Dangers and problems of ungrounded PC
PC equipment are ment to be connected to grounded power outlets. All office buildings have grounded outlets, but is some residential installationd don't have propwe grounding. Why it is important to have your PC properly grounded ?
PCs are typically packed to metal case with exposed metal connectors on the back. If some wire will go loose insed the power supply or some isolation breaks, the metal case can get full mains potential to it which is very dangerous. And this kind of fault would not even burn the fuse unless someone gets electrocuted. A single fail inside PC power supply RFI filter capacitor can make this happen and you will not notice this until you touch your PC. In case of such fault if the PC were grounded, the fault would just blow the fuse.
Floating voltage on the metal case
PC power supply has RFI filter which consists of capacitors and indictors. Some of those capacitors are connected from live and neutral wires to safety ground wire which is permanentley connected to the metal case of the computer.
All of the current which passes through those capacitors in the RFI filter goes to the case of the PC, which is not good thing.
Many other equipment also have this type of filters. Many items used to be fitted with a "radio frequency suppression unit" consisting of two 0.01uF capacitors in series across the mains leads, with the centre point connected to ground through a 0.005 uF capacitor.
Leakage current voltage in case
Typically when your computer is connected to ungrounde 230V AC power source you have 115V AC on the case of your computer ! This is caused by the leakage current capacitors inside the mais filter.
This voltage itself is not dangerous, because the current is limited (around 0.5 mA maximum). If you touch your computer you might feel some tickeling in your fingers when you move your hand because of the potential in the case. This can be annoyning and even little painful if you touch something which is connected to ground at the same time (not recommended for your safety).
Safety standards now impose limits on leakage current. 5 nF is the largest value used from mains to ground at 230 V. Obviously that PG would have had to be carefully bonded to ground (in europe the leakage current should be less than 0.7 mA measured through 2 kohm resistor to ground in computer (EN60950) and AV equipments, laboaratory equipments (IEC61010) can have higher leakage currents).
The 110V AC at small <0.5 mA current is not dangerous to human beings but can easily damage dedicate electronics like PC parallel or serial ports. If you connect other equipment (grounded or ungrounded) to your ungrounded PC when it is connected to mains you will risk damaging the interfaces in your PC or the equipment you are conncting. This applies to serial ports, parallel ports and even soundcard audio lines.
The only safe way to prevent unnecessary equipment damages is to connect all computer equipment you wan to connect to your PC to same grounded extension lead which is then connected to grounded electrical outlet.
Operation of peripherals can be disturbed
When every card inside PC and the peripherals are floating at 110V AC potential that can easily cause them to pick interference. Cheap audio equipment which are floating at 110V potential can easily pick up mains humming. Some modems refuse to operate reliably (or not at all) when they are connected to ungrounded PC.
Grounded metal case in the computer is very good shield against radio interferences. When the case not properly conncted to the ground, the shielding is not as good as it was designed to be. When the PC is not connected to grounded outlet, it's operation can be more easily disturbed by external radio interference and electrostatic discharges.
Leakage to the case and power switch
The European Standards require that the mains switch break both of the power wires, so the entire machine is disconnected when the power is off. The power switch switches disconnects the power from the computer electronics, so if the filter is after the switch then the power is disconnected. But in some equipments the filter can be directly built into the power input socket (combined input socket and filter) and in those cases the leake current continues to flow even if you have the switch in off position.
Gound fault interrupters and leakeage current
Ground fault interrupters interrupt the mains current in the power outlet when they detect that some current is leaking to the ground (they sense the difference in currents flowing in live nad neutral wires). Typically those Ground Fault Interrupters trigger at 30 mA leakage current (some are more sensitive) so single computer equipment does not cause problems. If you have lots of computers connected to outlet protected with ground fault interrupter then you might have problem that the Ground Fault Interrupter occasionally trips and cuts the power from your computer).
Tomi Engdahl <Tomi.Engdahl@iki.fi>