Dielectric Voltage - The dielectric (insulation of Potentiometer "guts" to the body) rating is especially important if the Potentiometer is connected to mains operated, non-isolated equipment. Wall mounted lamp dimmers and such are typical examples. This is not commonly specified, but for safety, should be at least 2.5kV. A common way to achieve this is to use a plastic shaft, with the body of the Potentiometer insulated from the chassis, and inaccessible by the user - even if the knob falls off or is removed! This point cannot be stressed highly enough.

Most standard Potentiometer will safely withstand (maybe) 100V or so between the resistance element and terminals, and the body and shaft. Miniature types will usually be less than this. Never, ever, use a standard Potentiometer with a metal shaft to control direct mains operated equipment. Even if the Potentiometer case is earthed, the voltage rating between the internal element(s) and terminals to the case is often unspecified, and is almost always completely unsuited to mains voltages.