What’s wrong with Appendix B of AS/NZS3551:2012
In a word, LOTS!
Firstly all of the performance specifications have been left out from the 2004 version. The accuracy tolerance for the Ground Wire Resistance Test is missing along with the voltage requirements for Insulation Resistance Testing and the regulation required for Mains Contact Current Testing.
CONTACT CURRENT TESTING:
Contact Current Testing has been added but it is flawed. Applying Ohm’s Law one can see that you have a 1Kohm Measuring Device in parallel with 0.3Ohm medical device return earth and to get 100uA fail, 100mV would have to be on the protectively earthed case but this would generate 333mA current flow through the medical device being tested. This then increases to 1.5A when the neutral is opened.
To really add insult to injury, the last Contact Current Test opens the earth line so you now have exactly the same test as the Earth Leakage Test but under Contact Current the fail level is 500uA, (same as earth leakage fail level in 2004), and later under Earth Leakage Tests the fail level is 5000uA. How you can do an identical test and have two different fail levels, a factor of 10 times different in the same standard has everyone mystified.
APPLIED PART TESTING:
Applied Part Current Testing & Mains Contact Current Testing made perfect sense as API was current sourced from the medical device & MCI was current sinking into the medical device.
To open the neutral and earth with API testing also made sense because you are presenting the applied parts with the only return path via the measuring device to ground, what is commonly referred to as the worst case condition.
Similarly, the worst case condition for MCI testing is having all of the mains conductors present as this gives the applied parts the maximum pathways for the current to sink into the medical device.
So adding for MCI testing a Neutral Open Test and an Earth Open Test is a useless waste of testing time.
Then of course we have to keep up with fashion and change the names of the tests. It took me ages to change my thinking in the 1980s to calling what were referred to as Patient Circuits to Applied Parts.
Now they want to call API testing, ‘Patient Leakage Current’ and are you ready for this, MCI testing they want to call ‘Patient Leakage Current – Mains on Applied Parts’.
I cannot believe that our smart engineers and scientists have disappeared and we have to follow IEC like sheep.
For a more detailed review of what I see as problems with AS/NZS3551:2012 along with my recommendations to fix them, email me and request a copy.
The paper I will send you was the one I delivered at the SMBE (NSW) Training Workshop at Mudgee NSW, March 2013.
Kevin E. O’Brien
MEMIE A’ASIA Pty Ltd