App note from CalRamic Technologies LLC on detecting defects in high voltage capacitor by Partial discharge. Link here (PDF)
High reliability life critical systems require enhanced reliability high voltage multilayer ceramic capacitors, thereby necessitating improvements in materials, design, process control, and in-process screening procedures. Given that energy stored in a capacitor is proportional to the square on the operating voltage, these capacitors can become their own source of destructive energy if internal defects are present. Consequently critical high reliability systems mandate even greater restrictions be placed on capacitors to be defect free.