A probe cell monitors conditions within electrical power transmission and switchgear apparatus to detect degradation of stressed components. The probe cell is a hardware simulation of components of a specific unit of electrical power apparatus, including electrodes between which an electric field gradient is established. The probe cell electrodes accumulate contamination at a rate related to accumulation rate on components of working apparatus. In a typical embodiment, a probe cell installed within an enclosure shares the insulating, hydrocarbon-based immersion fluid of the apparatus. Because the probe cell can be invasively tested and results recorded as often as desired without deenergizing the actual apparatus, degradation of the apparatus can be predicted with high confidence, avoiding unnecessary maintenance as well as unexpected catastrophic failure. Potential reduction in unproductive preventive maintenance while avoiding emergency repairs can offset probe cell cost.