BF-type applied part MOPP vs secondary

[NeoKrunch]

Hello everyone,

I am designing a medical device with BF-type applied part (surgical power tool with micromotor).
The applied part is so small that I cannot implement more than 1 MOPP (working voltage of 48V) inside its metallic body (barrier C in attached picture).

The power supply is medical grade with BF class insulation. It provides 2 MOPP (4000VAC) from input to output (barrier A) and 1 MOPP (1500VAC) from output to ground (barrier D).

The applied part is driven by a 48V floating circuit with creepage distances between GND and protective earth for 1 MOPP at main voltage (barrier E).



To me, the design above respects clause 8.5.1.1 on the applied part for 2 MOPP at 48V with C + E.
The clause 8.5.2.1 for 1 MOPP at mains voltage is also respected with E.

But an expert asked for 2 MOPP at 48V between the applied part metal body and the secondary voltage (barrier B).
Wouldn't be C + E or C + D sufficient ?

P.S. : Patient leakage current limits are respected in Normal Condition and Single Fault Condition (even with SFC = shorted barrier C).

 

[NeoKrunch]

Hello again,

A short update on my issue. The expert statement is that MOPP C + E (or C + D) cannot be considered together as they are not rated for the same working voltage.

I disagree as barrier D or E are better than C. Patient safety is even better no ?

 

[Peter Selvey]

The expert might be used to dealing earthed voltage sources. Most sources are earth referenced, so it's easy to fall into the trap of applying this in all cases.

If the voltage is earthed, you need 2 MOP to prevent currents exceeding the limits from flowing. This is because the operator or patient is assumed to be earthed, providing a return path.

However, if the voltage source is genuinely floating from earth, then you only need 1 MOP, because, well, .... you know ... current always needs two points of contact to flow. Thus, a single fault to a floating circuit doesn't present any risk. Just like a bird happily resting on an 11kV line.

A caveat though is to make sure the circuit is genuinely floating, which can be verified by test. This would involve shorting insulation C and measuring the current. In this case, insulation E is actually forming the second MOP by preventing current from flowing. Note that circuits are rarely 100% floating as they are typically EMC or other capacitors to earth which bridge insulation E. While these may be isolating at dc, there is usually some impedance at 50/60Hz and could even be low impedance at the switching frequencies often found inside secondary circuits. Nevertheless, with C shorted the currents are usually still below the limits.

The argument that they are "not rated for the same working voltage" seems to be based on clause 8.5.4 2nd dash, the working voltage for each MOP in a 2MOP system should be the same as for the whole system. This is correct, but it applies for each voltage source individually. In a device there may be several voltage sources, and a single barrier can play insulating roles for two or more sources. The MOP analysis (insulation diagram) should, In principle, be done for each voltage source individually, rather than lumped together in a single diagram. In this case, the analysis for the 48V source would find that both C and E need to be each assessed based on (the same) working voltage of 48V.

Separately, for the purpose of F-type applied parts (a hypothetical single fault situation that the patient is raised to mains voltage), isolation E needs to be 1MOP for a working voltage of 230V (mains voltage). It's a difference source, so different analysis.

Then, if a single MOP is involved in multiple voltage sources (as is the case for E), we can of course test it only for the most strict requirements. It makes no sense to do a 1500Vrms test for the 230V and then a 500Vdc test for the 48Vdc. Same for creep/clearance. Note however that there are some case where a higher working voltage does not necessarily mean stricter requirements.

 

[william guo]

Peter's reply always enlightened. I have a question about insulation C and E. If E is 1MOPP based on mains voltage and 2MOPP based on working voltage (48vdc), so genuinely floating. Is insulation C (2MOPP@48V) necessary, or a "functional insulation" is sufficient?