I discovered in my new house that someone had incorrectly connected grounded outlets (without any ground connected) to the knob and tube wiring. I have read that the outlets can be made safe by adding a GFCI upstream.
However, electrical code requires that any ungrounded GFCI be labeled "No Equipment Ground." Why is this? What consequences as a homeowner do I need to be aware of to the lack of ground connection? Are there certain devices I shouldn't use on this circuit?
Some surge suppressors dump surges to the grounding conductor, some audio video equipment requires a grounding conductor, some "smart" devices trickle current to the grounding conductor, etc.
It also notifies folks doing work in this box in the future, that they shouldn't expect to connect the grounding conductor. The label is also applied to all other outlets supplied by the GFCI, since it may not be obvious in remote boxes what's going on.
GFCIs are a poor substiture for grounding. Proper grounding should disconnect a fault to the case pretty much immediately. If the case is not grounded A GFCI will only disconnect a fault to the case after your electric shock starts (but hoepfully before the sockhas persisted long enough to kill you)
It's better to have a missing ground and a GFCI than to have a missing ground with no GFCI. I assume that your safety regulators put these provisions in as a carrot to try and encourage people to add GFCIs and to discourage people from cutting earth pins off appliances.
A particular nasty situation can be if you have multiple items in touching distance some of which are connected to ground (either through electrical ground connectors or otherwise) and some of which have missing ground connections.
Some surge suppressors do indeed transmit some energy to the EGC -- however, this is not a major concern for typical units as MOVs absorb energy in addition to shunting it.
A/V (or other) equipment in metal chassis is not a concern as well -- it is the local equipotential bonding of the chassis that provides protection from EM and RF noise as the EGC is electrically "long" at any significant frequency.
This leaves the issue of devices that either inadvertently or intentionally leak excess current to the EGC -- Tester101 mentioned some "smart" home automation equipment that uses the EGC instead of a neutral return. There are also some pieces of test equipment that leak current to ground as an unavoidable consequence of their normal functioning. The usual sign that an incompatibility with the GFCI protection is present is simply that the GFCI trips (either immediately, or when some poor sod pokes the device) -- either that, or a device that leaks operating current to ground will not function without a way for the current to get back home, of course.
(Of course, Tester's concerns about communicating the situation to the next poor sap who has to work on your house wiring are very important as well.)
