Australian trip currents for GFI/RCD

[daddles]

This is a just-curious question for the down-under folks: why do your RCDs trip at 30 mA? This was stated in this thread.

It's usually agreed that a person can't let go of a conductor at around 10 mA. I was supposing that half that value is what the US manufacturers chose for their GFI trip points (the ones I've measured have all tripped at 4.5 to 5 mA). (A similar argument is used for the typical AC leakage current spec of 500 uA, but instead the 1 mA level is the generally-agreed sensibility threshold for humans.) Now, of course, if a GFI/RCD trips in 40 ms or so, the current level probably isn't terribly important. I was just curious why the Aussies chose 30 mA. Anyone know? Or is it one of those bureaucratic things designed by committee so that no one is responsible?

 

[davenn]

dont know why that level is set,
but this doc from Western Australia State Govt says 30mA.

A snippet from the doc......

"Residual Current Devices
Test 1: Residual non operating current -
A current between 40% and 50% of the rated tripping current should be
passed between active and earth for 5 seconds. The R.C.D. should not
trip.
Test 2: Tripping current and time test -
A current equal to the rated tripping current of the R.C.D. should be
passed between active and earth. The R.C.D. should trip within its
specified time - usually 30 milli-seconds.
Note:
The recommended tripping current for RCD’s is 30 milli-amps. Test instruments are readily
available from electrical wholesalers. In addition to the above tests the inbuilt test facility
should be operated each time that the unit is in use.
Portable outlet devices require a combination of all of these tests."

Dave

 

[(*steve*)]

I seem to recall that the RCD devices built into houses allow a relatively high current due to the high leakage present in some older devices, particularly refrigerators.

This leaves the possibility of getting just enough of a shock to kill you, but not quite enough to trip the breaker.

Considering the higher voltage, and the relatively low resistance of a human once you've overcome the dry resistance of the skin, you would have to be very unlucky to achieve this.

I guess you have to set a limit that is low enough to provide protection, yet not so low as to get people so annoyed by false triggering that they disable them.

 

[daddles]

Leakage currents and capacitive loads were the things that came to my mind because of the nuisance trips. Frankly, as a technical person, I like the 30 mA design much better than the US's 5 mA design. As long as the GFI/RCD trips in the mandated time, most users are probably still protected pretty well.

I was interested in the rationale behind the design trade-offs. To reduce the nuisance trips, set the current level higher. But this then likely is in tension with a desire to keep the current low to avoid physiological problems when a human gets shocked. I wonder if there are some clinical data that justify picking the 30 mA point. At around 100 mA is where a human can start having ventricular fibrillation. An obvious question is whether a 100 mA shock for 40 ms or so can still lead to this arrhythmia. If so, then some committee made a design decision to limit things to about 1/3 of this value.

Just curious...