GFCI vs RCD trip threshold-why?

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mbrooke

mbrooke

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I don't want to hijack a GFCI thread already in progress but I still want to ask. Why are GFCIs (RCDs) outside of North America set to trip at 30ma while GFCIs in the US are set to trip at 6ma? Isn't 30ma dangerous being past the let go threshold?
 
mbrooke said:
I don't want to hijack a GFCI thread already in progress but I still want to ask.
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Thank you. :) Really

I know I am guilty of sidetracking threads too and am trying to get better about it.


Why are GFCIs (RCDs) outside of North America set to trip at 30ma while GFCIs in the US are set to trip at 6ma?
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I think it would be more surprising if they were the same.:)


Isn't 30ma dangerous being past the let go threshold?
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Our bodies did not come with a printed set of specifications, anything about the body is subjective. Consider poisons, they are ranked by the "LD50" which is the amount of an ingested substance that kills 50 percent of a test sample. Some people will die ingesting less and some will be able to ingest more.


So in this case one persons study pointed to 30mA while another persons study likely pointed at more than 100 mA.


That's my story, I have no proof it is true.
 
The IEC Publication 479 gives details of the effect of current passing through the human body and relates current flow to time. Essentially the longer the current flow and the larger the current, then the grater the probability of heart fibrillation (asphyxiation as the muscles surrounding the heart pulse at different rates to the heart) and even death.

A RCD with a trip range of 30mA greatly reduces the possibility of heart fibrillation and death.

Why NIC dictates 6mA I don’t know
 
Tony S said:
A RCD with a trip range of 30mA greatly reduces the possibility of heart fibrillation and death.

Why NEC dictates 6mA I don’t know
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Perhaps because a GFCI with a trip range of 4-6mA greatly reduces the possibility of heart fibrillation and death.
 
Tony S said:
The IEC Publication 479 gives details of the effect of current passing through the human body and relates current flow to time. Essentially the longer the current flow and the larger the current, then the grater the probability of heart fibrillation (asphyxiation as the muscles surrounding the heart pulse at different rates to the heart) and even death.

A RCD with a trip range of 30mA greatly reduces the possibility of heart fibrillation and death.

Why NIC dictates 6mA I don’t know
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Tony, is this the curve you speak of?


520px-FigF01.jpg


2000px-IEC_TS_60479-1_electric_shock_graph.svg.png


And am I correct to assume this is for a healthy adult?
 
mbrooke said:
... 6 is still better than 30ma IMO.
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Duration could play a significant role in in the mA rating. I don't know the criteria of the associated standards, but the 30mA-rated IEC devices may be required to trip substantially quicker than the 6mA-rated NEC devices.
 
Smart $ said:
Duration could play a significant role in in the mA rating. I don't know the criteria of the associated standards, but the 30mA-rated IEC devices may be required to trip substantially quicker than the 6mA-rated NEC devices.
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Anyone know the trip time of a NEC/UL listed GFCI?
 
One would think, after 1/2 century of existence in the field, that some statistical comparatives for GFCI vs RCD exist

Sadly, i'm unable to surf up any.....

~RJ~
 
Smart $ said:
Duration could play a significant role in in the mA rating. I don't know the criteria of the associated standards, but the 30mA-rated IEC devices may be required to trip substantially quicker than the 6mA-rated NEC devices.
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If someone knows more about this, please share. I think Smart money is on to something.

Does the HZ rating of the source have an effect (60 vs 50)?
 
Both 50Hz and 60Hz are right in the peak sensitivity range for causing heart rhythm disruption. I am not sure if there is much difference between them.

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Don't most places that use 30mA also use 230 volt systems? If so the same resistance to ground would result in double the ground fault current, likely increasing the number of nuisance trips.
 
don_resqcapt19 said:
Don't most places that use 30mA also use 230 volt systems? If so the same resistance to ground would result in double the ground fault current, likely increasing the number of nuisance trips.
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Correct- but it does not stop use from using 5ma on 240 volt hot tubs.
 
GoldDigger said:
Both 50Hz and 60Hz are right in the peak sensitivity range for causing heart rhythm disruption. I am not sure if there is much difference between them.

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so the inventers of AC should be shot..... why didnt they pick 400Hz, things would be so much better :blink:
 
an engineer in the 30's did a lot of research
it is pretty much the basis for shock injury
Dalziel http://www.electriciancalculators.com/dalziel/dalziel_study.pdf
look at him messing with grad student volunteers at the end of the paper lol

he came up with an approximation to fibrillation

i = body wt/sqrt t

i in mA
body wt in lbs, most use 165
t is the time you can tolerate with little chance of injury

assume a trip time of 10 Hz 0r 0.167 sec
i = 165/sqrt 0.167 = and individual could take 400 mA before fibrillation so with 30 mA you should be fine

20 Hz or 0.333 sec
mA 285
drops off quickly

working backwards
say 120 vac across a 1000 Ohm 165 lb person, 120 mA
must trip in less than 1.9 sec
 
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