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Sample Size justification for IEC 60601-1 Testing

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Related Topic Tags
design validation, design verification, design verification testing (dvt - medical devices), fda requirements, iec 60601 - medical electrical equipment, sample size, samples and sampling plans
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  #1  
Old 20th April 2013, 11:35 AM
QualityVic

 
 
Posts: 3
Please Help! Sample Size justification for IEC 60601-1 Testing

My company sends one device for 60601 testing. Our "user need" is that the device must be safe and we use the 60601 testing results to validate this.

In terms of the FDA, all design verification and validation activities must include sample sizes that were determined using some statistical method (i.e. binomial sampling plan). In some guidance, we found that if a statistically significant sample size in not possible, one should provide a scientific rationale to support the number of samples tested in the test summary and protocol.

What I am asking is, how do other companies handle this? My experience is that usually one or two samples are sent for 60601 testing, which is not a statistically significant sample. Do we have to go clause by clause of the standard justifying with science why a sample size of more than one is not necessary?

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  #2  
Old 20th April 2013, 12:21 PM
QualityVic

 
 
Posts: 3
Re: Sample Size justification for IEC 60601-1 Testing

Would another approach be to remove the 60601 requirement from design validation/verification and somehow validate safety through a subset of 60601 tests which can be done in-house (dielectric strength and ground continuity)? We'd still do the 60601 testing to be certified.

Or could we say to validate the safety, the device must be certified to be 60601 compliant, where the lab requires only one or two samples be provided?
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  #3  
Old 21st April 2013, 08:23 PM
Peter Selvey

 
 
Posts: 763
Re: Sample Size justification for IEC 60601-1 Testing

It's good question, and an interesting subject ...

Yes, it's not only FDA, but in Europe the directives refer to compliance with standard for both design and production, a point which is often overlooked.

Also, IEC 60601-1:2005 uses the following definition of a type test (bold is my emphasis):

Quote:
3.135 TYPE TEST
test on a representative sample of the equipment with the objective of determining if the equipment, as designed and manufactured, can meet the requirements of this standard
From fundamentals, number of samples depends on variability of results and the margin to an applicable limit. That depends a lot on the specific design aspect being tested. So you cannot decide for the whole standard.

For example, for earth leakage current, if the manufacturer uses 1nF EMC capacitors, including capacitor tolerance (say 20% including temperature effects), and stray leakage (varies usually 20-40uA) we can expect the actual production results to be variable between 100-140uA. Since the limit is 500uA, a single test result of say 127uA proves that the design is implemented as expected, and "representative" of production given the huge margin to the limit. Additional samples are not going to increase confidence in a pass / fail result.

Many safety tests are like this, in practice there will be mild to moderate variability in the test results on each production sample, but the margin to the limits will be far higher than the variability.

But, sometimes there will be a close result, or large variability. In that case there may be a few options:
- redesign with a greater margin or reduced variability
- run several tests until you get enough data to be confident of pass/fail result
- determine that the test method/criteria is already conservative, so there is no significant risk due to production variations.

Standards are usually based on a series of worst case assumptions which are necessary for consistency but rarely occur in the real world. Also the allowable limits often include some margin to allow for production variations. In that case, worrying about whether a test result is representative of production might be overkill. For example, in the temperature rise test, we assume worst case supply voltage, cooling, operating mode, loading, operating conditions, length of loading, room ambient etc etc. If under all of those conditions a test result is marginally below the limit, production variations are unlikely to introduce any significant risk in the real world.

On the other hand, we may have a test result where a PCB track fused during a switching transformer short circuit test. Since it was an unintended part of the circuit that provided protection, it's reasonable to ask if the result is representative of production. In that case, running the test on 10 samples might be a reasonable approach.

So to summarize, it's a case by case, taking into account the test, the design, expected variability of the result, margin to the limit, and how critical the limit/test is in the real world.

Because it is built in to the standard (via the definition of a type test), this is something that the test laboratory should consider when each test is performed. They should inform the applicant if they feel a test result will not be representative of production.
Thank You to Peter Selvey for your informative Post and/or Attachment!
  #4  
Old 22nd April 2013, 03:39 AM
Pads38

 
 
Posts: 521
Re: Sample Size justification for IEC 60601-1 Testing

As 60601 is intended as a 'Type Test' it is expected that a single unit will be subject to all of the applicable tests and at the end still be safe and functional. You can see this in Annex B - Sequence of Testing where the final verification of the markings is the very last thing to be done after all rubbing/heating/poking/impacts/drops have been completed.

I usually find that the test piece is not in a condition where it could be sold. It may be scratched and dented. Remember also that the full dielectric test, held for 1 minute, can over-stress insulations without obvious damage.

If you are looking for a sub-set of the 60601 type tests to form production tests then have a look at IEC 62353 (recurrent test and test after repair). For instance, there the dielectric test voltage is the same but is only held for 5 secs. and there are alternative techniques described for the measurement of leakage currents.

The test equipment manufacturer Rigel do a handy little (free) guide to 62353.

www.rigelmedical.com

(No connections)
Thank You to Pads38 for your informative Post and/or Attachment!
  #5  
Old 22nd April 2013, 09:26 AM
QualityVic

 
 
Posts: 3
Re: Sample Size justification for IEC 60601-1 Testing

Thanks to both of you Peter and Pads. I believe Peter may have provided the rationale needed here. It sounds like I'm going to have to sift through the results and determine the margin to the limit, but this makes absolute sense!

Thank you very much both of you!
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