Isolation of secondary grounded circuits

[Schwarzyzi]

Hello Everyone,

We are designing electronics for a ME equipment, the device is class I, the applied parts are BF.
Our power supply is medical grade and provide a low DC voltage (<50Vdc) to power the secondary circuits and our secondary circuits are reliably connected to ground (which is PE) to provide 0V.
We have designed our isolation diagram with 1 MOOP between secondary circuits and the enclosure of the device, that is reliably connected to PE, as stated in annex J.1 of 60601-1 :

We are having a hard time figuring how to isolate a grounded secondary circuits from PE since PE is part of the circuit. We thought that each conductive part of the circuit board should be under the creepage and clearance of the isolation diagram from PE but we have boards with digital electronics that have small integrated components, it is impossible to maintain the requirements of 1MOOP on such circuits.

Do you see anything that we missed ? Is it normal to try to isolate a grounded secondary circuit from ground ?

 

[Peter Selvey]

No, it's not normally required to have any safety related insulation (MOP) in secondary circuits, except in some cases between that circuit and applied parts (the patient).

Annex J is informative only and some of the diagrams don't really make sense. The normative requirement is that a MOP is only required to prevent electric shock. If there is any doubt about whether an insulation needs to be a MOP, you can short circuit it (as a thought experiment, or if necessary by actual test) and then see if there is any way for the operator or patient to get a shock.

One possibility for Annex J.1 is that the authors assumed that mains-secondary has no safety insulation (a functional transformer). In this case, the secondary is treated as mains as well and 1 MOOP would required to the earthed frame. But such a case would be rare, normally transformers provide 2 MOOP so we don't care about the secondary.

 

[Benjamin Weber]

Peter is absolutely correct. You don't need to isolate the secondary from PE , because... well.... in your case it not possible to be PE connected and isolated at the same time ;-)

But you have to isolate your BF applied part from (cl. 8.5.2.1):
- the secondary circuit by 2 MOPP based upon the working voltage,
- all other parts, including the PE by 1 MOPP based on the maximum mains voltage (usually this is more than the 2 MOPP vs. secondary).

 

[Schwarzyzi]

Thank you very much for your answers, it's much more clear. But I think the standard is missing something, if we short circuit the said secondary PE connected circuit with PE, this would not be considered single fault since no MOPs are implemented for this circuit.

So the test could be short circuit + PE loss on the enclosure, and in that case, we would have to demonstrate that the touch current doesn't exceed the limit in single fault so 10mA through the circuit in 8.7.3 ?

 

[Schwarzyzi]

So we had a discussion with our lab, it appears that connecting the secondary to the protective earth through a Y capacitor would be a solution. For the electrical shock hazard, all the creepage and distance would be reported on this capacitor.
For the fire hazard, the short circuit shall be prevented by implementing the correct creepage and clearance, however, if not possible, as state in cl 13.1.2, the short circuit shall not dissipate more than 15 W.

 

[Felipe Schneider]

Thank you so much for the discussion here, it is very clarifying to me.

@Schwarzyzi, connecting the PCB ground using a Y capacitor would not be an EMC problem for you?
Another question, how would you keep a proper isolation and creepage distances in all PCB connectors and the metal frame?