Common faults during the first power-on after magnesium oxide refilling include insulation breakdown, short-circuit sparking, leakage protection tripping, abnormal temperature rise, and tube shell rupture. These are mostly caused by defects in the sintering process or improper operation.
1. Insulation Breakdown and Short-Circuit Sparking
Symptoms: Smoke, sparks, or popping sounds appear instantly upon power-on; equipment trips.
Causes: Insufficient sintering (insufficient temperature, short holding time) leads to a loose magnesium oxide layer; Uneven filling or voids create localized electric field concentration; Use of low-purity magnesium oxide powder allows impurities to form conductive paths.
Prevention: Ensure sintering temperature is maintained at 800–1000℃ for 1–2 hours; use ≥99.9% high-purity powder.
2. Leakage Current Protection Trip
Symptoms: The leakage current protection trips immediately after power-on, failing to start normally.
Causes: The component was tested or powered on before it was fully cooled, resulting in insufficient internal moisture removal; The seal was not tight, allowing moisture to seep into the insulation layer along the leads; Power was applied despite the cold insulation resistance not meeting the standard (<500 MΩ).
Risk: This indicates a serious hidden danger in the insulation system; forced operation may lead to a safety accident.
3. Abnormal Heating or Insufficient Power
Symptoms: Slow heating, temperature failing to reach the set value, and large current fluctuations.
Causes: Low filler density (<2.8 g/cm³), resulting in decreased thermal conductivity; The magnesium oxide layer is damp or carbonized, affecting heat conduction; Poor lead contact or excessively high crimping resistance (>0.03Ω).
Recommendation: Check the filler density and lead crimping quality before powering on; prioritize automated compaction processes.
4. Tube Shell Cracks or Bulging
Symptoms: Localized expansion, cracking, and even powder ejection from the tube body after energization.
Causes: Rapid cooling leading to thermal stress cracks; moisture seeping in and vaporizing at high temperatures during operation; excessive heating/cooling rates during sintering (>5℃/min), resulting in unreleased internal structural stress; microcracks or uneven wall thickness in the tube material itself, leading to insufficient strength.
Natural cooling with the furnace is mandatory; forced cooling is strictly prohibited.
5. Sudden Drop in Hot Insulation Resistance
Symptoms: Passes cold-state test, but insulation resistance drops from ≥500 MΩ to <200 MΩ after 30 minutes of operation.
Causes: Incomplete sintering; microporous structure enlargement at high temperatures leading to leakage paths; failure to pass moisture test, with residual moisture released during heating; aging or failure of the sealing adhesive, allowing continuous moisture intrusion.
These types of faults are delayed and easily overlooked, but pose a very high risk in long-term operation.
