A rule-of-thumb for determining whether additional encapsulation is required is the 10KV/inch isolation in air rule, which assumes relatively low humidity and clean air (no particulates on the diode). For example, using the 10kV/inch rule for the 1N6517 diode (5kV, 1A, 70ns), based on body size and distance between leads, it could operate up to 2900V in clean, dry air. Above 2900V, or in less clean conditions, the diode should be encapsulated or operated in oil or some other dielectric fluid.
Many factors can effect whether the 10KV/inch rule is effective in applications. Additional factors include humidity levels, thickness of the glass passivation, body length between leads, ambient temperatures, vacuum levels, and power levels.
Generally speaking, additional isolation should be provided for diodes operating a levels of 5KV or higher.