Can I use varistor on medical devices?

M

MSouza

#1
My concern is about 8.11.1 h) that says:

h) * ME EQUIPMENT shall not include a device that causes disconnection of the ME EQUIPMENT from the SUPPLY MAINS by producing a short circuit that results in operation of an overcurrent protection device.

So, varistor is prohibited to use on supply mains??

Thanks!
 
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Peter Selvey

Staff member
Super Moderator
#2
They should not be used.

There is some history of fire for varistors, but I think the primary reason is that placing a short on the mains could cause not only the medical device in question to stop operating, but could potentially trip out the room breaker which could have significant implications in a medical environment.

Actually I just read the rationale, it says the same thing. If the clause has an "*" mark, it means there is an explanation in Annex A.
 
M

MSouza

#3
Tanks Peter! Thats what I thought, but one lab here in Brazil thinks different. They say to me that I can use varistors after the fuse.

I will maintain my project as is, without varistor.
 
N

nikolaos

#4
Varistors can be used after the fuses. Without a varistor between L/N the EMC Surge test will be most probably fail. Transient surges can cause fire, short-circuits...in a ME. A Varistor is a protection against this. Varistors can be destroyed by too many surges so the probability of a short-circuited varistor is very low. From my experiance Testing Labs accept the presence of varistors.
 
M

MSouza

#5
Nikolaos, tank you for your coment, I'm already certified a product with a off the shelf medical power supply that came with no varistors on mains input, passed with no problems on EMC surge tests.
But good to know that possibility.


Tanks!
 
#6
Varistors in the mains part have been identified as an "unaddressed safety aspect" in edition 3. Guidance on this is provided in IEC TR 60601-4-3:2015 at clause 3.2.168 (I only have the preview so cannot say what the guidance is!).

Link:
https://webstore.iec.ch/publication/22123

IEC TR 60601-4-3:2015
Medical electrical equipment - Part 4-3: Guidance and interpretation - Considerations of unaddressed safety aspects in the third edition of IEC 60601-1 and proposals for new requirements
 
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