Transformer Bushing C1 and C2: Test Report Review
Review bushing C1 and C2 capacitance and dielectric loss against factory records, test conditions and model-specific instructions.
Selection note: use this guide to compare technical requirements. Confirm catalog ranges, typical values and standard clauses against the model, edition and agreed test conditions; project acceptance follows the approved drawings and technical agreement.
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In short
Compare C1 and C2 readings with the same bushing’s factory record and test history under comparable conditions.
- C1 and C2 describe different parts of a capacitance-graded bushing.
- A single reading is not a complete diagnosis or a clearance to energize.
Capacitance and dielectric loss reports help buyers and maintenance engineers review bushing condition. Start with the exact unit, reference values and measurement conditions before comparing results.
What do C1 and C2 measure?
For a capacitance-graded bushing with a suitable tap, C1 generally represents main insulation between the conductor and tap. C2 generally represents tap-side insulation towards the grounded flange and surrounding materials.
Construction and available measurements vary by model. A bushing without a suitable accessible tap requires a different assessment; do not assume separate C1 and C2 readings are available.
How should test results be compared?
Compare with the same unit’s factory or nameplate values and previous measurements. Record the method, units, frequency, test voltage, temperature and installation condition.
Capacitance changes may indicate a change in the graded structure, but test connections and configuration can also affect results. Increased dielectric loss may have several causes, including moisture or contamination. Investigate unexpected trends with the manufacturer or a qualified test engineer.
Power factor and dissipation factor (tan delta) are related but not identical. Confirm the reported quantity, units and temperature correction before comparing values.
| Item | Review |
|---|---|
| C1 capacitance | Compare nameplate, factory and prior values for the same unit. |
| C1 dielectric loss | Check quantity, temperature and correction method. |
| C2 and tap | Confirm tap type, applicable measurement and manufacturer instructions. |
Why does the test tap need special care?
Tap types and permissible conditions differ by design. An incorrectly handled or ungrounded tap can create a serious hazard and damage the bushing.
Only qualified personnel should test the equipment using the specific manufacturer’s instructions and approved site procedures. Never use a test voltage or connection from an unrelated model as a universal rule.
Buyer prompt: match each report to the offered bushing and its factory baseline.
What should buyers request?
Request the complete model, serial number, drawing revision, manufacturing entity and applicable test reports. Confirm whether a test or voltage tap is supplied and request available factory C1 and C2 reference values.
Agree the standard, edition, tests and acceptance documents in the contract. For replacement, verify the equipment interfaces and installation constraints separately; a matching voltage class does not establish interchangeability.
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Conclusion
Review each reading against the correct bushing, reference and test conditions before making an engineering decision.
Discuss your specificationFrequently Asked Questions
Does increased C1 always mean a shorted layer?
+
No. Review the baseline, setup and repeatability with a qualified engineer before diagnosing the cause.
Is there one pass or fail limit for every bushing?
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No universal limit is proposed here. Apply the contract, relevant standard and manufacturer guidance to the specific unit.
Source documents
- [1]1: Transformer bushings
Megger
Supports: Overview of C1 and C2 insulation and diagnostic measurements.
- [2]
- [3]3: Checklist to Reduce Bushing Testing Issues
Hubbell
Supports: Test tap handling and model-specific instructions.
Editorial guidance. Confirm the bushing model and test records with the manufacturer.
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