I was under the impression that an "RCD" was the same as our "GFCI"
Effectively, yes, although as already stated our general-purpose RCDs are usually 30mA or 100mA trip rather than the 4 to 6mA trip of the American GFCI.
Current practice is to use a 30mA RCD where necessary (e.g. for receptacles used to power equipment outdoors). 100mA RCDs are still available and commonly used where RCDs have to be cascaded. A typical example would be a system where the main grounding of the installation is just to an earth rod rather than to the supply. The 100mA RCD would then be a delayed-trip type to provide protection to the entire system, with a 30mA RCD then used to provide the higher level of protection as needed.
10mA RCDs are also available, but rare.
if the genset replaces the mains supply and is wired up correctly, RCDs or GFCIs should react the same way, surely?
The key point is "wired up correctly." It's all going to depend on if and how the system is grounded.
Scenario #1. Genset frame is grounded by way of a rod, and one side of the 240V output is also bonded directly to the frame/ground. In this case the source will behave exactly like a normal (TN) supply, and the RCD will "see" any fault which exceeds its trip level.
Scenario #2. The 240V winding is not bonded at the genset (as is common on many small generators sold here), then somebody installs a bond to ground on the load
side of the RCD to reference the system to earth. The RCD is now useless, since it cannot detect an imbalance in the installation's line/neutral current.
Scenario #3. The 240V output is completely floating, with no bonds anywhere. Again, the RCD would be ineffective. A bolted ground fault on either side of the installation would -- effectively -- just turn the system into scenario #2 above.
The main difference between the typical RCD and a GFCI is that the first is typically passive. It doesn't contain much electronic stuff and would work at 20 VAC as well as at 230V as long as more current is passing than allowed and written on it.
I'm not sure about the RCDs usually available in Germany, but in the U.K. there are plenty of active RCDs which incorporate an amplifier to increase sensitivity to the required level.
It can be compared with "separating transformers" which we use here in Germany f.i. in tv repair shops. This issue is limited by a maximum power of some kW and a maximum length of the isolated mini-"grid". Otherwise a capacitive coupling (correct word?) could produce dangerous voltages against ground also with an isolated system.
is the right word.
Your "separating transformer" (direct translation from German?) is what we would refer to as an isolating transformer. Of course, this is exactly the principle used in our bathroom shaver outlets in Britain: The transformer is located right at the outlet so that the shaver supply is completely floating, and there is not enough wiring between the transformer secondary and the shaver for capacitive effects to become significant.
[This message has been edited by pauluk (edited 11-24-2006).]