Maximum temperature during NORMAL USE

[HelviReg]

Hello,

Our product is currently being tested per IEC60601-1. The testing laboratory changed how they address section '11.1.1 Maximum temperature during NORMAL USE' and identified much more (non-accessible) parts than during our previous certifications.

According to the test operator, this identification was driven by the question "can this part heat up during product use?". Moreover, when determining the allowable maximum temperature, the ambient operating conditions from datasheets were taken into account by the lab for some parts (when nothing else would apply). It then led to several fails where parts with max operating temperature of 40°C had their surface at 42°C.

So, I got 2 questions:
- Table 22 of the standard mentions the maximum operating temperature (b). Does it refer to the ambient temperature or the surface part temperature ?
- Subclause 11.1.1 mentions that Table 22 "address limits for parts that could affect compliance of the ME EQUIPMENT with this standard in general (e.g. electrical BASIC SAFETY)". It sounds different than the test lab mindset described above. Do you have some hints on how to apply this principle when selecting the parts to be tested ?

Thank you in advance for your help on that,

Vivien J.-

 

[Peter Selvey]

Most likely "ambient operating conditions" from parts datasheets refers to use in a low air flow environmental chamber set to the specified ambient, where the part itself may have higher temperatures than the ambient in the chamber, due to self heating. If this is correct, then the test lab should be measuring the temperatures close by the part, but not the surface of the part itself.

However, note that even this is tricky.

If the end product is specified for 40°C ambient, and the part inside is also specified for 40°C ambient, then logically it would be impossible to comply if there is any heat generated inside the case of the end product, even with forced cooling. To comply it would be necessary to drop the end product's ambient to, for example, 35°C, so as to allow for some margin.

Also "ambient operating conditions" is problematic from a measurement point of view. It's fine in the context of a controlled low flow environmental chamber, but in a typical end product there may be no clean "ambient" condition to measure inside the case, air flow, other hot parts nearby would yield an uncertainty of ±5°C at least. Measuring the surface of the part as a proxy for ambient might be necessary, but even that raises the question of where to measure it, and if it is a heat source, the method of attachment greatly influences the result, and the attachment itself could interfere with heat flow. Overall, it could be a ±10°C uncertainty, so if it's fine margin to compliance, it's going to be a nightmare.

In the end, there may need to be a grain of salt taken if a part manufacturer specifies a 40°C environment. Probably, the part is OK for 50°C, but they say 40°C to allow for the uncertainty discussed above and then just hopes that nobody really thinks about it too much. Like me.

 

[HelviReg]

Hi Peter,

Thank you very much for your response. I agree with what you said, the solutions are quite limited. Any thoughts on the 11.1.1 subclause precision ?

Thanks,