Discharging is a common reason of causing electron-filament failure in EMs. Corona discharge is not encountered at 100 kV and below. However, for high voltage EMs, discharge occurs in the high-voltage-generating stack and/or in the accelerator. Actually, the most common phenomena in high-voltage TEMs is still discharge, for instance, discharging often occurs when the high voltage is turned on.

Discharge in the high voltage area in EMs should be prevented because this discharge can induce damage to the insulators, even though it also cleans the inner surface of the electron gun. To minimize this, the high voltage EMs often use multi-stage systems, with only a moderate voltage (e.g. 50-90 kV) across each stage. The earlier HT (high tension) sets had fewer stages with up to 150 kV per stage, and thus required higher vacuum in the accelerator and greater attention to cleanliness of insulators and corona shields to avoid flash-over.