CT FAT is a wiring-and-zone proof, not merely a ratio reading. The test must show that the correct physical core and tap serve the correct relay/meter, with correct P1/P2–S1/S2 polarity, continuous secondary wiring, one intentional earth point, safe shorting facilities and an acceptable connected burden.
This practical guide covers current-transformer ratio, polarity, continuity and secondary-earthing tests in MV switchgear under IEC 61869-1/2. It distinguishes primary injection from low-signal CT analysis, explains multi-core/tap and residual/CBCT cases, safe test blocks, burden, demagnetisation, differential-zone checks, records and restoration.
Executive conclusions
- Never open a CT secondary while primary current can flow; short it with a rated facility before disconnection.
- Start from the approved CT schedule: ratio/tap, core, class, burden, polarity, phase, destination and earth point.
- Verify identity and physical orientation before electrical tests; a correct ratio on the wrong core still fails the scheme.
- Use primary injection to prove the installed primary-to-relay path and phase mapping; use low-signal/equivalent-circuit analysis for detailed CT characteristics where appropriate.
- Polarity must be verified end to end, not inferred from terminal markings alone.
- DC continuity/winding resistance does not prove AC burden or ratio accuracy.
- Provide one intentional secondary earth per galvanically connected CT circuit unless the approved design explicitly differs; search for duplicate/hidden earths.
- Test shorting links in the safe operating sequence: short first, then isolate; reconnect before removing the short.
- Demagnetise after tests that can leave remanence, particularly DC resistance/excitation procedures.
- Record actual primary/secondary quantities, phase angle/polarity, wiring path, earth state and as-left links.
1. CT nameplate and schedule baseline
- manufacturer/type/serial and IEC 61869 compliance;
- rated primary and secondary current;
- multi-ratio taps and corresponding terminal pairs;
- core number/function: metering, protection, differential, REF, backup;
- accuracy class, rated burden, accuracy limit/safety factor and any transient class;
- rated frequency and insulation data;
- P1/P2 and S1/S2 terminal markings/orientation;
- secondary winding resistance/reference data where declared;
- physical phase and installation direction;
- approved relay/meter destination and single earth point.
Photograph the nameplate and primary orientation before compartments close. Reconcile full nameplate data with the CT schedule and relay settings; do not accept only the ratio.
2. Safety: the non-negotiable rule
A CT behaves approximately as a current source. Opening its secondary while primary current flows can create hazardous voltage, insulation stress, arcing and remanence. Controls include:
- de-energise/isolate/earth primary equipment for factory tests;
- use rated CT shorting/test blocks and an approved sequence;
- apply the short at the source side before opening the relay/meter side;
- never rely on a loose jumper or ordinary disconnect terminal;
- verify current is zero before changing connections;
- earth test equipment and manage induced/return voltage;
- discharge/demagnetise after excitation or DC tests;
- restore covers, links and terminal screws with independent check.
3. What each test proves
| Test | Proves | Does not prove alone |
|---|---|---|
| Visual/identity | Correct CT/core/tap/orientation/terminal | Electrical ratio or wiring continuity |
| Continuity/DC resistance | Closed winding/path and gross connection quality | AC burden, class or saturation |
| Ratio | Transformation ratio for tested tap/path | Correct polarity/phase destination unless included |
| Polarity/phase angle | Direction relation primary-to-secondary | Accuracy over full range |
| Burden | Connected secondary impedance/VA | CT internal excitation characteristic |
| Excitation/equivalent circuit | Knee/accuracy/saturation-related parameters | Installed end-to-end wiring/relay trip by itself |
| Primary injection | Installed CT, wiring, relay scaling and phase path | High-fault saturation unless specifically assessed |
4. Test methods: primary versus secondary-side analysis
- Primary injection: inject known current through the actual primary path and measure each secondary/IED value. Strongest proof of installed ratio, phase and wiring.
- Secondary voltage/current method: apply controlled low signal under a validated CT analyser procedure to derive ratio and equivalent parameters without high primary current.
- Polarity pulse: momentarily apply a controlled DC source and observe direction; simple but must manage remanence and instrument polarity.
- Phase-angle comparison: AC injection with reference channel proves ratio/polarity quantitatively.
- DC winding resistance: four-wire measurement per method, followed by demagnetisation where required.
OMICRON’s CT Analyzer official description states that it injects low test signals into the secondary, derives equivalent-circuit parameters, assesses ratio/phase, winding resistance, excitation and burden, and demagnetises after the test. This complements—rather than automatically replaces—installed primary injection.
5. Ratio equations and interpretation
Nominal ratio:
Kn = Ipn / Isn
Measured ratio:
Kmeas = Ip / Is
Approximate ratio deviation for commissioning comparison:
ε% = 100 × (Kmeas − Kn) / Kn
- Use IEC 61869 definitions/method for formal accuracy, including phase displacement and burden.
- Test the actual connected tap; unused terminal markings can be misleading.
- Low primary injection may be influenced by magnetising current and test-set accuracy.
- Compare all phases/identical CTs and investigate outliers.
- Verify relay CT ratio setting separately from physical CT ratio.
6. Multi-ratio and multi-core CTs
- Map every core/tap terminal to function before connection.
- Test each used ratio and identify unused taps safely.
- Never parallel taps of one winding unless explicitly designed.
- Check shorting arrangement independently for every core.
- Confirm metering/protection cores are not swapped.
- Measure resistance/ratio with other windings in the state specified by the test method.
- Record core labels at both CT and relay/test block.
OMICRON’s CT SB2 official information illustrates automated multi-ratio testing and includes separate connections for primary resistance and secondary burden. Automation reduces rewiring, but terminal mapping still requires independent engineering verification.
7. Continuity and winding resistance
- Isolate/short safely and confirm primary is de-energised.
- Use a suitable low-resistance four-wire method where quantitative winding resistance is required.
- Record winding/lead temperature and correct only by approved method.
- Measure CT winding alone and complete loop separately if possible.
- Compare phases/identical cores; locate high-resistance terminal/test-link joints.
- Control DC test current to avoid excessive magnetisation/heating.
- Demagnetise after DC measurement when required by the CT/tester procedure.
A continuity beeper can find an open path, but it cannot quantify a poor connection or demonstrate rated burden. A good DC loop resistance also does not prove polarity.
7A. Excitation, knee point and remanence boundary
Detailed excitation testing is not required merely because a ratio/polarity FAT is witnessed, but it may be needed for protection-class verification, an unknown CT, a suspected saturation problem or contractual routine/type evidence. The method must match the IEC class and protect connected equipment:
- isolate the selected winding and short/protect other windings as the test procedure requires;
- increase secondary excitation voltage/current in a controlled way with the primary open/de-energised;
- record the full curve and determine parameters using the applicable IEC definition—not a generic visual knee;
- do not exceed winding/test-set thermal or voltage limits;
- compare with nameplate/design and identical phases/cores;
- demagnetise using a controlled decaying alternating method after excitation or DC resistance tests;
- verify all secondary links, earths and relay circuits after reconnection.
Remanent flux can reduce transient saturation margin and alter later tests. A successful ratio check after magnetising the core is not proof that it was left in an acceptable magnetic state.
8. Polarity test
- Confirm approved P1-to-P2 primary direction and S1/S2 reference.
- Connect a traceable primary/current or controlled pulse reference.
- Connect secondary reference instrument with known polarity.
- Apply stimulus and observe signed current/voltage or phase angle.
- Repeat every phase/core/tap used.
- Trace polarity through test block, terminal, relay input and star point.
- Label/correct only under approved drawing change.
- Demagnetise after DC pulse/excitation if required.
For differential/REF/directional protection, one reversed core can cause false differential current or reverse direction. Test scheme polarity end to end with expected phasors, not only at the CT terminal box.
9. Secondary-earthing verification
- Locate the single intentional earth point on the approved schematic.
- Verify conductor/terminal/PE identification and secure connection.
- Disconnect the designated earth under de-energised safe conditions and test for unintended earths.
- Check relay internal/chassis earth, test equipment, cable screens, terminal bridges and external marshalling.
- Restore the intended earth before injection/energisation.
- Do not combine protective earth, CT star-point earth and screen earth without design review.
- For segregated cores, verify whether each galvanically isolated circuit has its own defined earth.
Two earth points can circulate current and distort protection; no earth can allow hazardous secondary potential during a primary insulation fault. Record both intended location and duplicate-earth search method.
10. Shorting/test-block sequence
- Verify CT primary current is zero for FAT setup.
- Operate the test facility through its documented sequence.
- Confirm CT side is shorted before relay side opens.
- Verify visual/positive indication of short and isolation.
- Connect injection equipment to the intended relay side.
- After testing, reconnect relay side before removing CT short.
- Restore cover/seal and verify continuity/normal relay current path.
Test the sequence mechanically/electrically. A bridge drawn on a schematic but omitted in the delivered terminal block is a severe commissioning hazard.
11. Secondary burden
For an approximately resistive burden at rated secondary current:
Sburden ≈ Is² × |Zburden|
- Include relay/meter inputs, both lead conductors, terminals/test blocks and shared circuits.
- Measure AC impedance/VA/power factor under the approved method; DC resistance is not full burden.
- Use maximum design loop length/temperature for calculation.
- Compare operating burden with CT class/rated burden and saturation study.
- Check 1 A versus 5 A secondary selection—burden rises with current squared for the same resistance.
- Do not leave temporary meters/test leads as permanent burden.
12. Residual connections and CBCTs
- For Holmgreen residual summation, verify all phase CT ratios/polarities and star-point wiring.
- Inject balanced three-phase current; residual input should remain near expected zero within method limits.
- Inject one phase and verify residual magnitude/direction.
- For core-balance CT, pass all phase conductors through in correct direction; route earth/screen conductors according to the installation design.
- Verify CBCT ratio/polarity and relay scaling.
- Check no primary return conductor bypasses or incorrectly passes through the core.
13. Primary-injection end-to-end check
- Inject each phase through the actual primary conductor/CT window.
- Measure source current with traceable reference.
- Read CT secondary/test-block and relay metering simultaneously.
- Verify phase name, magnitude, angle/direction and correct IED input.
- Operate protection/logic/trip where within FAT scope.
- Test lowest practical current that gives adequate accuracy; state test limitations.
- Use OMICRON CPC 100 or equivalent only within its output/safety method; CPC 100 official data identifies CT/VT and primary injection applications.
14. Differential and REF zone checks
- Map CT location/orientation to the protected-zone boundary.
- Verify phase rotation and relay vector/ratio compensation.
- Inject through-current and confirm restraint/low differential current.
- Inject internal-zone equivalent and verify operate direction/path.
- Check neutral CT/residual connection polarity for REF.
- Verify no shared CT earth or test link bypasses the intended circuit.
- Record relay phasors/differential/restraint quantities and final trip.
15. Test record and restoration
- CT type/serial/phase/core/tap/nameplate data;
- physical orientation and terminal photographs;
- test method/equipment/software/calibration;
- primary/secondary magnitude, ratio error and phase/polarity;
- winding/loop resistance, temperature and burden;
- intended earth and duplicate-earth result;
- shorting/test-block sequence result;
- relay settings/meter reading and end-to-end path;
- demagnetisation completed;
- all links, covers, earths and relay circuits restored with sign-off.
Common mistakes
- Opening a secondary before shorting.
- Testing ratio but not core/function/phase identity.
- Assuming nameplate polarity equals installed scheme polarity.
- Using DC continuity as burden evidence.
- Leaving two secondary earth points.
- Testing only one multi-ratio tap/core.
- Forgetting demagnetisation after DC/excitation tests.
- Primary injecting one phase and copying results.
- Failing to prove test-block short-before-open sequence.
- Leaving a CT short after FAT.
Official standards and primary references
- IEC 61869-1:2023 — current general instrument-transformer requirements.
- IEC 61869-2:2012 — additional requirements for current transformers.
- IEC TR 61869-100:2017 — guidance for application of current transformers in protection.
- IEC 62271-200:2021+AMD1:2024 — current MV assembly integration/routine requirements.
- OMICRON CT Analyzer official information — low-signal CT equivalent-circuit, ratio/phase, excitation, burden and demagnetisation capabilities.
- OMICRON CT SB2 official information — multi-ratio CT connection/testing example.
- OMICRON CPC 100 official information — primary-injection and CT/VT application example.
Engineering note: Use IEC 61869 class-specific methods for formal CT accuracy. The FAT objective is also to prove the installed core, polarity, circuit and earth point that the protection scheme will actually use.