Secondary Circuits, Voltage Test and Clearance/Creepage

A

antonoia

#1
Hi All, I would like to hear your opinions and help me to clarify some doubts of some 60601-1 clauses.
We have a device, class 2 metallic enclosure with a PSU already having 2MOOP, secondary at 12Vdc that goes to some internal circuits.
According clause 8.9.1.12, where a secondary is not earthed and derived from Supply Mains, the circuit shall be subjected to the requirements for primary circuit in table 13 and 14. Clause 8.9.1.12 also states ?a secondary circuit derived from a supply mains will normally be overvoltage category I according to IEC 60664-1 if the mains part is overvoltage category II?. Table 13 starts with a voltage of 210V dc, would I consider this for my small 12 Vdc towards accessible parts? If my understanding is correct , should I select the value form the column ? Nominal mains Voltage <150V?? ( even I still have a mains of 240Vac) but just because this column contains ?(Mains transient Voltage 1500V)? ? In this case I should provide 2mm (2MOOP overvoltage cat.II). Am I correct?
And for the test voltage according table 6 seems will not be required between my secondary circuit of 12Vdc inside the device and the accessible parts. Table 6 says no test for voltage below 60Vdc, but table 7 provides test voltage also for 34Vdc, I did not understand here, one table says no test for voltage below 60Vdc while the other one expects test for 34Vdc. Could you please light up me from the darkness around this controversy? What is the minimum values for a dc so will not be required test for protection from secondary circuits?
I will like also to talk about clause 8.4.2 ?Accessible parts and applied parts? since seems to fall on same arguments of above. Here are given limit values of 60Vdc and 42.4Vp, same starting values of table 6. Are this two clause is some way related? For my understandings after all see clause 8.1, we can have a device that have high voltage on accessible parts, clearance and creepage not satisfying requirements, important are leakage currents are whiting the give limits? Is all about the requirement of 60601-1 ? leakage currents? Thank you and sorry for been so dispersive!
 
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N

NickM

#3
Just some quick feedback on your post on why you probably are not getting any responses. Need to be more concise and not ask so many questions. I got lost very quickly reading your post and because of that, I did not want to spend all the time needed to figure out your situation and then have to write a long response to answer all your questions. Most people are willing to help if they can provide quick answers. Thus, I would suggest, updating your post to be more clear and concise. Sorry, I know this doesn't help answer your questions but thought it might be helpful for you and others that may be looking for assistance.
 

Peter Selvey

Staff member
Super Moderator
#5
It's a little confusing.

The theory is that the mains supply there can be transients which are usually a few hundred volts but in rare cases can be up to a few kV. So any insulation needs to be designed with that in mind. In an earthed secondary circuit these transients are reduced so the insulation level can be dropped. But in an unearthed secondary circuit it's assumed that the transients are transferred in full. Thus, any insulation needed between the secondary circuit and other parts needs to take into account as to whether the circuit is earthed or not.

But ... here's the thing. In most cases we don't need any insulation from the secondary circuit, it's already safe for the operator to touch.

The problem with IEC 60601-1 is it does not say this directly, even though in practice it is necessary. Clause 8.4.2 lists some cases for exclusions for accessible parts. But really, it should have a general MOOP exclusion for the whole circuit, not just the accessible parts. This avoids the need to assess touch screens, secondary switches, enclosures around secondary parts, accessible secondary wiring and so on. None of these are dangerous to the operator and can't practically transfer any current/voltage to the patient. And the number of parts are far too high to be controlled as safety insulation even if they were assessed in a type test.

Most test agencies use this interpretation even though they might not say so or even be aware of it.
 
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