Can a fail still be a pass? (Criteria Level B for 61000-4-6 Conducted Immunity)

qu1nn

Involved In Discussions
#1
Hello,
My company has been trying unsuccessful in passing precompliance testing conducted immunity (61000-4-6). We are prepping up for our 3rd attempt.



The product is an electronic weight scale the stated accuracy in the manual is 0.2% . The output is a LCD display showing weight.


During the conducted immunity testing the output of the scale went well outside the stated accuracy. After the test no intervention is required (the displayed weight error goes away when the test is complete) thus, the degradation of performance ceases after the disturbance ceases and there is full recovery to normal performance, without operator intervention.


Is it possible to fail the conducted immunity testing (Criteria Level A) at various frequencies but yet still be able to move forward by utilizing Criteria Level B?


where
Criteria level A: normal performance withing equipment specifications
Criteria level B: degradation or loss of function or performance which is self recoverable when the interfering signal is removed.
Criteria level C: degradation or loss of function or performance that requires system reset or operator intervention when the interfering signal is removed.


It sounds straight forward.... (thus written into the spec, thus in answering my own question .... YES)...

I am assuming that it would have to be clearly stated in the Guidance and Manufacturers Declaration contained in the manual....


Are there any pitfalls that I should watch out for if going with Criteria Level B that I am missing?


I look toward your response,


qu1nn
 
Last edited:
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Peter Selvey

Staff member
Super Moderator
#3
I guess these are medical scales, so you are looking at IEC 60601-1-2.

In theory the EMC immunity levels are intended to represent the "normal environment", so there should be no degradation.

Lets say the scales also need to be waterproof, as the intended purpose requires use while the patient is wet (for whatever reason). It would not be acceptable if the accuracy of the scales went outside of spec during the waterproof test, even if it returned to specified accuracy after being dried out.

In principle it is the same for EMC.

The difference with EMC is that in reality the type and nature of electrical noise is so wide and varied, so the tests try to simulate this with a wide range of frequencies, amplitude modulation and direction of the EM field.

So there is two broad possibilities: one is that your design is really poor and there are a lot of EM field conditions that cause problems. In this case, it really is hard to justify Criteria B. If during the whole tests the scales showed an error, it makes no sense. They are basically useless in the real world.

Another possibility is that it just happens that a particular frequency, modulation or field direction that resonates with your sensor circuit and causes an error result. In this case, Criteria B can make sense if the severity is low and probability is low. Good design would be to at least look into the problem and try to understand the cause and effect. For example, it may be that some sensor types unavoidably show errors with certain EM field conditions, but these conditions are relatively rare.

Simply taking a fail result, and automatically switching from Criteria A to Criteria B would not fit with the fundamentals of risk management.
 
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