Separation requirement for Class II equipment with functional earth

[Roland chung]

Hi folks,

I have a question about the Class II equipment with functional earth. It is easy to think that functional earth does not provide MOP at location where large fault current (e.g. short circuit) may occur. The standard therefore requires two MOPs between internal screens, including internal wiring connected to them and ACCESSIBLE PARTS.

But I am a little bit confused on the wording for insulation the standard used which mentioned above. I can at least think of 3 scenes:

1) The internal screen functionally earthed is not separated from MAINS, then two MOPs required from internal screen to accessible part. (In line with standard requirement)

2) The internal screen functionally earthed is separated from MAINS by one MOP, then an additional one MOP from internal screen to accessible part is sufficient.

3) The internal screen functionally earthed is separated from MAINS by two MOPs, then no insulation required between internal screen and accessible part.

Any other opinions?

Thanks,
Roland

 

[raymond02302]

From the meaning of wording of standards, opinion 3 should be right. It is only such condition for equipment also with acceessible functional terminial. For other equipment without accessible functional terminal, I can't foresee any hazard that will arise for equipment structured as per opinin 1 and 2.

BTW, this structure is not allowed for medical equipment used in home healthcare environment.

 

[Roland chung]

That is my question. Why does the standard stipulate one situation only?

 

[Peter Selvey]

I haven't checked the standard on this one, but I think the term "functional earth" basically assumes it's part which is accessible, i.e. (e.g.) an externally accessible terminal for the user may optionally use to connect a ground wire to for noise reduction.

I know for Class II equipment, it may get confusing, because the design may use the 3rd pin of the power supply cord as the FE terminal, which means the user may not be able to directly touch it. But the original concept was for a terminal separate to the power cord, like you see on many sensitive instruments.

Does this make sense?

 

[Roland chung]

You may be right regarding the original concept. We could find some clues from Figure 2 to Figure 4 in IEC 60601-1:2005. All the functional earth described in the Figures are exposed.

Let's look back to the question, the standard says the insulation between internal screens, including internal wiring connected to them and ACCESSIBLE PARTS, shall provide two MEANS OF PROTECTION. According to the requirement, the earth conductor of power supply cord shall be used only as the functional earth connection to a FUNCTIONAL EARTH TERMINAL for these screens. That means the internal screens are functionally earthed.

There is obviousely no insulation between internal screens and accessible parts (functional earth), since they are connected directly. I am totally confused.

 

[Dimitri2]

I see it as follows: your functional earth and all other ASSESSIBLE parts must be 2 MOOPs away from the MAINS as well as from any other power entry point.

Would you please explain what your "internal screen" really ist?

 

[Roland chung]

The "internal screen" is from standard. It seems that Figure 2 of IEC 60601-1:2005 gives us a hint.

 

[Dimitri2]

Figure 2 does not show any insulation of FE from the MAINS. It just explains the terms.
At Fig. 3 you can see that between MAINS and FE there are at least 2 MOPPs whereas a screen between windings is connected to PROTECTIVE EARTH CONDUCTOR.

 

[Roland chung]

I was talking about the internal screen, not the insulation diagram. The dotted line border in Figure 2 could be the screen.

 

[Dimitri2]

so this one must have 2 MOPPs against MAINS.