Search the Elsmar Cove!
**Search ALL of Elsmar.com** with DuckDuckGo Especially for content not in the forum
Such as files in the Cove "Members" Directory

IEC 60601 subclause 8.9.1.12 - Earthed secondary circuits

eldercosta

Starting to get Involved
#1
Dear all,

I have a question regarding the interpretation of table 15, referred to on subclause 8.9.1.12, namely “Where the SECONDARY CIRCUIT is earthed or the ME EQUIPMENT is INTERNALLY POWERED, Table 15 applies”, which is the case of my device. Table 15 (Minimum air clearances for MOOP in secondary circuits) has clearances but what these clearances are from, considering enclosure (or the metal parts of it) are earthed?
 

eldercosta

Starting to get Involved
#3
Hello, Marcelo, thank you for chiming in.

So, in this case (i.e. earthed secondary circuit), it is redundant with MOOP between mains and secondary circuits and MOPP between applied parts and secondary circuits. Is it right?
 

Marcelo

Inactive Registered Visitor
#4
No, in the case of MOOP you still have to have one mean of protection (basic insulation) from mains (and guarantee that this basic insulation is maintained thru the device expected service life as per 4.7 b), otherwise you would have mains on the secondary circuit in normal condition, and in this case you would introduce the failure of the earth, with the leakage current limits being exceeded).

In the case of MOPP, if the applied part is type F, it has to be separated particularly from earth (but also from all other parts) as per 8.5.2.1 by one MOPP, and even if it's a type B, if it has patient connections not protectively earthed, it has to comply with 8.5.2.2.
 

eldercosta

Starting to get Involved
#5
Hello, Marcelo.

I think we are talking of the same thing but my previous message was not clear whatsoever (BTW there is a 2xMOPP isolation between secondary and AP in my design). What I was thinking of and did not express in an appropriate way was that there might be some cases where isolation requiments superimpose or intersect on the isolation diagram and the most stringent (whether MOOP or MOPP) applies.

Anyway your first response helped to clarify the subject to me. Also I made a small mental simulation where the secondary circuit has a high voltage source instead of the low voltage of my design and it made clearer why and how some isolation requirements would apply even if the secondary was earthed. The standard tries to be as comprehensive as possible (cover every possible scenario) and sometimes a some requirements may not be clear for a particular use case.

Thank you very much for your help.
 

Marcelo

Inactive Registered Visitor
#6
Hello, Marcelo.

I think we are talking of the same thing but my previous message was not clear whatsoever (BTW there is a 2xMOPP isolation between secondary and AP in my design). What I was thinking of and did not express in an appropriate way was that there might be some cases where isolation requiments superimpose or intersect on the isolation diagram and the most stringent (whether MOOP or MOPP) applies.

Anyway your first response helped to clarify the subject to me. Also I made a small mental simulation where the secondary circuit has a high voltage source instead of the low voltage of my design and it made clearer why and how some isolation requirements would apply even if the secondary was earthed. The standard tries to be as comprehensive as possible (cover every possible scenario) and sometimes a some requirements may not be clear for a particular use case.

Thank you very much for your help.
Ah, ok, sorry for that.

Anyway, maybe the thing is to understand that the insulation coordination is more like a puzzle that you have to create for each device. For example, you mentioned that you have 2 MOPP between earthed secondary and AP, but you in principle need only 1 (as long as your secondary has the 1 MOP between mains and 4.7 is fulfilled), so these options are you to make, and you create the insulation system based on your needs (the main problem with having more insulation than needed is that it costs more without any gain to safety). Surely, and particular in the case of MOOP and MOPP, there are superimposed requirements (because an insulation can function as both), but it's not that hey superimpose by default, it's your design that dictates that.
 
Last edited:

eldercosta

Starting to get Involved
#7
Anyway, maybe the thing is to understand that the insulation coordination is more like a puzzle that you have to create for each device.
Puzzle perfecly describes it. :)

you mentioned that have 2 MOPP between earthed secondary and AP, but you in principle need only 1 (as long as your secondary has the 1 MOP between mains and 4.7 is fulfilled)
That is one of the points I found very confusing in the standard. 8.5.2.1 states 1MOPP but the figures in Annex J state otherwise e.g. J5 and J6. It is not clear to me why.

Still about Annex J, J4 requires 1MOPP for signal I/O for maximum mains voltage but 2MOPP for working voltage. That seems odd to me, especially if the working voltage of secondary or AP is a few volts (case of my designs). What am I missing?

the main problem with having more insulation than needed is that it costs more without any gain to safety
In my design, using 2MOPP between secondary and AP is fairly trivial as there are components that meet the requirements at an affordable cost. Also, in the dielectric strength test between mains and AP, most of the voltage goes over the AP isolation as its parasitic capacitance tends to be much smaller than the one of the AC/DC converter.
 

eldercosta

Starting to get Involved
#8
That is one of the points I found very confusing in the standard. 8.5.2.1 states 1MOPP but the figures in Annex J state otherwise e.g. J5 and J6. It is not clear to me why.

Still about Annex J, J4 requires 1MOPP for signal I/O for maximum mains voltage but 2MOPP for working voltage. That seems odd to me, especially if the working voltage of secondary or AP is a few volts (case of my designs). What am I missing?
OK, I think I figured it out. If the working voltage is low, say less than 43V, then 2 MOPP creepage/clearance are similar (but not equal) to 1 MOPP for maximum mains voltage. If working voltage is higher, things may change and distances increase accordingly. Does that make sense?
 
Top Bottom