Primary injection is the strongest practical proof that current entering an MV panel reaches the correct CT core, polarity, test block, relay algorithm, trip output, DC circuit and breaker. It closes gaps that secondary injection cannot—but its available current is usually far below fault level, so it does not prove CT saturation or breaker interruption capability.
This guide explains primary-injection and end-to-end scheme testing for MV switchgear FAT/SAT. It covers objectives, source sizing, current paths, CT safety, phase/polarity mapping, residual/differential schemes, pickup strategies when current is limited, complete trip timing, breaker operation, SCADA/SOE, IEC 61850 interfaces, risk, records and restoration.
Executive conclusions
- Draw the exact primary current loop; injection through the wrong return path can bypass the CT or create a false residual current.
- Measure injected current with a traceable reference and compare CT secondary plus relay primary/secondary displays.
- Test every phase and every protection core/path needed by the scheme; one phase cannot prove phase mapping.
- Keep CT secondaries closed/shorted safely. Never change test-block wiring while primary current flows.
- Primary injection proves installed transformation/wiring/polarity and end-to-end response at the test current; it does not reproduce high-fault CT saturation, arc energy or breaker short-circuit interruption.
- If the source cannot reach the actual pickup, use an approved scaled-setting strategy or combine ratio/polarity proof with secondary injection—then restore and independently verify settings.
- Separate CT/relay operate time, output/lockout delay, breaker opening time and total end-to-end time.
- Control test-lead heating and electromagnetic forces; required source VA increases rapidly with current and loop impedance.
- Verify final breaker, feedback, target, alarm, SOE, disturbance record and remote indication—not only relay pickup.
- Remove all primary links, temporary CT shorts, setting changes, blocks and test modes before release.
1. What primary injection proves
| Can prove | Cannot prove alone |
|---|---|
| Actual primary conductor passes through correct CT/CBCT | CT accuracy/saturation at full short-circuit current |
| Ratio, polarity, phase/core and installed secondary path | All relay characteristic points if current range is limited |
| Relay scaling, logic, output and breaker trip path | Breaker short-circuit making/breaking performance |
| End-to-end time at the tested magnitude/condition | Power-system current-zero/arcing time |
| SCADA/SOE/disturbance and feedback chain | Every communication/failure scenario unless tested |
2. Test boundary options
- CT window/conductor injection: strongest CT polarity/ratio proof; may bypass breaker/bus joints.
- Panel bus-to-cable path: includes primary disconnects, breaker and CT but requires suitable loop terminals.
- Cable-compartment loop: practical factory/site method through feeder CT/CBCT.
- Multiple-panel/bus differential loop: proves zone CTs and differential current direction.
- Low-current CMC primary check: can prove wiring/plausibility where source capacity suffices.
- CT-analyser plus secondary injection: alternative composite evidence when a safe primary loop cannot be formed.
Mark the source, outgoing conductor, CT orientation, return conductor, earths and measurement point on a single-line diagram. State whether the breaker is closed and which primary contacts/joints are included.
3. Source sizing and loop impedance
Required source voltage is approximately:
Vsource ≈ Itest × Zloop
and apparent power for a mainly resistive loop:
S ≈ Itest² × Zloop
- Use short, large-section injection leads and low-resistance bolted/clamped contacts.
- Include source leads, breaker/links and return path in impedance estimate.
- Check source compliance voltage at target current.
- Control current ramp and duration to avoid heating.
- Rate leads, clamps, test adapter and return bar for RMS current/time.
- Secure conductors against electrodynamic movement.
- Measure actual current; do not rely only on setpoint.
- Allow cooling between high-current tests and inspect joints.
OMICRON’s CPC 100 official information describes output up to 800 A (and higher with boosters) for primary injection; CMC 500 documentation describes up to 450 A in certain commissioning configurations. These are equipment examples, not universal required test currents.
Perform a low-current continuity/polarity trial before increasing current. It can reveal a wrong loop, open link or reversed clamp without exposing test adapters to the full planned thermal and mechanical duty.
4. Safety and preparation
- Switchgear de-energised, isolated, proved dead and earthed under approved programme.
- All backfeeds, generators, cables, VTs and station interconnections controlled.
- Test loop physically segregated and rated; enclosure/protective earth intact.
- CT circuits complete; unused cores safely shorted/earthed as designed.
- VT circuits isolated from unintended induced/injected voltage.
- Breaker remote/local operation and stored energy controlled.
- Trip outputs to adjacent equipment blocked under a signed matrix.
- Test area fenced; high current and moving-breaker hazards communicated.
- Source output zero/off before lead changes.
- Thermal cooldown and emergency stop defined.
5. Configuration baseline
- CT schedule/core/tap/polarity/earth and shorting-test block;
- relay settings/logic/I/O/active group and checksums;
- trip matrix, breaker coil/channel and lockout logic;
- SCL/GOOSE/SCADA alarm/SOE configuration;
- test pickup strategy and temporary settings if any;
- source/reference/test-template calibration;
- breaker operation count and rated auxiliary voltage;
- cause-effect expected results and timing boundaries.
6. Phase-by-phase ratio and mapping check
- Form the L1 primary loop through the actual CT direction.
- Inject a stable current high enough for accurate reference/relay measurement.
- Record primary reference, CT secondary/test-block current and relay display.
- Verify expected ratio, sign/angle and only L1 channel response.
- Verify relay primary-value scaling and SCADA metering.
- Reduce current to zero and prove safe before moving leads.
- Repeat L2 and L3 with identical path/current.
- Investigate any residual/negative-sequence quantity during single/balanced tests.
Ratio comparison uses K = Ip/Is, with accuracy interpreted under the selected low-current method. Primary injection at a small fraction of rated current is a wiring/ratio plausibility check, not a full IEC CT accuracy calibration.
7. Polarity and direction
- Use a signed phase/current reference or controlled multi-phase injection.
- Verify P1/P2 orientation relative to bus/cable or protected zone.
- Check directional overcurrent/earth-fault forward and reverse logic with voltage reference where needed.
- For differential, inject through current that enters/leaves the protected zone and confirm restraint/near-zero differential.
- Reverse/relocate the loop to simulate internal-zone current and verify operate direction where safe.
- Record relay phasors/differential/restraint, not just trip/no trip.
8. Residual and core-balance CT tests
- For phase-CT residual summation, inject equal balanced currents and verify low residual.
- Inject one phase and verify calculated/residual channel magnitude and polarity.
- For CBCT, pass test conductor through the core in the intended direction and return outside the core to create residual current.
- For a zero-residual check, pass outgoing and return conductors through the core so ampere-turns cancel.
- Verify cable screen/earth conductor routing does not defeat residual measurement.
- Test relay pickup/trip and alarm path at appropriate current.
9. When source current cannot reach pickup
High CT ratio or high-set protection may require primary current beyond portable source capability. Options must be approved and transparently combined:
- prove CT ratio/polarity/wiring by feasible primary current;
- prove actual protection pickup/timing by secondary injection at final settings;
- temporarily reduce pickup in a controlled test setting group to obtain an end-to-end primary trip;
- calculate scaling and retain margin from source/instrument uncertainty;
- restore final settings, read back/checksum and repeat a final secondary sanity point;
- never report the reduced-setting trip as proof of actual high-current pickup.
A temporary setting change is a configuration-controlled exception with approval and restoration—not an informal way to make the test set sufficient.
10. End-to-end trip test
- Set breaker closed/charged, correct DC supply and trip path healthy.
- Arm source current, relay element/output, coil voltage/current, main contacts and SOE capture.
- Inject from below pickup/no-trip state to the approved operate quantity.
- Record CT/relay measurement and element pickup.
- Verify logic/output/86/interposing contact and correct trip coil.
- Verify breaker opens and injected current falls/opens safely.
- Confirm 52a/52b, target, lockout, alarm, SCADA and disturbance record.
- Verify adjacent breakers/outputs did not operate.
- Return source to zero, discharge/earth and restore breaker state.
11. Timing boundaries
Total measured end-to-end time can be decomposed:
tE2E = tCT/measurement + trelay + tlogic/output + tinterposing/86 + tbreaker-open
- Define start: current threshold crossing, source command or steady fault application.
- Define stop: relay output, coil voltage, contact separation or injected-current interruption.
- Use a common time base/synchronised channels.
- Do not call contact separation total fault clearing; no short-circuit arcing/current zero is reproduced.
- Compare each component to its own declared condition and the protection-study total where applicable.
- Record test-current multiple because inverse-time relay delay depends on it.
12. Differential, busbar and REF schemes
- Identify all zone CTs, disconnect points and check zones.
- Inject through-current pairs to prove stability and polarity.
- Inject one zone-end current to create internal differential operate.
- Verify isolator/CT selection and dynamic zone logic from actual position contacts/GOOSE.
- For REF, prove phase/neutral CT polarity and internal/external fault response.
- Verify correct trip matrix: all breakers bounding the faulted zone, no healthy-zone breaker.
- Test breaker-failure backup/retrip as a controlled scheme sequence.
- Record differential/restraint phasors and zone membership.
13. IEC 61850 and remote interfaces
- Where the relay trips by GOOSE, keep the real publisher/subscriber path in the end-to-end test.
- Record GOOSE dataset/quality, relay logic, output and breaker response.
- Verify test/simulation flags do not operate non-test equipment.
- Check SCADA primary current, protection target, breaker status and SOE order.
- Verify time synchronisation and timestamp source.
- Capture disturbance records with correct CT ratio/phase and trigger.
- Test loss/degraded communication separately; primary injection alone does not cover it.
14. Source and test-lead thermal control
- Calculate/record duty cycle and maximum on-time.
- Monitor lead, clamp, test adapter and breaker joint temperature.
- Stop on rising resistance, smoke, insulation softening or abnormal vibration.
- Keep high-current leads separated/secured according to test-set guidance.
- Allow cooldown between phases.
- Inspect contact surfaces after test and restore protective covers.
- Do not use switchgear earth bar as return unless it is the intended/rated test path.
15. Failure analysis
| Finding | Checks |
|---|---|
| Low relay current | Primary reference/path, CT ratio/tap, secondary burden/open/high-resistance link |
| Wrong phase/sign | CT orientation, secondary terminals, test loop and relay mapping |
| Relay operates, breaker does not | Output block, trip matrix, 86/interposing, DC/coil/TCS |
| Residual during balanced injection | Current balance, polarity, return routing, unequal CT/phase path |
| Source cannot reach current | Loop impedance, lead size/contact, source compliance, breaker/path state |
| Unexpected adjacent trip | Shared trip bus, GOOSE subscription, output mapping/zone logic |
16. Report and restoration
- single-line current-loop diagram and included/excluded equipment;
- panel/breaker/CT/relay serials, core/tap and settings checksum;
- source/reference/test equipment/calibration and lead arrangement;
- injected primary current/duration and CT secondary/relay phasors;
- pickup strategy and any temporary setting with approval;
- relay/output/coil/breaker timing boundaries;
- breaker, target, alarm, SCADA/SOE and disturbance result;
- NCR/change/retest/regression;
- temporary primary links removed and covers refitted;
- CT shorts/test blocks/earths restored;
- final settings/group/blocks/test modes read back and signed.
Common mistakes
- Injecting through a return path that bypasses the CT.
- Testing one phase and copying results.
- Changing CT links while current flows.
- Claiming full CT accuracy/saturation proof at low current.
- Reducing relay setting without controlled restoration.
- Stopping at relay trip indication.
- Calling contact-separation time fault-clearing time.
- Overheating test leads/clamps.
- Using earth bar unintentionally as current return.
- Leaving CT shorts/output blocks/test mode active.
Official standards and primary references
- IEC 61869-1:2023 and IEC 61869-2:2012 — instrument-transformer and CT requirements.
- IEC 60255-1:2022 — current protection-equipment framework.
- IEC 62271-200:2021+AMD1:2024 — current MV switchgear assembly requirements.
- IEC 62271-100:2021+AMD1:2024 — current breaker operation requirements.
- OMICRON CPC 100 official information — primary-injection source and automated reporting example.
- OMICRON CMC 500 official information — current multi-purpose protection/primary plausibility test example.
Engineering note: State exactly what current path and pickup condition were tested. Primary injection is compelling evidence only when its limited fault-level representation is acknowledged, quantified and documented.