Double-insulated (class 2) mains equipment

A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth.
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation. The IEEE regs say that one of the layers can be an air gap and so as long as the chassis is predominantly made of plastic it's not hard to get class 2 certification for a piece of equipment (you'll see the sign on your DVD and probably even your TV). Now then, read the article linked (above) - it's possible to get residual voltage on the screens of video and audio cables, but once connected to other equipment (particularly broadcast VTRs which are class 1 - proper mains earth) you'll not even be able to measure them as they have no current carrying capacity.
Today I had to go to a facility where folks were getting shocks off the audio jackfields and at the edit desk monitor power supplies were blowing. I traced the fault to the DVD recorder AND the VHS machine - both were dumping around 90 volts onto the screen of the signal connectors.
I thought i was missing something fundamental - my first action was to PAT test all of the bay and desk mains and it all checked out. In the end it really was those two domestic machines. I got another DVD machine off the client which didn't have the problem and so I can only assume that some event in the past has compromised the internal isolation.