So, here's the shocker (literally): the aluminum case of electrolytic capacitors is not isolated from the circuit.
Normally, this does not present a hazard, because the caps' negative leads are connected to ground, thus to the chassis, thus to earth, and there's no danger to you if you touch the cap. (The exception is in reverse-biased bias circuits, where the positive lead is connected to ground).
However, when you use PAIRS of radial caps in series (very common for B+ reservoirs), the B+ voltage is split evenly across the two caps, and half the voltage is present on the body of the cap furthest from ground.
This information (that the cap body is not isolated) does not appear in typical data sheets. I did find mention of this in a Nichicon document: http://www.nichicon.co.jp/english/produ ... uminum.pdf
2.1.1(7) Aluminum electrolytic capacitors must be electrically isolated as follows:
The aluminum case and the cathode foil are connected by the unstable resistance of a naturally formed oxide layer inside the aluminum case and the electrolyte.
2.1.1(8)Outer sleeve of the capacitor is not guaranteed as an electrical insulator. Do not use a standard sleeve on a capacitor in applications that require the electrical insulation. When the application requires special insulation, please contact our sales office for details.
So, any time you see something like this, with pairs of caps with exposed aluminum bodies...BEWARE!