In the recent ElectroIndustry article [September 2008 Issue, "International Standards: A Look Back to the Future"] there is a highlight of an important distinction between GFCIs and RCDs. However the explanation – "…each technology can ultimately comply with the basic requirements in both North American and IEC product standards,…" – is not entirely accurate. Sticking with the basic differential current threshold, we have the 30 mA vs. 5 mA debate which is ostensibly based on Let-Go vs. fibrillation. While this seems to be true and fit nicely, the 30 mA IEC level was not arrived decided upon based on the Fibrillation criterion, it was selected because the current-driven electro-mechanical relay needed at least 30 mA to operate and 30 mA protection was considered better than nothing. In the USA, the requirement is for tripping at 6 mA.
Simply, when non-electronic RCDs were being developed in Germany, the most sensitive solenoid coil needed at least 30 mA to operate. At that time, they could not wind any finer wire to accommodate lower operating currents. This is the sole reason why 30 mA became the de-facto current limit in IEC standards. The fibrillation threshold was a nice afterthought that added to make it seem plausible. Remember that the ground fault current is all there is to energize the solenoid directly so that the electric shock current is interrupted.
It is true that in extreme low voltage circumstances a (voltage dependant, electronic) GFCI may not operate. There are trade-offs. No one ever qualified 30 mA fibrillation in babies!
While it is easy to raise the threshold on a GFCI, it is extremely difficult, if not impossible, to lower the threshold on an RCDs below 30 mA (a design constraint). Simply, an electro-mechanical relay driven RCD cannot be made sensitive enough to trip at 5 mA.
Theoretically, with a 30 mA RCD, a person can be energized at 25 mA, for example, indefinitely without tripping the device. This is an extremely painful situation and should be considered a safety issue to allow for a Technical Barrier to Trade Agreement argument. If someone thinks having 30 mA course through your body is safe, I would like to see how long they can tolerate it before passing out from the pain or stop breathing. In addition, electric shock levels well below 30 mA can cause either a startle reaction or tetanus of muscles. The startle reaction could cause someone to fall off of a ladder or have a similar accident. If the tetanus of muscles were the result of the current flow through the body, a person could not remove themselves from the source of current and could ultimately suffer injury up to and including death from asphyxiation.
The following website has information about the effects of different levels of electric shock:
Additional research can be discovered by reviewing the work of Charles F. Dalziel, Dr. Gottfried Biegelmeier, William B. Kouwenhoven and even a UL President – Baron Whitaker.