Choosing correct MOPs

EMengineer

Involved In Discussions
Hi,

I am a little confused about the requirements of the MOP.
insulation.jpg

Should the insulation between the primary and secondary circuit be 2 MOPP despite the fact that the applied part is the enclosure part made of insulation?
The secondary circuit is connected to the PE as is the enclosure. Thus applied part's only contact with secondary is through earthed enclosure.
It is unclear do 2 MOPP are really needed between primary and secondary circuits (or should be 2 MOOP?). In this case the non-medical PSU is used. Or just use medical grade PSU?
And how to determine MOP between applied part (Type B) and secondary? The applied part is integrated in the metal enclosure which is earthed so as the secondary circuit.
 

Benjamin Weber

Trusted Information Resource
In my oppinion you have to implement the following MOP:

  • 1 MOOP between L and N (see cl. 8.9.1)
  • 2 MOOP between accessible enclosure and mains voltage (1 MOOP is aachieved by PE)
  • 2 MOOP between accessible enclosure and secondary (1 MOOP is achieved by PE)
    • I hear you say: "Why? It's below 60 Vdc so it is SELV!" (btw: There is no such term as SELV defined in 60601-1)
    • According to cl. 8.4.2 c) this voltage is only safe, if the corresponding power is less than 240 VA for 1 minute (in NC and SFC). So it depneds on the maximum power output of the PSU and if this is limited to the power value even in SFC.
  • 2 MOPP between the patient connection of the applied and mains voltage
    • If the applied part is only the insulated part, PE cannot be considered as MOPP (only metal parts can be protectively earthed)
    • If the whole enclosure part at the right side can be seen as the applied part and the insulating part is the patient connection only, PE could be considered as 1 MOPP
  • 2 MOPP between the patient connection of the applied part and the secondary voltage
    • see both additional bullet points above
Regarding your question on the 2 MOPP to mains in particular:
  • You can implement these at the AC-DC-converter. But you will still need 2 MOPP regarding the secondary voltage.
  • If the insulating material is strong enough for 2 MOPP regarding mains voltage (i.e 4 kV dieelctric strength), then you could fulfill the 2 MOPP for mains and secondary at one point.
 

EMengineer

Involved In Discussions
Thank you Benjamin for clarification.
Here is the updated insulation diagram:
insulation.jpg
According your suggestion:
  • 1 MOOP between L and N (see cl. 8.9.1) - Point A
  • 2 MOOP between accessible enclosure and mains voltage (1 MOOP is achieved by PE) - Point B, in this case enclosure is of insulating material thus 2 MOOP.
  • 2 MOOP between accessible enclosure and secondary (1 MOOP is achieved by PE) -Point C
  • 2 MOPP between the patient connection of the applied and mains voltage - Point D
    • If the applied part is only the insulated part, PE cannot be considered as MOPP (only metal parts can be protectively earthed)
    • If the whole enclosure part at the right side can be seen as the applied part and the insulating part is the patient connection only, PE could be considered as 1 MOPP
All other parts are clear, here is the part where I got confused originally. I tried to apply 2 MOPP requirements for insulation in PSU between primary and secondary, but a.c. and c.d. come a little short (around 7 mm). If I consider earth as 1 MOP does it mean that earthed secondary circuit could be separated from mains by c.d and a.c for 1 MOPP?

As you mentioned:
If the applied part is only the insulated part, PE cannot be considered as MOPP (only metal parts can be protectively earthed)
At first I considered this case, the only possible contact to mains is through earthed enclosure. In this case I could not decide whether I should apply 2 MOPP (a.c and c.d) between PSU primary and secondary (secondary is connected to enclosure through earth), or should I consider earth as 1 MOP.

Applied part is glass where patient puts his body part (through that glass camera takes pictures). But patient connection is in the middle of the glass, so does the edges of the glass plane can be treated as an additional insulating measure?:
insulation3.jpg
In that case this additional distance would sum up with distance between mains part and secondary thus forming adequate distance for 2 MOPP between mains and patient?

  • 2 MOPP between the patient connection of the applied part and the secondary voltage - Point E
 
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