Who expects datacom problems to come from "Ground"? Of course "the last place you are looking"! The reason why many GPD problems stay hidden in a system.

GPD is seldom recognized or tested, often it's solved changing equipment, changing wall outlets. Many assume the distance does not support the communication, but the real reason is often that at less distance the chance working with the same Ground Potential is bigger. 

GPD comes from improper grounding, or equipment leaking to ground. Ground Potential in the same room is most often the same (this is even true having a bad ground), but the chance that you have GPD increases with the distance between terminals. 

Your data communication works well if S.G. is connected to F.G. only in one place in your system, or if all equipment is fed through the same wall outlet. A GPD of already 2 volt does change to unbalanced data communication (RS232) considerably. A signal of +/- 6 Volt with 2 volt GPD is seen by the receiver as an signal +4/-8 Volt. Modern balanced data communication circuitry works at differences up to +/- 7 Volt and protected better against small GPDs. 

You would say, "never had this problem", but watch out: many Electricity Boards accept GPDs UP TO 50 VOLT (a maximum of 24V should make a Ground Fault breaker cut the mains supply). More equipment is destroyed by GPD than by Lightning, anyway, with bad grounding a strike will take your equipment for sure (Surge Projectors need good ground).

GPD failures often show after many months perfect communication.

Most important:  TEST GPD, BEFORE IT TESTS YOU.

1. Test for GPD if more than one wall outlet is used in the system. 
2. Watch out: systems are often tested before delivery! GPD tests have to be done in the ready installation at customer, re-test GPD moving or adding devices.

What about a PC connected to an outlet without ground, or a broken ground?
The RFI filter uses de-coupling capacitors between the hot wire, the neutral wire, and Ground. Ground has a potential of 1/2 the mains voltage (at low current, but it tickles your fingers), what if S.G. and F.G. are connected together in this PC?

For more information about GPD check following websites:

• Dangers and problems of ungrounded PC!  http://www.hut.fi/~then/mytexts/ungrounded_pc.html
• The How's and Why's of isolated grounding! http://www.powerquality.com/art0043/art1.html
• Why grounding is used? http://www.hut.fi/Misc/Electronics/docs/groundloop/why_grounding.html
• Ground loop basics!                    http://www.hut.fi/Misc/Electronics/docs/groundloop/basics.html
• Ground loop problems and how to get rid of them. http://www.hut.fi/Misc/Electronics/docs/groundloop/index.html