Author: Leon Zhang — Power Monitoring and Testing Equipment Team, Baoding Houfeng Trading Co., Ltd., sourcing CT and VT test equipment for utilities, relay engineers and calibration laboratories
Technical review: reviewed by the in-house high voltage test engineering team against IEC 61869-2:2012, IEC 61869-3:2011 and IEEE C57.13.1-2017 practice
Last updated: 19 October 2026
Instrument transformer testing is the metrology layer of a substation, and it is bought for the one question a ratio meter cannot answer: whether a current or voltage transformer will still hold its accuracy class after years in a switchyard. Every meter reading, every protection decision and every energy settlement number passes through a CT or a VT. They are also the part of the system that is easiest to take for granted. A CT with the right nameplate ratio can still be wired with reversed polarity, carry the wrong burden, or saturate under fault current, and none of that is visible from the outside.
That gap between nameplate and behaviour is why the test list is longer than most people expect. A ratio check confirms one number at one operating point. The remaining tests confirm that the transformer still behaves correctly when the current is twenty times rated and the relay is deciding whether to trip. IEC 61869-2:2012 covers current transformers and IEC 61869-3:2011 covers inductive voltage transformers, and both documents are written to be read alongside the general requirements part.
Key takeaways
- A ratio check and an error measurement are different tests with different instruments; a turns ratio tester does not produce an accuracy class result.
- Polarity is a protection test, not a metrology one. A reversed secondary inverts every current-derived decision at once while normal load looks entirely plausible.
- Burden is the half of metering error that is usually assumed rather than measured, and on long secondary runs the lead drop is often the largest single term.
- IEC 61869-2:2012 and IEC 61869-3:2011 are still the current parts, but both are written against the 2007 first edition of IEC 61869-1:2007, which was replaced by a second edition in 2023.
- Laboratory calibration establishes the device, field testing proves the installed circuit. A programme that only does one of them is measuring half the error.
- Excitation, ratio and winding resistance can all pass on a CT that is wired into the wrong core of a multi-core unit, which is why identification steps belong in the procedure.
What instrument transformer testing actually covers
The eight tests above are not alternatives to each other, and the reason they are all in the list is that each one closes a different failure mode, which is what makes the list read as repetition when it is in fact eight different questions. A procedure that skips one of them is not a shorter procedure, it is a procedure with a known blind spot. IEEE C57.13.1-2017 sets out the field test set that a crew is expected to be able to perform on a relaying current transformer, and in practice a CT or VT test set is bought to carry out some or all of the eight measurements below.
- Ratio test. The actual transformation ratio, and the deviation from the marked or nameplate ratio.
- Ratio error and phase error. The two components of composite error, which is what an accuracy class is actually a statement about.
- Polarity test. Confirming that primary and secondary are referenced the same way. A CT with a correct ratio and reversed polarity feeds protection a signal inverted by 180 degrees.
- Winding and lead resistance. The internal resistance of the secondary circuit, which sets how much of the available driving voltage is lost before the relay or meter sees it.
- Excitation test. The excitation characteristic including the knee point, which is what predicts behaviour when the current runs far above rated.
- Admittance test. The secondary circuit admittance, most relevant where long secondary runs are involved.
- Burden test. The actual load hanging on the secondary — cables, relays, meters, transducers — measured rather than taken from a datasheet.
- Insulation resistance. The routine check that a winding is sound before anything else is trusted.
| Test | Failure mode it closes | Goes wrong when skipped |
|---|---|---|
| Ratio | Wrong nameplate or turns fault | Metering error appears as an unexplained constant offset |
| Ratio and phase error | Class drift, core degradation | Billing error accumulates with no single cause |
| Polarity | Reversed secondary | Directional, distance and differential protection all invert |
| Winding and lead resistance | High-resistance secondary path | Relay sees a low signal at exactly the wrong moment |
| Excitation | Early saturation | Protection refuses to operate during a real fault |
| Admittance | Excessive secondary loading | Long runs attenuate the signal below detection |
| Burden | Overloaded secondary | Accuracy class moves without any device fault |
| Insulation resistance | Moisture or insulation damage | Every other result becomes unreliable |
The instrument transformer testers category covers all eight, but no single box does them equally well. That is the practical reason instrument transformer testing equipment gets bought by measurement rather than by model number, and why the same crew often carries two sets to one commissioning job.
Ratio is not the same as ratio error
The two get confused often enough that the difference decides which instrument gets ordered, and the confusion is expensive because the wrong purchase looks entirely reasonable on paper until the first accuracy audit arrives. A transformer turns ratio tester answers a single question: what is the ratio of this winding pair. That is the question that comes up during commissioning of a power transformer, on a suspect unit, or after a tap change. It is a power-transformer test, and a general ratio meter performs it correctly.
Ratio error and phase error are the metrology question. They ask how far the actual ratio and the phase displacement deviate from the ideal, expressed against the accuracy class the transformer is certified to, and they are stated at defined points of rated current and rated burden. Producing a calibrated error figure needs a reference standard, a burden, and a comparison — not a ratio meter.
| What instrument transformer testing asks | Test name | Instrument class | Result |
|---|---|---|---|
| What is the ratio of this winding pair | Turns ratio | Ratio meter or TTR | A dimensionless ratio |
| How far is the ratio from nameplate | Ratio error | CT/VT calibrator with reference | An error in percent at a stated point |
| How far is the phase from ideal | Phase error (phase displacement) | CT/VT calibrator with reference | An angle in minutes |
| Is primary referenced to secondary correctly | Polarity | Polarity test set or analyzer | In phase, or reversed |
| What load is actually connected | Burden | Burden tester | An impedance in VA and power factor |
| How does it behave above rated current | Excitation | Excitation test set | A knee point on a curve |
Read the first two rows as the trap. A unit can pass a turns ratio test and fail its accuracy class, because the turns ratio test is taken at low voltage with no burden and says nothing about core behaviour under excitation. The second pair of rows is the reverse: a calibrator can confirm an error figure on the bench and say nothing about whether the installed secondary wiring still delivers it, which is what the burden rows address.The three things a ratio meter cannot tell you. It cannot tell you the error at a stated burden, because it applies none. It cannot tell you the phase displacement, because it measures magnitude only. And it cannot tell you the knee point, because it never drives the core near saturation. Those three gaps are the reason a calibrator costs more than a ratio meter, and they are the three questions an accuracy class is actually about.
Polarity and phase error: the failure that gets found late
Polarity errors are expensive because they are invisible in normal operation and catastrophic in abnormal operation, so the test that finds them is really a protection test wearing a metrology label, and it belongs in the protection procedure rather than the calibration one. A reversed CT secondary produces a current signal that looks entirely plausible at steady state and inverts the direction of every current-based decision at once: directional overcurrent, distance protection and differential protection all misoperate together. The ratio test cannot detect it, because a reversed secondary still transforms the right magnitude.
That is why the polarity test sits beside the ratio test in the standard rather than being treated as a one-off at installation. It is also why the check has to be repeated after any work that opens the secondary circuit, including a relay replacement, a wiring modification or a test block operation that was not restored correctly. The polarity failures we see returned to us are almost never a wired-backwards transformer — they are a transformer that was correct at commissioning and had its secondary disturbed later. The test record shows a pass in year one and nothing after it.
Phase error is the related quantity on the metering side, and it fails differently. A CT with excellent ratio accuracy but a phase displacement outside its class produces a steady, directionless billing error. Small per reading, uncollectable in aggregate after the fact. That is what makes re-verification cycles matter commercially, and it is also why a dispute over an energy account is usually settled by a laboratory calibration rather than a site test.
| Branch | How it presents | Which test finds it | Why the ratio test misses it |
|---|---|---|---|
| Polarity reversal | Protection trips in the wrong direction | Polarity | Magnitude is still correct |
| Burden overload | Metering error grows slowly over years | Burden | The transformer is fine; the circuit is not |
| Early saturation | Protection fails to operate on a fault | Excitation | Needs current far above rated to show |
| Wrong core | Both cores pass, the wrong one is wired | Core identification | Every electrical test passes on both |
Burden: the half of the error that instruments usually miss
Metering error is not one number, and the burden test exists precisely because one term in that sum belongs to the wiring rather than to any instrument on the bench, so no amount of instrument accuracy removes it. It is the sum of several contributions: the energy meter itself, the combined error of the current transformer, the combined error of the voltage transformer, and the error introduced by voltage drop on the VT secondary leads. Where the secondary runs are long, that last term is frequently the largest of the four. It is the one nobody checks.
Burden testing brings that term under control. The secondary load — relays, meters, transducers and the cable between them — is measured as an actual impedance instead of being taken from a datasheet, then compared against the rated burden the transformer was selected for. The comparison has a hard limit: a CT rated for 15 VA carrying 30 VA does not simply read low, it moves its accuracy class, and the error curve at high current turns upward in a way no calibration offset can fix.
The HF-IT-207 CT burden tester and the HF-IT-206 voltage transformer burden tester are built for exactly that measurement. The HF-IT-208 burden box supplies the standardised loads needed to load the circuit up deliberately during verification, so the error is observed at the burden the transformer is rated for rather than at whatever happened to be connected.Why the rated burden is a grid, not a number. An accuracy class is only meaningful at a stated burden and a stated current. A CT marked 0.5 at 100 percent rated current and 25 percent rated burden is not guaranteed to hold 0.5 at a quarter of that burden. When a burden test shows the installed load is far below rated, the result is not automatically better — the transformer is simply being measured outside the conditions its class describes. Record the burden alongside the error, or the error figure cannot be interpreted later.
The burden on a CT secondary is usually lowest the day it is commissioned and highest five years later, and nothing in the test record shows the drift. Relays get added, meters get upgraded, a spare transducer is wired onto the same core during a retrofit, and the CT is still rated for a load it no longer serves. A burden measurement taken at acceptance describes a circuit that no longer exists, which is the argument for repeating it at every verification rather than trusting the original sheet.
Site testing or laboratory calibration
Both, and for different reasons, because the two answer questions that cannot substitute for each other and a programme that runs only one of them is measuring half the error. That is not a rhetorical point: the device and the circuit each contribute, and a method that looks at only one of them cannot tell you which one moved. A portable CT/VT test set is carried to the substation for commissioning, acceptance and periodic verification. Its advantage is not convenience but coverage: the reading includes the actual secondary wiring, the actual burden and the actual installation conditions, which is where most real failures live. An installation fault in a terminal block or a test link simply does not exist on a bench.
In the laboratory, a calibrator working against a reference standard establishes the error of the transformer itself, free of site effects. This is the traceable measurement, and it is what a dispute or an accreditation requirement will ask for. It is also the only way to separate a transformer that has drifted from a circuit that was wired wrong.
| Question on the job | Where it is answered | Why the other location cannot | Typical interval |
|---|---|---|---|
| Does this new unit meet its class | Laboratory | Site burden and wiring add error that is not the transformer’s | Once, at acceptance |
| Is the installed circuit still accurate | Site | Laboratory removes the wiring being questioned | 1–6 years, per policy |
| Has the transformer drifted over time | Laboratory | Needs a reference standard, not a working circuit | On dispute or audit |
| Why did protection misoperate | Site, plus records | The event happened on the installed circuit | After the event |
| Can the error trend be trusted | Both, paired | One without the other cannot separate device from circuit | Every cycle |
A practical programme pairs them: laboratory calibration establishes and documents the device, field testing proves the installed circuit still delivers that accuracy. The pairing is what makes a trend line meaningful, because it keeps the two error sources from being mixed into one number.What a paired record looks like in practice. One line per test, four fields: the calibrated error from the laboratory, the burden measured on site, the wiring condition at the time, and the date. Everything else is commentary. When a metering dispute arrives three years later, those four fields answer the question, and a stack of certificates without them does not.
Which instrument performs which test
Most CT/VT test sets are bought by test type rather than by model number, so the fastest route is to start from the measurement and work back to the instrument rather than compare datasheets line by line and hope the coverage difference becomes obvious. The list below follows that order, and the eight entries above map onto it.
- Excitation, ratio, error and phase measurement, polarity, winding resistance and hysteresis on one unit. The HF-IT-210 instrument transformer analyzer is a multi-function CT/PT analyzer covering the full laboratory and field test set.
- On-site verification of CTs and VTs with automatic range adjustment and PC data output. The HF-IT-201 instrument transformer calibrator is the core calibrating instrument of a CT/PT test bench.
- Automatic ratio and volt-ampere characteristic curves, variable frequency, no manual voltage regulation or wiring changes. The HF-IT-202 variable frequency instrument transformer tester drives the excitation curve without the operator hand-cranking a regulator.
- Portable on-site CT and VT verification. The HF-IT-203 on-site current transformer tester.
- Full voltage transformer calibration — error and phase angle deviation, winding resistance, polarity and excitation in one field device. The HF-IT-204 voltage transformer tester.
- Online ratio and polarity without opening the primary circuit. The HF-IT-205 wireless CT ratio tester clamps the high and low voltage sides and measures current, ratio, phase identification, polarity and leakage current.
- Burden measurement and standardised loads. The HF-IT-207 CT burden tester, the HF-IT-206 voltage transformer burden tester and the HF-IT-208 CT burden box.
- The variable voltage a VT test needs. The HF-IT-209 voltage transformer test power supply supplies it.
| Product | Main measurement | Bench or field | What it leaves to another instrument |
|---|---|---|---|
| HF-IT-210 analyzer | Ratio, error, polarity, excitation, R | Both | Nothing in the eight, at reduced accuracy on long runs |
| HF-IT-201 calibrator | Calibrated error and phase | Bench | Excitation and burden |
| HF-IT-202 variable frequency | Excitation curves | Both | Error figure and burden |
| HF-IT-203 on-site CT tester | On-site CT verification | Field | VT work and burden |
| HF-IT-204 VT tester | Full VT calibration | Field | CT work |
| HF-IT-205 wireless ratio | On-line ratio and polarity | Field, energised | Any calibrated error figure |
| HF-IT-207 / 206 burden | Secondary load | Field | Everything except load |
| HF-IT-208 burden box | Applies standard loads | Both | Measurement — it is the load, not the meter |
The standards behind the test list
Three bodies of standards matter, and it is worth knowing which generation you are working to before writing a procedure, because the edition question is not administrative. A clause reference carried over from a superseded text can point at a requirement that no longer exists, and the parts of the 61869 family sit on two different bases at the moment: IEC 61869-2:2012 and IEC 61869-3:2011 on the 2007 general part, everything newer on the 2023 one.
- IEC 61869-2:2012 covers additional requirements for current transformers and replaced IEC 60044-1:1996 and IEC 60044-6:1992 (IEC 61869-2:2012). It remains the current part, with Interpretation Sheet 1 issued in 2022 — and here is the detail most procedures miss. It is written to be read in conjunction with the 2007 first edition of IEC 61869-1:2007.
- IEC 61869-3:2011 covers inductive voltage transformers and replaced IEC 60044-2:1997 (IEC 61869-3:2011). It carries the same 2007 base. A second edition is in development at IEC but has not been published, so the 2011 text is still the reference to cite in a procurement document.
- IEC 61869-1:2023 is the general requirements part, now in its second edition, replacing the 2007 first edition that parts 2 and 3 still point to (IEC 61869-1:2023). This is the version trap in the series: a procedure that cites parts 2 and 3 without a year, and assumes they inherit the 2023 general part, is describing a document set that does not exist yet.
- IEEE C57.13-2016 covers requirements, the test code for ratio and phase angle, demagnetisation, excitation, impedance, polarity and resistance measurement (IEEE C57.13-2016). IEEE C57.13.1-2017 is the field-testing guide for relaying CTs and is the reference to follow when deciding what to test on site (IEEE C57.13.1-2017).
- JJG 313:2010 and JJG 314:2010 are the Chinese verification regulations for current and voltage transformers, with the GB 1208-2006, GB 1207-2006 and GB 16847-1997 family covering instrument transformer requirements. Where a Chinese verification certificate is the deliverable, these govern the procedure even when the equipment is specified to IEC.
One line on edition discipline. Write the year into the work method statement, not just the quotation. A method that names a part without its edition year invites two crews to test to different documents on the same site, and nobody notices until the results disagree.
| Document | Edition in force | Replaced | Base it is written against |
|---|---|---|---|
| IEC 61869-1:2023 | 2nd edition | IEC 61869-1:2007 | Standalone general part |
| IEC 61869-2:2012 | 1st edition + ISH1:2022 | IEC 60044-1:1996, IEC 60044-6:1992 | IEC 61869-1:2007 |
| IEC 61869-3:2011 | 1st edition | IEC 60044-2:1997 | IEC 61869-1:2007 |
| IEC 61869-4:2013 | 1st edition + COR1:2014 | IEC 60044-3:2000 | IEC 61869-1:2007 |
| IEEE C57.13-2016 | Current | C57.13-2008 | ANSI practice |
| IEEE C57.13.1-2017 | Current | C57.13.1-2006 | C57.13-2016 |
Limitations and boundaries
A test set answers only the question its physics allows. These boundaries are the ones that most often get lost between the test report and what someone takes the report to prove.
A ratio test says nothing about saturation.It is taken at low voltage with no burden, so it cannot show where the knee point sits or how the core behaves at twenty times rated current. A CT that passes ratio and saturates early will still operate correctly at load and fail to operate during the fault it exists for.
Polarity results are only valid for the wiring as found.A correct polarity reading confirms the circuit at the moment of the test. Any subsequent work on the secondary — a relay change, a test link left in the wrong position, a terminal re-made — voids it. The check is a statement about a configuration, not about a device.
Burden measurement needs the circuit as it will operate.Measuring burden with the trip circuit isolated, or with a spare core in parallel, gives a number that does not describe service. The measurement is only meaningful with the actual relays, meters and cabling connected and the circuit in its operating state.
Excitation test voltage can be dangerous on high-ratio CTs.The knee point of a high-ratio CT sits at a voltage that will injure someone who touches an open secondary. The test is routine; the hazard is not, and the earth and isolation steps belong in the procedure rather than in the crew’s memory.
Laboratory calibration cannot see installation error.A traceable certificate establishes the transformer. It says nothing about a 2.5 mm² secondary lead run of 200 metres, a burden that was never re-measured after a retrofit, or a core wired to the wrong relay. Treating a calibration certificate as evidence about the installed circuit is the most common over-reading of a test report.
On-line ratio measurement is a comparison, not a calibration.A wireless clamp-on set measures ratio, phase identification and polarity without opening the primary, which is what makes it usable on an energised circuit. It does not produce an error figure against a reference, and a pass from it should not be recorded as an accuracy class result.
Test frequency and test current both change the answer.Instrument transformer error is a function of current, burden and frequency. A result taken at 50 Hz, at one current point, at one burden, is a point on a surface. Recording the conditions matters as much as recording the number, because a result without its conditions cannot be compared to the next test.
| Boundary | What the test cannot deliver | What to do instead |
|---|---|---|
| Ratio test on its own | Nothing about knee point or saturation | Add an excitation test on the same visit |
| Polarity pass at commissioning | Nothing after secondary work | Repeat it after any relay or link change |
| Burden measured in isolation | Nothing about the operating circuit | Measure with relays, meters and cabling connected |
| Laboratory certificate only | Nothing about installed wiring | Pair it with a field test on the circuit |
| On-line clamp-on ratio | No calibrated error figure | Record it as a comparison, not an accuracy result |
| One result at one current | Nothing about the other stated points | Take the error at the points the class covers |
A passing test is a statement about the point that was tested.Instrument transformer error is a surface, not a number: it changes with current, with burden and with frequency. A report that says “passed” without the current and burden at which it passed cannot be compared with the next report, and a trend built from such reports is a trend in nothing.Before you write the procedure. Name the failure mode first, then pick the test. Polarity protects the protection scheme, burden protects the metering account, excitation protects the fault response, and calibration protects the audit trail. A test list that was not built from failure modes is a test list that will be trimmed by whoever pays for the outage.
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Frequently asked questions
What is instrument transformer testing equipment?
It is the test equipment used to verify current transformers and voltage transformers rather than power transformers: calibrators that measure error and phase displacement, ratio testers, polarity and excitation test sets, and burden testers and burden boxes that measure and apply the secondary load.
Is a transformer turns ratio tester the same as an instrument transformer tester?
No. A turns ratio tester measures the ratio of power transformer windings during commissioning or fault investigation. Instrument transformer testing is a metrology task on a device that must hold its accuracy class in service, and it needs a calibrator, a calibrated error and phase measurement, and burden testing in addition to a ratio check.
Why test CT polarity if the ratio is correct?
Because a reversed secondary inverts every current-derived decision in the protection scheme. Directional overcurrent, distance and differential protection all misoperate while normal load current looks entirely plausible. The ratio test cannot detect it, because a reversed secondary still transforms the correct magnitude.
What does a burden test measure?
The actual impedance of the secondary circuit as installed — relays, meters, transducers and cabling — compared with the burden the transformer is rated for. On circuits with long secondary runs the voltage drop in the leads is often the largest single contribution to metering error.
Can CT ratio be measured without disconnecting the primary?
Yes, with a wireless CT ratio tester that clamps the high and low voltage sides and measures current, transformation ratio, phase identification and polarity on-line. That matters where the primary cannot be taken out of service for a conventional test, but the result is a comparison rather than a calibrated error figure.
Do IEC 61869-2:2012 and IEC 61869-3:2011 still apply after the 2023 revision of Part 1?
Yes. Parts 2 and 3 were not reissued with the second edition of Part 1. IEC 61869-2:2012 and IEC 61869-3:2011 remain the current parts, and both are written against the 2007 first edition of the general requirements. Cite the part with its year, and check clause references against the edition the part actually references.
Conclusion
Instrument transformer testing is bought one failure mode at a time, and the specification should be written the same way: which decision depends on this transformer, what measurement proves that decision is sound, and which instrument performs it. A ratio test without a polarity test leaves protection able to trip in the wrong direction. A ratio error figure without a burden measurement leaves metering error unaccounted for. A calibration certificate without a field test describes a device that may no longer be connected the way the certificate assumes.
If you are not sure which test set the job needs, describe the situation rather than the specification: the transformer type and ratio, the tests your procedure requires, whether the unit is in service or on the bench, and what the last result showed. The instrument transformer tester range covers calibrators, ratio testers, burden testers, the power supplies and burden boxes that complete a CT/PT test set.
Request a quotation for instrument transformer test equipment matched to your transformer type, accuracy class and test procedure, and we will confirm the instrument combination your procedure actually requires.
Technical references
- IEC, IEC 61869-1:2023: Instrument transformers — Part 1: General requirements, 2nd edition — webstore.iec.ch.
- IEC, IEC 61869-2:2012: Instrument transformers — Part 2: Additional requirements for current transformers, 1st edition, with Interpretation Sheet 1:2022 — webstore.iec.ch.
- IEC, IEC 61869-3:2011: Instrument transformers — Part 3: Additional requirements for inductive voltage transformers, 1st edition — webstore.iec.ch.
- IEEE, IEEE C57.13-2016: IEEE Standard Requirements for Instrument Transformers — standards.ieee.org.
- IEEE, IEEE C57.13.1-2017: IEEE Guide for Field Testing of Relaying Current Transformers — standards.ieee.org.
- SAC, JJG 313:2010: Verification Regulation of Current Transformers and JJG 314:2010: Verification Regulation of Voltage Transformers — samr.gov.cn.
- SAC, GB 1208-2006 and GB 1207-2006: Power Transformers — Instrument Transformers requirements, with GB 16847-1997 for protective current transformers — openstd.samr.gov.cn.