It is about these capacitors that the users often ask. Clearly, if your computer’s case isn’t grounded, it will have half the voltage of the power grid, i.e. 110 volts, due to these capacitors. You would feel the current tickling you if you grasped any grounded item (for example, the heating radiator) with one hand and touched the computer’s case with the other hand. The capacitance of these capacitors is very small, however, so the maximum flowing current is negligible and is not dangerous to you in any way. It is somewhat dangerous to various peripheral devices. For example, if you don’t disconnect your ungrounded computer from the wall outlet before attaching an LPT printer to it, those 110 volts may find themselves on the signal pins of the printer’s LPT connector, resulting in damage to the LPT port of the printer or computer. You don’t have to ground everything up, though. It would suffice to have the cases of all devices properly connected electrically, for example through attaching them to one surge protector with three-wire sockets – the devices will be connected through the “ground” wire of the sockets, and you won’t be running the risk of damaging any port. Nothing also threatens “hot-plug” ports (like FireWire or USB) as the pins in these ports are designed in such a way as to lock the “ground” first.
Another question is about the probability of a disruption of one of these capacitors – if this happens, the full 220 volts will be on the computer’s case. I can calm you down here: such circuits make use of special high-voltage self-repair Y-class capacitors with double insulation that are specifically intended for circuits where a disruption of a capacitor is not allowable for safety considerations.
The only situation where it would be really necessary to ground the computer is when your computer creates noise that affects the surrounding equipment (an FM receiver or a TV-set), since, as I mentioned above, common-mode interference cannot be fully avoided without grounding. External surge protectors are helpless here – their schematic is fully analogous to the above-described one, so they don’t work without grounding, either. If you’ve got three-wire electric wiring in your house (with a ground wire), you should just use appropriate power cords, but if you’ve got two-wire wiring, you should contact qualified electricians. Manual grounding is not only unsafe (for example, an egregious mistake is often committed by inexperienced users who attach the computer’s “ground” to the neutral wire in the socket, which is absolutely unacceptable), but may not bring the effect expected since grounding should have as low as possible resistance to be efficient at suppressing electromagnetic interference.
Near the power filter in the PSU there’s usually a safety fuse and varistors (nonlinear resistors whose resistance diminishes abruptly if the threshold voltage is exceeded) attached in parallel to the capacitors Cy. There’s a common misunderstanding about this fuse: some people claim it protects the PSU from breaking down. That’s not true. The safety fuse of a switch-mode power supply melts only after the switching transistors of this PSU fail. In other words, it protects the power grid from the consequences of the PSU’s failure, rather than otherwise. There’s a widespread misunderstanding about the varistors, too. Some people think they can protect the PSU if the voltage in the power grid is high above the norm. That’s again not true since the varistors can only consume rather short-term voltage surges that are provoked by a nearby stroke of lightning, for example. And if you need protection against long-term voltage surges that can occur as air wires close (which is typical for rural areas) or as a mistake of electricians (which are humans and hence can err), you should consider specialized devices for which the manufacturer explicitly declares such protection, for example APC Line-R regulators and others of that kind. Once again – a computer power supply doesn’t have any protection against long-term voltage surges, and it just breaks down if there’s no external protection device.