Switching Power Supplies Certified to IEC 60601 3rd Edition

Roland chung

Trusted Information Resource
#1
Hello folks,

As you may know, some switching power supplies contain Y2 capacitors between the secondary part and the earth. These Y2 capacitors are considered as 1 MOOP according to 60601-1: 2005. If the secondary part of SPS is regarded as PATIENT CIRCUIT, the question arises if such Y2 capacitors in SPS shall be changed by Y1 capacitors which are considered as 1 MOPP.

Any thoughts on this would be appreciated.

Roland
 
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Peter Selvey

Staff member
Super Moderator
#2
It's not unusual that the "floating" secondary of a SPS has a capacitor to earth, but this capacitor normally does not have a safety function. It's very rare (in fact, never seen) to have the output of a SPS connected directly to the patient.

Normally there would be either an additional barrier (e.g. F-type insulation, 1.5kV/4mm), the applied part is earthed (e.g. operating table, X-ray), or otherwise well removed from the secondary. In such a case, the capacitance between SPS secondary to earth is not relevant for safety.
 
R

raymond02302

#3
I guess roland means the capacitor between secordary part (reference earth) and the primary part (earth).Just a Y2 capacitor is deemed as 1MOOP if it can withstand the dielectric strength for 1MOPP. 2 Y2 capcitor with same capacitance can be used to 2MOPP, they can be used in adapter to protect operator. But for patient operating, there shoud be 2 Y1 with same capacitance if each capacitor can withstand dielectric strength for 2MOPP.
SPS can bear an CB Cert excluding risk analysis, So we can't know it's intended use.I think the manufacturer shoud be responsible for choosing which SPS is appropirate for the end product and assuring if additional MOP is needed.
 
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Roland chung

Trusted Information Resource
#4
Thanks for all the input.

Actually, the SPS is used to supply the evoked response electrical stimulator (this device with BF type applied part is not uncommon in Japanese market). In my experience, there are at least two kind of products (these products normally employ skin electrodes) that the secondary circuit is regarded as the patient circuit.

I also agree that the capacitor between secondary and earth in SPS is mainly for EMC purposes and not relevant to safety. But such SPS is custom-made for the equipment mentioned above, so my question raised.

I took part in a 60601-1_2ED training in TUVPS Munich few years ago. The expert said capacitors between the secondary circuit and PE shall comply with B-d requirement (4mm/ 2.5mm, 1500V), if the secondary circuit will be a part of the F type floating applied part.

Now, since the equipment is in progress of to apply 60601-1_3rd compliant, I am wondering if Y1 capacitor is a must in my case.
 

Peter Selvey

Staff member
Super Moderator
#5
In the case the SPS output is directly connected to the patient:

Firstly, if it is electrode connection, BF is a must for 3rd edition. Type BF isolation needs, 1 MOPP, which in turn means a Y1 cap.

Secondly, for a stimulator, to prevent expected current in the applied part 2 MOPP is likely to be required. Consider if the output is floating 100V; two electrodes are applied to the patient, with the intention that the current flows in between the electrodes. If however there is only 1 MOPP to earth, and this shorts (SFC) the output gets earthed, and the patient is for example standing on an earthed floor, the current could flow in unintended path (e.g. electrode -> feet -> wall earth -> equipment earth -> SPS output).

But, the reference voltage for this is 100V. Also, the transients are lower for secondary (Cat I). The transient withstand is a main reason Y1 cap is OK for 2 MOOP but not 2 MOPP. So I think a Y1 would be OK for double insulation for 100V Cat I. Of course, this is just example. Actual case we need to know the output voltage of the stimulator.
 
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