v(t) is winding terminal voltage, i(t) is excitation current, and R is winding resistance. Neglecting the resistive term adds error, especially where current is high. Converting flux linkage to flux density requires the winding turns and effective core area. Converting current to magnetic field requires the magnetic path geometry.
Gapped cores require particular care because the air gap changes the magnetic circuit and the relationship between excitation current, field strength, and retained flux. Use the customer-defined calculation and the CT design parameters rather than applying an acceptance ratio developed for another core construction.
Controlled Measurement Procedure
- De-energize and isolate the primary circuit. Establish the required safe state using the site electrical procedure, then verify the absence of primary current with an appropriate method.
- Keep the CT secondary shorted while preparing connections. Identify every secondary terminal and remove or isolate connected relays, meters, and burdens as required by the approved test circuit.
- Connect the excitation source, voltage measurement, current measurement, and integration channel with the source at zero output. If using dedicated equipment, select the applicable CT test function and enter only values taken from the CT record or approved test specification.
- Remove the temporary secondary short only after the test source controls the complete secondary circuit. Never open a CT secondary while its primary is energized.
- Demagnetize the core before establishing a repeatable baseline. Drive the core through controlled excitation and reduce the excitation gradually to zero. The decreasing sequence matters because an abrupt interruption can leave another residual magnetic state.
- Apply the magnetizing sequence required by the customer specification. Record secondary voltage and excitation current throughout the rising and falling portions of the cycle.
- Integrate the corrected winding voltage to obtain flux linkage. If absolute B-H results are required, apply the documented turns and core-geometry values. Read remanence at the zero-magnetizing-force point defined by the approved method.
- Return excitation smoothly to zero, demagnetize the CT if the production process requires it, restore the secondary short, and only then disconnect the test equipment.
A digital oscilloscope with integration can perform the measurement when the excitation source and CT design data are available. The setup is sensitive to channel offset, integration drift, scaling, and operator technique. An Omicron CT Analyzer or Omicron standalone remanence-measurement equipment is a candidate for a production station, but the selected configuration must report the exact quantity required by the customer.
Result Verification
Repeat the complete magnetize-measure-demagnetize sequence rather than repeating only the final reading. Two runs that begin from different magnetic states are not a repeatability test. Compare the integrated waveform, excitation curve, zero point, ramp direction, and reported remanence result.
Check the integrator with zero applied excitation. A drifting calculated flux trace indicates voltage-channel offset or numerical integration error. Confirm voltage and current polarity as well; a reversed channel can mirror the loop or produce an incorrect residual-flux sign.
For a manufacturing fixture, verify a known CT at the start of a test interval and after connection or software changes. Store the raw voltage-current traces with the calculated result when practical. The traces separate a CT change from a wiring error, clipped measurement channel, unstable source, or incorrect scaling factor.
Recurring Measurement Pitfalls
| Pitfall | Observed effect | Correction |
|---|---|---|
| Opening the secondary with primary current present | Hazardous voltage, possible instrument or winding damage, and additional core magnetization | De-energize the primary and maintain a secondary short during setup and removal |
| Comparing different excitation points | An apparent remanence change caused by test-condition variation | Use the same voltage, frequency, ramp, polarity, and initial state |
| Uncorrected integrator offset | Flux trace drifts even when physical flux is not changing | Zero the channels and check the integrated baseline before testing |
| Ignoring winding resistance | Terminal voltage is treated entirely as induced voltage | Use v - iR when the approved calculation requires corrected flux linkage |
| Using unknown core geometry | Excitation data is presented as absolute B-H data without a valid conversion | Obtain turns and core dimensions from controlled design records |
| Undefined pass criterion | A repeatable number that cannot be accepted or rejected | Resolve the required quantity, magnetic history, and threshold with the customer before production release |
Frequently Asked Questions
Can I measure CT remanence with an oscilloscope?
Yes. Use controlled secondary excitation, measure voltage and current, and digitally integrate the corrected winding voltage. Absolute B-H conversion also requires the CT turns and core geometry.
Does an excitation test reveal remanent magnetism?
Abnormally high excitation current at the same known excitation point can reveal abnormal remanence. Match voltage, frequency, ramp direction, and starting magnetic state before comparing results.
Can I open the CT secondary during the test?
Only after the primary is de-energized and the controlled test circuit is ready. An open secondary with primary current flowing can generate hazardous voltage, damage equipment, and magnetize the core.
Does slowly reducing excitation remove CT remanence?
Driving the core into controlled excitation and gradually reducing the excitation to zero provides a demagnetizing sequence. Use the same approved sequence before every comparison measurement.
Can I use a CT Analyzer for production remanence testing?
An Omicron CT Analyzer or its standalone remanence equipment may reduce operator-dependent integration work, provided it reports the customer's required quantity. Stop testing if the primary cannot be positively isolated, the secondary connections are uncertain, or the acceptance definition is missing; escalate the circuit, method, and captured traces to official manufacturer support.