Humidity Pre-Conditioning before leakage current and dielectric strength tests

Pads38

Trusted Information Resource
#1
Our (small) company does our own 60601-1 testing in-house. For the last couple of years our Notified Body has asked us to have our Test Report expert reviewed. This we have done by the test house arm of the NB. (It costs - but much less than an external test).

Up until now, when it comes to the requirement for humidity pre-conditioning (before leakage current and dielectric strength tests) I have marked it as N/A, stating that "there are no hygroscopic parts; no natural rubbers or sintered insulations and it is not for use in atmospheres where condensation may occur".
And this has been accepted.

However, our expert reviewer has now decided that that is not a strong enough argument. Has anyone got any useful "extra ammunition" in this argument?

He does seem to suggest a possibilty of using 'robust risk management' to justify the N/A status, but any suggestions would be most welcome.
 
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Peter Selvey

Staff member
Moderator
#3
There are a few interesting issues here ...

First on the direct subject: For a long time I have used the rationale of "non-hygroscopic materials" to skip the RH test, without any problems, and other agencies do as well. The key point is that the dielectric strength test is a test of solid insulating materials (not along the surface, but through the insulation). As long as the materials (wiring, capacitors, optos, transformer insulation etc) are well established the humidity test is not important. The test is important for new or novel materials, impregnated paper and for powdered insulation such as sealed heaters. Some agencies assume the dielectric strength test is intended for the finished product, but this is a mistake, it is a material test.

While the humidity test may also be similarly skipped for leakage current, it may be important to check the leakage current when the equipment is cold. Materials, and in particular EMC capacitors can have a high temperature co-efficient. Changes in leakage of 10% between hot/cold conditions are not unusual, with the maximum often being in the cold condition.

On a technical point, I would use a verdict of "Pass" rather than N/A. The requirement is applicable, it is just the test itself has been waived. This would be the same as, for example, a stability test on a device shaped like a old VCR - the center of gravity is so low that the test is a waste of time, reasonably waived, but the verdict is still Pass.

On the subject of the "expert review" - I think this is not a bad idea, but some care is needed to define what it really means. Does the reviewer's report form part of the technical file? If so, does the reviewer take responsibility for the results in the report(s)? Or does the review form part of the audit (and hence end up in the audit report)?

Mostly likely the review is just a side service using an "all care, no responsibility" model, which is why it is cheaper. If so, the auditors should ignore this report, they should only refer to the report that is in the technical file.

It means you can take the expert review as feedback, but you don't have to follow all of their recommendations. The person that signs the report that ends up in the technical file has the final say.

If the review is part of the audit (which it could be, but there are procedural requirements the NB must follow to do this), then in effect the auditor is challenging your engineering judgement. This is also fine, but the rules state that the auditor must produce "objective evidence" for a non-conformity. In other words, if you have followed the procedures correctly, and written up a plausible rationale for waiving the test, then they need the ammunition, not you. "Objective evidence" is more than a feeling that it "... is not a strong enough argument", particular in a case like this where waiving a test is common practice.

:2cents:
 

Pads38

Trusted Information Resource
#4
Thanks for your reply Peter.

The reason for the report review is that the NB auditors have decided that they, as primarily quality system people, do not have the technical knowledge to pass judgement upon the adequacy of our type test reports. Instead, they ask an engineer from their test laboratory to review and make the decision.

As for what goes in the Tech File - at the moment we are limited to just adding a print out of the e-mails. I have wondered whether we should get the test lab. engineer to countersign the report. Although that has logistical challenges as the lab is in another country.

As for the need to do the conditioning - It is difficult to argue against the Test Lab's position as they state that they never waive the test. And they keep on bringing up condensation as being an issue, even though the test is clearly designed so that condensation cannot occur.

I expect that we will be stuck with doing some testing just to prove our point, and fortunately, I have identified a couple of local places that have the equipment to be able to do this. When we have proven that it makes no difference we may have more success with our arguments, provided we don't introduce any "unusual materials / components.

I take your point about marking it as "Pass" rather than "N/A". Perhaps with my own testing in place that will make the most sense, such that only other contrary "objective evidence" could force us to repeat the tests for other devices.
 

Kevin Mader

One of THE Original Covers!
Staff member
Admin
#5
Nice job Peter. I agree it's a Pass using mitigating rationale. I'd say that would settle a number of auditors who might see N/A as objectionable.

And any of us should require objective evidence for any allegation made by an auditor or technical reviewer. Gut feel is never enough. "In God we trust...all others bring data!" WED
 

swathi.sp

Starting to get Involved
#6
Dear folks,

We are in to radiotherapy device manufacturing and as part of mains isolation, we have been using Isolation Transformer at mains input and also it was tested as part of end system as per IEC 60601-1, 3.1 ed. All the electrical safety testing like Dielectric strength , Leakage Current , Transformer Overload and short circuit testing after Humidity Preconditioning treatment have been performed. Now we are in plan to change the transformer rating, however dielectric strength remains the same. In this scenario, do we need to perform all the electrical safety testing related to Isolation Transformer from the test lab? Or can we test internally. If in that case, is that mandate to get tranformer subject to humidity preconditioning.

Can any one , please throw some light on this.
 

Peter Selvey

Staff member
Moderator
#7
There's no strict rule for a design change (such as this), it's just engineering judgement. In principle, if a test is not done, you need to find a qualified person willing to sign their life away (= take responsibility) and say that the tests on the old design are representative the new design. So it's not a case of a mandatory rule to test or not, but rather finding the person who will be held responsible and tying them down to a decision.

Personally, I would expect to be OK with a spot check dielectric and skip the humidity preconditioning, but I would want to formally review the detailed documents such as drawings, specifications, material list, description of changes etc (which is not what this forum is for ;)).

Another engineer might say better just to do all the tests again. Which is fine - it just depends on what the individual is willing to take responsibility for.
 

Benjamin Weber

Trusted Information Resource
#8
Dear folks,

We are in to radiotherapy device manufacturing and as part of mains isolation, we have been using Isolation Transformer at mains input and also it was tested as part of end system as per IEC 60601-1, 3.1 ed. All the electrical safety testing like Dielectric strength , Leakage Current , Transformer Overload and short circuit testing after Humidity Preconditioning treatment have been performed. Now we are in plan to change the transformer rating, however dielectric strength remains the same. In this scenario, do we need to perform all the electrical safety testing related to Isolation Transformer from the test lab? Or can we test internally. If in that case, is that mandate to get tranformer subject to humidity preconditioning.

Can any one , please throw some light on this.
I as a tester highly recommend to retest the transformer incl. humidity preconditioning. But in my opinion it is sufficient to test the transformer separately, not the whole system.

In the unlikely case of an incident, you don't want to explain yourself based on a pure engineering judgement.
 
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