CT and PT Testing Guide: Ratio, Polarity, Burden and Knee Point
Protection relays and meters never see the real currents and voltages of a high-voltage system. They see scaled-down copies produced by instrument transformers: current transformers, or CTs, that step large primary currents down to a standard secondary current, and potential transformers, or PTs (also called voltage transformers, or VTs), that step high voltages down to a standard secondary voltage. If those copies are wrong — wrong ratio, reversed polarity, excessive burden, or a saturated core — then every relay decision and every meter reading built on them is wrong too. That is why instrument transformer testing is a core part of commissioning, and why it deserves the same care as testing the power transformer itself.
This guide explains CT and PT testing in plain language: ratio, polarity, burden, saturation and knee point, and insulation checks, plus the one safety rule that matters above all others. As with everything on this site, always work to your project specification, the manufacturer's instructions and the applicable standards for your region; the general guidance here never overrides those documents. If you are new to transformer testing as a whole, start with our power transformer testing and commissioning guide, which puts instrument transformer testing in context alongside the power transformer tests.
What CTs and PTs do, and why accuracy matters
A current transformer has a primary winding carrying the system current and a secondary winding delivering a standard current — commonly 1 ampere or 5 amperes at rated primary current — to relays and meters. A potential transformer works the other way round for voltage, delivering a standard secondary voltage — commonly 110 volts — from the system voltage. Both exist for the same two reasons: safety, because they isolate secondary circuits from high voltage and high current, and standardisation, because relays and meters can then be built for one standard input regardless of the system they serve.
Accuracy matters because protection depends on it. A CT that saturates during a fault delivers a distorted, reduced secondary current, and a relay seeing that distorted current may operate too slowly or not at all. A CT with reversed polarity makes a differential relay see a fault current where none exists, or miss a real one. A PT with the wrong ratio makes every voltage-dependent measurement and protection function read wrong. Instrument transformer testing exists to prove, before energisation, that the copies the relays will see are faithful — and to catch wiring and ratio errors while they are still cheap to fix.
Safety first: the CT open-secondary rule
There is one safety rule in CT testing that must be understood before anything else: a CT secondary must never be left open-circuited while primary current is flowing. A CT is designed to deliver its secondary current into a low-impedance load. If the secondary circuit is opened, the CT still tries to drive that current, and the voltage across the open terminals rises to dangerous levels — high enough to injure anyone touching the terminals and to break down the CT insulation. This is not a theoretical risk; it is one of the best-known hazards in substation work.
The practical consequences run through every CT test and every CT handling procedure. Unused CT secondary windings must be short-circuited. Before disconnecting any CT secondary wiring, short the winding first. Before removing a short, make sure the secondary circuit is complete. And during testing, any test that involves opening the secondary circuit must only be done with the primary de-energised or with the CT shorted according to the approved procedure. PTs have the opposite hazard to respect: a PT secondary must never be short-circuited, because a PT is a voltage source and a short across its secondary draws a large fault current. Remember the pair together — CT secondaries open is dangerous, PT secondaries shorted is dangerous — and most instrument transformer accidents are already prevented.
CT ratio test
The ratio test proves that the CT delivers the secondary current its nameplate promises. The standard method is primary injection: a known primary current is passed through the CT primary — using a high-current test set for the larger ratios — and the secondary current is measured. The ratio of primary to secondary current should match the nameplate ratio within the CT's accuracy class, and the test is normally repeated on each tap where the CT has multiple ratios. An alternative for smaller CTs is secondary injection with a ratio test set, which verifies the turns ratio by a different route, but primary injection remains the most convincing proof because it tests the whole CT in its real operating configuration.
Ratio errors have common causes worth checking before concluding that a CT is faulty: the wrong tap selected, a wiring error in the test setup, or a secondary circuit that is not the one you think it is. Always confirm the tap position and the wiring against the drawings before recording a failure. A ratio result that is wrong by exactly a tap step, or by a factor that matches a wiring mistake, is almost always a setup or selection error rather than a defective CT. Record the tap, the test current, the measured secondary current and the calculated ratio for every winding tested.
CT polarity test
Polarity proves that the secondary current flows in the expected direction relative to the primary current — that the terminal markings on the CT correspond to the actual winding sense. Polarity matters most for differential and directional protection, where the relay compares currents from two or more CTs and expects them to cancel under normal conditions. If one CT is reversed, the relay sees a differential current during normal load and may trip, or it may fail to recognise a genuine fault.
The classic polarity check uses a battery and a DC milliammeter or a dedicated polarity tester: a brief DC pulse applied to the primary produces a kick in the secondary instrument whose direction reveals the polarity. Modern test sets perform the same check automatically as part of a CT test sequence. Whatever the method, the result for each CT should be recorded against the drawings, and any reversal found must be corrected — either by correcting the wiring or by formally documenting a deliberate reversal that the protection design accounts for. An undocumented reversal is a trap for the next engineer who works on the scheme.
CT burden test
Burden is the total load connected to a CT secondary: the relays, meters and transducers, plus the resistance of the wiring and connections, expressed in volt-amperes at rated secondary current. A CT only maintains its stated accuracy while the connected burden stays within its rated burden. Exceed the rated burden and the CT's errors grow, its saturation behaviour worsens, and protection performance degrades — often silently, because nothing trips or alarms to tell you the burden is too high.
The burden test measures the actual connected burden, usually by injecting rated secondary current into the complete secondary circuit and measuring the voltage drop, from which the burden in volt-amperes is calculated. The measured burden is then compared with the CT's rated burden and with the design value. Common findings include wiring longer than designed, extra devices added to a circuit over the years without a burden recalculation, and high-resistance connections. Where the burden exceeds the rating, the fixes are practical: shorten or upsize wiring, remove unnecessary devices from the circuit, or replace the CT with a higher-burden unit. Burden should be rechecked whenever devices are added to a CT circuit, not just at commissioning.
CT saturation and knee point test
The saturation test, often called the excitation or magnetisation test, measures how the CT core behaves as secondary voltage rises. The test applies a gradually increasing AC voltage to the secondary winding, with the primary open, and records the excitation current at each voltage step. Plotted as a curve — the excitation curve — the result shows a region where current rises slowly with voltage, followed by a knee where the current begins to rise much faster. That knee is the knee point voltage, and it marks the onset of core saturation.
The knee point matters because it sets how much fault current the CT can reproduce faithfully. During a fault, the secondary voltage the CT must develop is proportional to the fault current multiplied by the burden. If that voltage exceeds the knee point, the core saturates, the secondary current waveform distorts and collapses, and the relay sees less current than the fault is really producing. Protection CTs are therefore specified with a knee point high enough for the maximum fault current and the connected burden, and the saturation test proves that the installed CT meets that specification. A knee point well below the expected value can indicate a wrong tap, a damaged core, or a CT that does not match its nameplate, and any of those must be resolved before energisation. The test also gives a useful by-product: the shape of the excitation curve itself, which can reveal shorted turns or core damage when compared with sister CTs or factory data.
PT testing: ratio, polarity and burden
Potential transformers are tested on the same principles, adapted to a voltage device. The ratio test applies a known primary voltage — or, more commonly on site, verifies the ratio by secondary injection and by checking the nameplate and wiring — and confirms that the secondary voltage matches the expected value. Polarity is checked by the same reasoning as for CTs: the secondary voltage must be in phase with the primary voltage as the markings indicate, because directional and synchronising functions depend on it. Burden is checked by measuring the connected load on each secondary winding against the PT's rated burden, since an overloaded PT loses accuracy and can overheat.
PT testing has its own practical points. PT secondary circuits are normally earthed at one point, and that earthing must be verified, because a lost earth changes the voltage relationships the protection sees. PT fuses and miniature circuit breakers in the secondary circuits must be checked, since a blown fuse silently removes voltage from a whole protection scheme. And the secondary wiring must be checked against the drawings just as carefully as for CTs: a swapped phase on a PT circuit produces exactly the kind of confusing, intermittent protection behaviour that wastes days of fault-finding after energisation. Where a PT has multiple secondary windings — for metering, protection and residual voltage — each winding must be tested and its burden checked separately.
Insulation and winding resistance checks
Completing the instrument transformer test set are the insulation and winding checks. Insulation resistance is measured between windings and earth and between primary and secondary, using a suitable insulation tester and following the same discipline as the insulation resistance test for power transformers: clean, dry terminals, correct test voltage for the winding, and discharge afterwards. Low insulation resistance on an instrument transformer is taken just as seriously as on a power transformer, because it indicates moisture, contamination or damaged insulation that will only worsen in service.
Winding resistance on CT and PT secondaries is a quick, useful check that the winding is continuous and that no connection is high-resistance, and it provides a baseline for future comparison. On CTs, it also feeds into burden calculations, since the winding's own resistance is part of the total burden the core must drive. Record all results — ratio, polarity, burden, knee point, insulation resistance and winding resistance — in one test record per instrument transformer, cross-referenced to the protection scheme drawings, so that any future investigation starts from known-good values rather than guesswork.
Reporting and commissioning sign-off
A useful instrument transformer test report contains, for each CT and PT: the identification and location, the nameplate data, the tap tested, the ratio result, the polarity result, the measured burden against the rated burden, the knee point and excitation curve for protection CTs, the insulation resistance results, and any deviations found and corrected. Deviations must be explicit, not buried: a corrected polarity reversal, a burden that exceeded the rating and was fixed by rewiring, or a knee point that did not meet specification and led to a CT replacement are exactly the findings the report exists to record.
For engineers building a commissioning programme, the practical routine is simple: test every instrument transformer before energisation, prove ratio and polarity against the drawings, prove burden and knee point against the protection design, and keep the records with the scheme. Combined with the power transformer tests in our transformer testing and commissioning guide, with mechanical condition checked by SFRA and insulation health by tan delta and partial discharge testing, and with ratings verified using our free transformer calculators, instrument transformer testing closes the loop: it proves that the eyes and ears of the protection system — the CTs and PTs — can be trusted before the system is asked to rely on them.
Frequently asked questions
Can CT ratio be tested without primary injection?
Yes, to a degree. Secondary injection ratio test sets can verify the turns ratio, and the excitation curve gives strong evidence of core condition. But primary injection remains the most complete proof because it tests the CT, its tap and its connections in the real operating configuration, and it is preferred wherever the test current can practically be provided.
What happens if CT burden is too high?
Accuracy degrades and saturation occurs at lower fault currents than designed. Relays may operate slowly or fail to operate on faults, and metering reads low. The remedy is to reduce the burden — shorter or larger wiring, fewer devices on the circuit — or to fit a CT with a higher rated burden.
Do PTs need a knee point test?
Not in the same way as protection CTs. PTs operate at essentially constant voltage, so saturation behaviour under fault current is not the design driver it is for CTs. PT testing focuses on ratio, polarity, burden, insulation and secondary circuit integrity instead.
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