Auxiliary dielectric verification checks wiring insulation and separation, but modern LVCs contain electronics that cannot withstand a blanket high-voltage test. The procedure must define included circuits and justified exclusions.
Learning objectives
Classify circuits, select the applicable test and protect incompatible devices while preserving meaningful coverage.
Core engineering principles
Not every auxiliary circuit has the same insulation domain
Station DC, AC heaters, low-voltage electronics and communication ports require different treatment.
Electronics and suppression can limit the test
IEDs, power supplies, SPDs, RC networks and filters may need disconnection.
Exclusion must be documented
Removing a device does not excuse testing its field wiring; boundaries and alternative evidence should be recorded.
Temporary links can create hidden damage
Test jumpers must be controlled and removed with restoration checks.
Functional tests follow dielectric work
Connections and electronics are reconnected and verified after testing.
Engineering application method
- Step 1: Create a circuit inclusion/exclusion schedule.
- Step 2: Select voltage, duration and source from applicable requirements.
- Step 3: Disconnect incompatible devices with controlled tags.
- Step 4: Test wiring groups to earth and between circuits as required.
- Step 5: Restore and function-test all affected circuits.
Practical example
Applying a high AC voltage through a numerical relay input can destroy it. Isolating the IED while testing the terminal-to-field wiring preserves both safety and test coverage.
Common mistakes
- Testing the complete LVC indiscriminately.
- Excluding all wiring with electronics.
- Forgetting suppression devices.
- Leaving temporary earths or jumpers.
- Skipping post-restoration function checks.
Design and review checklist
- Which circuits are included?
- Why is each device excluded?
- Is wiring coverage preserved?
- Are test links controlled?
- Are functions retested afterward?
Standards basis and official sources
- IEC 62271-200:2021 + AMD1:2024 — AC metal-enclosed switchgear assemblies above 1 kV and up to 52 kV.
- IEC 61439-1:2020 — General rules for low-voltage switchgear and controlgear assemblies where applicable.
- IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.