A current transformer that fails to deliver an accurate scaled replica of the primary current causes every connected device that depends on it to respond incorrectly — protective relays that do not operate when they should, or that operate when they should not, and revenue meters that measure the wrong value. CT testing verifies that ratio, polarity, saturation characteristics, and insulation condition are within specification, and identifies problems before they cause a protection failure or billing error.
Current transformers are passive devices with no moving parts and no routine maintenance interval the way mechanical equipment has. This makes them easy to overlook. A CT can remain in service for decades, and when it eventually develops a problem — a degraded secondary winding, a grounding issue, a wrong ratio after reuse — the problem is invisible during normal operation until a fault or audit exposes it.
CT testing is performed at several points in the equipment lifecycle: during commissioning of new switchgear, after any event that may have subjected the CT to abnormal current (fault, short-circuit), as part of a protection system audit, after a CT has been removed and reinstalled, and periodically on critical feeders where protection and metering accuracy are both essential.
A correctly specified CT that is tested at installation and verified at intervals provides a solid foundation for the entire protection and metering scheme it supports. A CT that has never been tested carries an unknown reliability.
Ratio test. The ratio test verifies that the CT is delivering the correct scaled output. A CT rated 600:5, for example, should produce exactly 5 A secondary current for every 600 A flowing in the primary. In practice, no CT is perfect — there is a small ratio error due to magnetizing current and winding resistance, and IEEE C57.13 defines the acceptable ratio error by accuracy class (0.3, 0.6, or 1.2% for metering classes; broader limits for protection classes).
The ratio test is performed by injecting a known low-voltage AC signal into the primary winding and measuring the resulting secondary voltage, or by using a dedicated CT analyzer that applies a test current. The measured ratio is compared to the nameplate ratio and to the accuracy class limits. A significantly wrong ratio — outside the accuracy class limit or substantially different from the nameplate — indicates a wrong tap selected on a multi-ratio CT, a shorted secondary turn, or a damaged winding.
Polarity test. Current transformer polarity determines the phase relationship between primary and secondary current, which directly affects differential protection and directional relay operation. A CT with reversed polarity feeds the protection relay a secondary current that is 180 degrees out of phase with the primary current. A differential relay connected to a reversed CT will fail to operate on an internal fault, or will operate on a through-fault, depending on the scheme.
Polarity is verified by passing a DC pulse through the primary and observing the direction of the secondary response — a positive primary current impulse should produce a positive secondary response at the H1/X1 terminal pair per the standard marking convention. The test takes seconds and eliminates polarity uncertainty for the protection scheme the CT serves.
The excitation test, also called the magnetization or saturation curve test, is the most diagnostic test for assessing the core condition and usability of a CT in a protection application. With the primary open, an increasing AC voltage is applied to the secondary and the resulting magnetizing current is measured at each voltage step. The result is a curve of excitation current versus applied voltage — essentially the B-H curve of the CT core, expressed in terminal quantities.
The knee point of the excitation curve is the critical value for protection applications. The knee point is the voltage at which the excitation current begins to increase rapidly as the core approaches saturation — roughly where a 10% increase in voltage requires a 50% increase in excitation current. A CT in a protection application must have a knee point voltage that is high enough to prevent saturation during fault conditions. If the CT saturates during a fault, the secondary current collapses and the protective relay does not see the fault current correctly.
What an abnormal excitation curve reveals. A curve that is shifted significantly below the nameplate excitation curve for that CT type, or one with a low knee point, indicates core degradation — often the result of a severe through-fault that drove the core hard into saturation repeatedly, or mechanical damage to the lamination stack. Shorted turns in the secondary winding will also change the excitation curve by reducing the apparent secondary inductance, though shorted turns are more visible in ratio testing as a ratio error.
Comparing the field excitation curve to the manufacturer's published excitation curve for that CT type — or to a previously tested baseline on the same unit — establishes whether the CT is performing to its design specification. A CT removed from one feeder and reused on another should be excitation-tested before reinstallation, since its history in the original application may not be known.
Burden is the impedance connected to the CT secondary — the wiring resistance, relay coil impedance, and any other secondary load. Burden matters because a CT is a current source driving a load impedance, and a higher burden requires a higher secondary voltage to drive the rated secondary current. If the burden is high enough that the required secondary voltage exceeds the CT's knee point voltage, the CT will saturate before delivering the rated current.
Burden measurement verifies that the actual connected secondary burden does not exceed the CT's rated burden. The total burden includes the relay input impedance, the lead wire resistance (which increases with wire length and decreases with conductor cross-section), and any series-connected devices such as ammeters or test blocks. Lead resistance is a common source of burden violations on long secondary runs between switchgear and a remote relay panel.
When the measured burden exceeds the rated burden, the options are to increase the secondary wiring conductor size, relocate the metering or relay equipment closer to the CT, or replace the CT with a unit that has a higher burden rating and a proportionally higher knee point voltage. Accepting an overloaded CT secondary circuit without correction means the CT will saturate during fault conditions.
Insulation resistance is measured between the secondary winding and the CT case (ground) and between the primary and secondary windings on units where the primary is accessible. The test uses a DC megohmmeter — typically at 500 V DC for low-voltage secondary circuits and at 1000 V DC or higher for the primary insulation, following the test voltage guidelines for the CT's voltage class.
A low insulation resistance reading on the secondary circuit indicates contamination, moisture ingress, or physical damage to the secondary winding insulation. The minimum acceptable value varies by equipment type and age, but a secondary insulation resistance below 1 megohm on a relatively modern CT is a red flag. Trending the value over time is more informative than any single reading — a CT that measured 500 megohms on the previous test and now reads 5 megohms has developed a significant insulation problem even though 5 megohms is technically above some minimum thresholds.
Grounding check. CT secondary circuits should be grounded at one point — and only one point — typically at the switchgear assembly or at the relay panel, depending on the utility's practice. Multiple grounds on a CT secondary circuit create a circulating current path that introduces measurement errors and can cause interference with protective relay operation. The grounding configuration should be verified and documented during testing.
Many CTs in distribution switchgear have multiple secondary taps that allow the ratio to be changed by reconnecting secondary terminal jumpers. This is useful for adapting to load growth or changed feeder configurations, but it introduces an error mode: the wrong tap is selected, either at installation or after a tap change that was not completed correctly.
A ratio test will immediately reveal a wrong tap by producing a ratio that does not match the relay or meter setting. When testing multi-ratio CTs, the test should be performed at the tap that is actually connected in service — not at all possible taps — and the result compared to the relay setting documentation for that circuit. A discrepancy between the measured ratio and the relay setting is a critical finding that requires reconciliation before the equipment is returned to service.
Note that unused taps on a multi-ratio CT secondary should not be left open-circuited — they should be shorted or terminated per the manufacturer's guidelines. An open-circuited secondary on an energized CT can develop dangerous high voltages at the open terminal, creating a safety hazard and potentially damaging the winding insulation.
CT test results should be documented with the test date, test equipment used, ambient conditions, CT nameplate data (manufacturer, serial number, ratio, accuracy class, burden rating), the connected burden measured, the measured ratio and ratio error, polarity result, the excitation curve data and identified knee point voltage, and insulation resistance values. This record becomes the baseline for comparison on all future tests and provides evidence of condition at the time of commissioning or maintenance.
Acceptance criteria vary by standard and application. IEEE C57.13 sets accuracy class limits for ratio error. NEMA and manufacturer guidelines establish minimum insulation resistance values. The knee point voltage requirement for protection applications is derived from the protection system design — specifically from the fault current magnitude, CT ratio, and total secondary burden — and should be specified by the protection engineer for each application rather than applied as a universal number.
We test CTs in the field — ratio, polarity, excitation curves, burden, and insulation resistance — on distribution switchgear, pad-mounted gear, and substation assemblies across Florida and the Southeast. Tell us what you have and we’ll put together a testing scope.