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
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.
 

nikolaos

Involved In Discussions
#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!
 

Pads38

Trusted Information Resource
#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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