2x MOOP for type B AP - Therapeutic device - Heater/cooler with pump

#1
Hello,

Currently we’re designing a therapeutic device with a nonconductive type B applied part. The device consists of a heater/cooler where cooled or heated water is pumped through a non-conductive type B applied part. The water reservoir is conductive and bonded to PE. There are no external electrical connections to the device, only a GUI for the operator.

The pump and cooler are 24Vdc and are supplied by a 2x MOOP rated 24Vdc supply, the device is class I. The heater is mains supplied and bonded to PE with 1 MOPP insulation (1.5kV).
The 24Vdc pump and cooler are uncertified components so if these fail it isn’t considered as a SFC. Since the supply is only rated for 2x MOOP another MOP is required. The easiest solution is for the AP to provide the additional MOP but it is desirable for the AP to have minimal requirements, I’m not sure if this is possible though.

Although the AP is nonconductive if I assume it is conductive and bonded to PE than he NC/SFC condition AC leakage currents are met since these are the same for a type B AP as for the earth leakage currents. This also applies when the SFC is a loose PE connection. Knowing this I would like to use a conductive tube connector (for the additional MOP) bonded to the chassis (PE) for connecting the AP because this would result in the water potential being lowest in a SFC.

Because the pump is uncertified it failing isn’t considered a SFC. If I assume the pump has failed (24Vdc on water) AND the SFC is a loose PE, then the only option is to leave the secondary GND floating to comply with the allowed DC leakage current but this seems counterintuitive to me.

The worst-case scenario would be that the water becomes live when the pump and PSU fail simultaneously. The water resistance is assumed to be > 50Ohm (depending on salt level) towards the grounded tube connector. For this fault connecting the secondary ground to PE is surely better.

Alternatively, the tube connector could be insulating (1 MOP 500V).


Hopefully someone can make sense of the above as it is difficult to describe and I’m going in circles with this problem.

After re-reading the above I think the best option is:
- Tube connector bonded to PE.
- Secondary GND connected to PE.
- AP should provide 24V/10uA = 2.4 Meg DC resistance.

Does anyone see any flaws in my assessment of the system or have some more insights?

Thanks
 
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#2
Hello Publius,

an insulation diagram would be helpful to illustrate your thoughts and might also help you to find flaws in the considerations. With the diagram the requirements are obvious and the solution then follows.

Best regards,
Martin
 
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