Delete comment from: Ken Shirriff's blog
@ Anonymous: You get shocks from your computer because you aren't using an earthed outlet.
The EMI filter in your PC PSU includes two Y class caps (the blue ones), connected from live to earth and neutral to earth. These caps play a part in keeping the switching noise away from the power grid (contrary to what some hardware sites say, the input filter is not there to protect the PSU from disturbances, it's to keep the PSU from disturbing other devices connected to the same outlet). The only component in the input filter that really does protect the PSU is the MOV (metal-oxide varistor), which absorbs voltage spikes coming from the mains supply.
When earth ground is disconnected from the switching power supply, the Y cap connected to live brings the case of the PC to line potential. This is at very low current so it will not hurt you, but it can zap some devices in unfortunate situations. Always plug your computer into a properly earthed outlet.
May 21, 2012, 7:09:08 AM
Posted to Apple iPhone charger teardown: quality in a tiny expensive package