Determining Working Voltage - Drawing a Medical Device Insulation Diagram

[Roland chung]

Hello all,

I am drawing an insulation diagram for class II equipment with BF type applied part which employs a 60950 power supply. Barrier A between Mains (240 V) and secondary circuit (48 V) meets 2 MOOP/ 1 MOPP (288 V). There is a barrier B between secondary (48 V) and applied part (12 V). Since the barrier A is 2 MOOP/ 1 MOPP @ 288 V, the barrier B shall be:
2 MOPP (60 V)
1 MOPP (XX V)
XX = either 240V or 288V.

My question is which working voltage shall be chosen and why.

Thanks,
Roland

[Pads38]

250V

8.5.2.1 * F-TYPE APPLIED PARTS
The PATIENT CONNECTION(S) of any F-TYPE APPLIED PART shall be separated from all other parts, including the PATIENT CONNECTION(S) of other APPLIED PARTS, by means equivalent to one MEANS OF PATIENT PROTECTION for a WORKING VOLTAGE equal to the MAXIMUM MAINS VOLTAGE

Maximum mains voltage is given in 8.5.3 and is 250V for just about everything.

[Roland chung]

Thank you pads.

I agree with the 250V BUT for the barrier between applied part and accessible part.

What I am concerned about in my case is barrier from Mains to applied part shall be 2 MOPP (barrier A + B). The working voltage for barrier B should seem to be same as barrier A based on the principle of DOUBLE INSULATION. But image barrier A fails, Mains voltage will stress the barrier B. Er, a bit confusing.

[raymond02302]

The working voltage (for 2 MOOP) for barrier A is not always the sum of mains voltage and secondary voltage. It should be the voltage actually stress barrier A. So you should measure the voltage.
Regarding your question, we should chose the least feavourable between the conditons below
1) We consider mains voltage as the hazard resource. To protect the patient, Barrier A+B should provide 2MOPP with working voltage mains voltage (240 V for your case), therefore, barrier B should provide 1MOPP with working voltage 240.
2) Considering the secondary voltage (48 V) as the hazard resource, So, barrier B should provide 2MOPP with working voltage actually stress barrier B.

[Roland chung]

Again, I am talking about the case that 2MOPP is divided to 2 single MOPP. However, working voltage of one concerned MOPP (288 V) differ from the working voltage of the whole 2MOPP (240V). In this case, how to consider the working voltage of the other concerned MOPP?

Peter, do you have any comments?

[Shine Zhang]

The working voltage you mentioned here might be different and it shall be decided case by case basing on the circuit analysis.
I suppose that barrier A is separated by a linear transformer since the working voltage was chose as 288V, so another MOPP on B could be 288V at the worst case in theory.
but in general, I think 250V is enough, especially, A is separated by SMPS. Although the priciple of DI-the working voltage is the whole to which it is subjected, however, B will not subjected 288V when failed in A (SFC) but 250V for AC mains.
The question is when failed in A, what voltage may stress on B considering the actual circuit and possible connection to secondary circuit, also earthed or not earthed of applied part.