Auxiliary/control-circuit dielectric verification is a boundary-management test. The challenge is not merely applying an AC voltage: it is proving insulation between the correct circuit groups and earth without overstressing relays, PLCs, meters, power supplies, communication ports, surge suppressors or functional-earth paths that were never intended to receive the assembly test voltage.
This guide explains how to plan and execute dielectric verification of MV switchgear low-voltage compartments (LVCs) under IEC 62271-1/200 and relevant component standards. It covers circuit grouping, electronic-device isolation, test diagrams, AC source/measurement, safety, acceptance, restoration, functional regression and auditable records.
Executive conclusions
- Use the exact voltage, duration, circuit grouping and exceptions from the contracted IEC/product standard and approved manufacturer procedure.
- Do not apply one blanket “2 kV megger/hipot” rule to every LVC circuit.
- Classify circuits by rated voltage, insulation system, galvanic relationship and connected components before creating the test matrix.
- Disconnect or otherwise protect electronic equipment, SPDs, RC suppressors, semiconductor interfaces, communication ports and sensitive sensors exactly as their instructions require.
- Test isolation between circuits as well as circuit-to-earth where required; tying all conductors together can hide an inter-circuit wiring error.
- Use controlled links and a restoration register. A forgotten disconnected trip circuit is a greater operational hazard than a perfect dielectric report.
- Measure high voltage with a suitable calibrated system and size the source for circuit capacitance/leakage.
- Earth accessible conductive parts and maintain a fenced/interlocked high-voltage test boundary.
- After dielectric testing, restore every device/link and perform point-to-point and complete functional regression.
- Preserve failed results and distinguish product failure from an invalid test connection or intentionally connected component.
1. Scope of the auxiliary/control circuits
- DC trip, close, spring-charge and trip-circuit supervision circuits;
- AC space heaters, lighting, sockets and auxiliary supplies;
- CT and VT secondary circuits;
- relay/meter analogue voltage/current inputs;
- binary inputs/outputs and interposing relays;
- PLC/RTU/HMI, annunciation and SCADA wiring;
- communications, Ethernet, serial, fibre transceivers and time-sync ports;
- transducers, temperature/pressure/density sensors and condition monitoring;
- functional earth, screen and protective earth networks;
- external customer interface terminals and multicore cables.
These circuits do not share one insulation design. A 220 VDC trip circuit, a CT secondary, a 24 VDC opto input and an Ethernet port must not be treated as one homogeneous bundle.
2. What the test proves
| Can provide evidence of | Does not prove alone |
|---|---|
| Insulation withstand between defined circuit group and earth/other groups | Correct control logic or trip operation |
| Detection of damaged wire insulation, wrong terminal bridge or contamination | Conductor ampacity, fuse selectivity or voltage-drop adequacy |
| Manufacturing conformity after wiring/assembly | EMC immunity or communication interoperability |
| Correct restoration when followed by functional tests | Long-term insulation ageing under all environments |
Insulation resistance can be a lower-stress pre-check, but it is not automatically the specified power-frequency dielectric test. Continuity/functional tests answer different questions.
3. Establish the normative baseline
- IEC 62271-1 common auxiliary/control-circuit test requirements;
- IEC 62271-200 assembly-specific provisions;
- component product/safety standards, especially IEC 60255-27 for protection equipment;
- manufacturer declared withstand/test-disconnection instructions;
- project auxiliary voltage and insulation-coordination requirements;
- approved schematics, terminal plans, BOM and I/O/interface schedules;
- test voltage, frequency, duration, permissible exclusions and acceptance;
- requirements for repeat testing after repair or at site.
The applicable IEC edition can define different treatment for circuits or components. Do not mix values from an old factory template with a newer contracted standard. Enter every value and its clause/source on the approved test sheet before connection.
4. Build circuit groups from the schematics
| Group example | Typical included conductors | Separation review |
|---|---|---|
| G1 DC control | +DC/−DC, trip/close, interlocks, TCS | Earth, AC, CT/VT, ELV/electronics |
| G2 AC auxiliaries | heater/light/socket/charger supply | Earth, DC and low-level signals |
| G3 CT secondary | phase/residual cores, test block, relay input | Earth point, other cores and VT/DC |
| G4 VT secondary | metering/protection voltage and fusing | Earth, CT, DC/AC auxiliaries |
| G5 SELV/PELV electronics | 24/48 VDC sensors/I/O | Higher-voltage circuits and earth classification |
| G6 communications | copper Ethernet/serial/time circuits | Usually excluded/handled per device; fibre has no electrical path |
A test group is not simply “all wires of the same colour.” Trace galvanic connections through relay coils, power supplies, converter inputs, suppression components and external terminal bridges.
5. Create a dielectric test matrix
| ID | Energised group | Return/bonded groups | Isolations | Voltage/time | Acceptance |
|---|---|---|---|---|---|
| LV-D01 | DC control conductors bonded together | PE/enclosure | IED/PSU/SPD per list | From approved IEC procedure | No disruptive discharge; limits met |
| LV-D02 | AC auxiliary conductors | PE/enclosure | Lamps/heaters/electronics as specified | Approved value | Approved criterion |
| LV-D03 | DC group | AC group + PE | Cross-connected devices isolated | Approved value | Approved criterion |
| LV-D04 | CT secondary group | Other circuits/earth per procedure | Relay/test block configured safely | Component/assembly value | Approved criterion |
The matrix should prove each required insulation boundary with the minimum safe number of configurations. Too few tests miss inter-circuit defects; uncontrolled excessive tests increase restoration risk and stress.
6. Electronic devices and suppressors
| Device | Possible problem during AC withstand | Control |
|---|---|---|
| Protection relay/meter/PLC | Input insulation or internal suppression below assembly test | Unplug/isolate terminals/module per manual |
| Power supply/DC-DC converter | Input-output/earth capacitors and surge components conduct | Define which side is tested; disconnect as instructed |
| SPD/MOV/TVS/RC snubber | Clamps voltage, heats or fails | Remove/bridge only under controlled diagram; restore |
| LED/lamp/buzzer | Rectifier/resistor path alters grouping | Isolate or test component circuit separately |
| Ethernet/serial port | Transformer/shield/circuit limits; connected cable creates path | Disconnect copper cables; follow product safety data |
| Sensor/LPIT interface | Low-energy electronic insulation limitation | Exclude/use specified test plug/configuration |
| Interposing relay | Coil-contact/coil-earth boundary may be part of test | Confirm product withstand and contact positions |
Do not assume an open MCB isolates both poles or all internal suppression paths. Verify with schematics and device data.
7. Protective earth, functional earth and screens
- PE: protective earth and accessible metal normally form the test return/reference.
- FE: functional earth may connect electronics or filters to chassis and may need controlled isolation.
- Cable screen: can be bonded at one or both ends and create a test path to external equipment.
- CT/VT secondary earth: must be controlled; duplicate earths can change the circuit boundary.
- Door bonds/rails: verify protective continuity separately; do not remove safety bonds merely to improve a dielectric result.
Record every lifted functional/screen/secondary earth and restore it. Colour alone is insufficient proof of its intended role.
8. Source, measurement and leakage monitoring
- AC source with suitable voltage, frequency, waveform and kVA for circuit capacitance.
- Approved high-voltage divider/measurement chain, serial/calibration and uncertainty.
- Current limiting and trip threshold selected so safety is maintained without masking acceptable capacitive current.
- Timer starting only after required voltage is established.
- Test leads/probes with sufficient insulation and physical clearance.
- Remote control, emergency stop and safe discharge/earthing arrangement.
- Record actual voltage and observed current; do not use leakage trip setting as the sole measured result.
Capacitive current is approximately I = 2πfCV. Long wiring and connected filters can create normal charging current. Set source/protection from an engineered estimate and approved procedure, not by raising the trip threshold after nuisance trips.
9. Safety and readiness
- De-energise and isolate all normal auxiliary supplies and external cables.
- Prove circuits dead and control backfeed from station battery, UPS, chargers, VT secondaries and remote systems.
- Fence/interlock the test area; exposed LVC terminals become high voltage during the test.
- Earth enclosure/PE and secure doors/covers in the required state.
- Use a controlled isolation/link register and independent verifier.
- Warn that normal wire colours/low-voltage labels do not indicate safety during test.
- Connect earth/return first, HV last; remove HV first after discharge.
- Discharge capacitance and prove residual voltage absent.
- Restore normal safety earths, fuses, links and plugs before functional energisation.
10. Pre-test inspection
- Wiring complete and point-to-point checks performed.
- Wire type, cross-section, voltage rating and terminal condition correct.
- No loose strands, exposed conductor, damaged insulation or trapped door loom.
- Clearance/creepage and segregation match design.
- Terminal bridges/test links/fuses/MCBs in the defined state.
- Electronics/SPDs/suppressors isolated and tagged.
- External field cables disconnected or included intentionally.
- Moisture, metal swarf and contamination removed.
- Test diagram and voltage/time independently checked.
11. Step-by-step test sequence
- Select the matrix row and verify exact panel/circuit group.
- Apply/verify all isolations and bond conductors within the energised group as specified.
- Bond return groups, enclosure and PE according to the test diagram.
- Inspect source, divider, leads and safety interlocks.
- Clear/lock the area and announce test start.
- Raise AC voltage smoothly to the specified value while monitoring current.
- Start timing at established voltage; maintain/record voltage for the specified duration.
- Reduce to zero, de-energise, discharge and prove safe.
- Inspect for damage/tracking/odour and record result.
- Reconfigure under the controlled matrix and repeat.
- After all rows, restore every isolation/link/device with independent check.
- Energise at normal auxiliary voltage and run complete regression tests.
Never leave a test jumper or bonded circuit group in place while moving to another panel without a link register. The jumper itself can bypass protection or create a short circuit on normal energisation.
12. Acceptance
- Specified voltage/frequency/waveform maintained for the required duration.
- No disruptive discharge, puncture or flashover in the test object.
- Leakage/protection behaviour within the approved method’s limits.
- No smoke, carbon tracking, component damage or persistent abnormal odour.
- Post-test insulation/visual/functional checks acceptable.
- All isolated components restored and their functions proven.
- The exact product-standard wording and manufacturer exceptions govern.
A trip caused by an inadvertently connected MOV does not prove the wiring insulation failed, but it does invalidate the test and reveal a configuration-control defect. Preserve and investigate it.
13. Functional regression after dielectric test
- AC/DC branch energisation, polarity and normal current draw;
- trip and close from all local/remote/protection paths;
- anti-pumping, trip-free and trip-circuit supervision;
- breaker/earthing switch position and interlock matrix;
- CT shorting and VT isolation/test-block sequences;
- relay analogue inputs, binary I/O and output contacts;
- alarms, LEDs, annunciation, SOE and HMI text;
- SCADA commands/feedback and IEC 61850 communications;
- heater, fan, thermostat and LVC lighting/socket;
- PE/FE/screen/secondary-earth restoration;
- fuses, MCBs, terminal bridges and device plugs in as-left state.
14. Failure diagnosis
| Symptom | Possible cause | Action |
|---|---|---|
| Immediate current trip at low voltage | Connected SPD/electronic device, wrong bond, gross short | Discharge; compare isolation register and circuit |
| Flashover at terminal | Stray strand, insufficient clearance, contamination or test-lead setup | Locate marks, preserve evidence, inspect design/workmanship |
| Current rises during hold | Heating suppressor, tracking/moisture or insulation breakdown developing | Stop safely; inspect and segment |
| One panel/group fails | Local wiring/component/terminal damage | Segment without uncontrolled repeated full-voltage stress |
| Passes after device unplugged | Device was outside test boundary or device insulation problem | Verify instructions and test device separately as applicable |
- De-energise, discharge and preserve first-failure data.
- Verify source/divider/test-lead validity.
- Review matrix, schematic and actual isolation state.
- Locate/inspect without repeatedly applying full voltage.
- Open NCR and assess common production population.
- Approve repair or component disposition.
- Repeat affected dielectric row and regression tests.
- Keep failure, correction and final result linked.
15. Factory, FAT, SAT and maintenance use
- Factory routine: required production verification using controlled device isolation.
- Customer FAT: may witness selected routine tests; witness sampling does not reduce manufacturer coverage.
- SAT: site test voltage/scope must be agreed; field cables/devices greatly change boundaries.
- Maintenance: do not repeat high-potential proof tests routinely without manufacturer/asset strategy; lower-stress diagnostic IR and targeted investigation may be more appropriate.
A new field cable may be tested under a cable standard, but the connected relay/LVC must be protected from that test. Coordinate external terminal isolation and temporary earths in the site method.
16. Test report and restoration record
- panel, circuit groups and schematic/terminal revision;
- matrix row and connection diagram;
- normal rated voltages and applicable test voltage/time/source;
- all disconnected devices, plugs, SPDs, suppressors, earths and external cables;
- source/divider serials, calibration and actual voltage/current/time;
- pass/fail observations and acceptance clause;
- NCR, repair and retest history;
- signed restoration checklist;
- post-test functional regression results;
- as-left configuration and independent verifier.
Common mistakes
- Applying one test value to all circuit types.
- Leaving relays, PSUs, SPDs or communication ports connected.
- Testing only circuit-to-earth and missing circuit-to-circuit insulation.
- Assuming an open MCB isolates every pole/internal path.
- Lifting PE bonds that must remain for safety.
- Using an insulation tester when the standard requires AC withstand.
- Starting the timer before full voltage is established.
- Ignoring normal capacitive current and raising protection blindly.
- Forgetting a disconnected trip/VT/SPD/earth after testing.
- Skipping complete functional regression.
Official standards and primary references
- IEC 62271-1:2017+AMD1:2021 — current common auxiliary/control-circuit dielectric requirements.
- IEC 62271-200:2021+AMD1:2024 — current assembly requirements for metal-enclosed MV switchgear.
- IEC 60255-27:2023 — current safety requirements for protection relays/equipment and associated auxiliary devices.
- IEC 60664-1:2020 with AMD1:2025 — current low-voltage insulation-coordination principles and tests.
- IEC 60060-1:2025 — current general high-voltage test terminology and requirements where referenced.
- IEC 60060-2:2025 — current high-voltage measuring-system requirements.
Engineering note: The licensed product standards and each component’s safety instructions govern exact test values and exclusions. The most important as-left result is both dielectric conformity and proof that every protective/control function was restored.