Hi Hans.
You wrote:
< I had read once (long ago) that fribilation would occur starting at 7
or so mA in a statistical significant portion of the populace. To hear
30ma is surprising!>
I'm not a Doctor, so I have to go to learned tomes for my data. This time
I used the IEC document "Effects of Electric currents on human beings", the
IEC number which I don't have to hand, but I can find it if you want it.
This is a morbid, but very enlightening document which goes into
significant detail about currents, times, heart phases, heating, voltages,
etc. etc. A torturers almanac no less. The forward describes how they got
the data for publication. Bleaahh!
The same data appears as a reference chart in the UK Machinery Guarding
standard, BS5304. In each case the current which can cause the onset of
Fibrillation is given as c. 30mA at 50Hz.
<I have several GFI outlets in my house and never
had nuisance tripping. Even with highly inductive loads I cannot see
how this will cause an imbalance of current unless there is a fault
path. Nuisance tripping can only occur if you indeed have an alternate
return path, lossy insulators, spark-overs etc.>
In the UK we tend to have only one non-electronic RCCD in our houses, and
this is usually on the main incomer to the house. Spur circuits to garages
and garden equipment should be connected by an RCCD, but this is not a
legal requirement.
The nuisance problem is primarily due to h.f. oscilatory currents in mains
leads etc. coupling to earth and providing sufficient imbalance to the
summing coil to cause a trip. As the UK RCCD's are non-electronic, they
are not frequency selective! Nowadays they have copper slugs/shading
pieces and stuff to slow down the response for a cycle of mains, this seems
to have pretty well stopped nuisance tripping. We have electronic RCCD
trips which are plug-in devices through which we power lawn mowers etc. The
one I took apart (it started to smoke!) had an RC circuit of about 100mS to
prevent nuisance tripping, it still tripped when used with long leads
sometimes.
Chris Dupres.
Surrey UK.
