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IEC 60243-2: Dielectric Strength DC Testing — Why DC Is Fundamentally Different From AC
Why DC Testing Can Be More Severe Than AC — Space Charge Effects Change the Breakdown Physics
IEC 60243-2:2013 specifies DC electric strength test methods for insulating materials. DC testing is not a simple substitute for AC — the physics is fundamentally different.
| Parameter | AC | DC | Reason for Difference |
|---|---|---|---|
| Breakdown vs. Ramp Rate | Little variation | Lower ramp rate → lower breakdown | Space charge accumulation time |
| Polarity Effect | None | ±30–50% difference | Space charge distribution asymmetry in non-uniform fields |
| Defect Sensitivity | Sensitive to voids | Sensitive to impurities and traps | DC charges accumulate in traps over hours |
Space charge effects explained: Under DC fields, charges are trapped in microscopic traps within the insulation — impurities, grain boundaries, or polymer chain defects. Accumulated space charge creates local field enhancement — while the macroscopic field is still far below breakdown strength, the local field may already exceed the critical value. This is why DC breakdown voltage decreases with decreasing ramp rate — slower ramping allows more charge accumulation — the opposite of AC behavior.
DC ramp rate selection:
Short-time test: 100/200/500 V/s (quick material screening)
20-second test: direct application to target (simulates real conditions)
Step-stress test: hold each level 1 min to hours (space charge research)TN Lab — DC dielectric testing is not AC testing simplified. It is fundamentally different physics.
