Class II ME EQUIPMENT and subclause 8.9.1.12

eldercosta

Involved In Discussions
My device (Insulation Diagram attached) is Class II, using an external Class I AC/DC converter . The converter (OEM supplied) is specified as 2xMOPP (passes the 4kVac test and construction meets requirements for 5000m and could theoretically be used on a BF APPLIED PART ). Our part of the device has an APPLIED PART that is also 2xMOPP compliant (it also passes the 4kVac test and meets creepage/clearance requirements for 240Vac - overengineered, but as it uses standard components, it comes as a bonus).

As the device is Class II, the PE of the AC/DC converter will work as FE, to reduce risk of failing on EMC. FWIW the AC/DC that will be treated as part of the ME EQUIPMENT.

I have been requested to analyze the impact of some requested design changes over the compliance with the general standard. The first one is that the SIP/SOP connectors should be exposed for convenience and usability (in the original design the SIP/SOP ports are protected by a cover that requires the use of a tool). The second change is the addition of an USB port to connect USB flash drives exclusively.

The only accessible metallic part of the original design was a fixture to mount the device on a support; insulation was easy as the fixture was fixed by some screws to the plastic enclosure; this fixture was encompassed by item C of the Insulation Diagram.

With the change, my undestanding is the Ethernet and Serial connectors (or their pins) meet the definition of accessible parts and they will need to comply with C requirements (more on this below).

The USB connector is problematic. It is part of the secondary circuit unlike the other two that are electrically isolated (there are air clearance and creepage distance issues though).

Item C of the Insulation Diagram (Separation between SECONDARY CIRCUITS and ACCESSIBLE PARTS non PROTECTIVELY-EARTHED) require 2xMOOP, 6mm creepage/clearance (for 5000m operation) and test voltage of 3kV for 240Vac working voltage I am interpreting the third paragraph of subclause 8.9.1.12, i.e.
Where the SECONDARY CIRCUIT is earthed or the ME EQUIPMENT is INTERNALLY POWERED, Table 15 applies.
is applicable.

Regarding the Ethernet/Serial connectors, one of the two options I see is either a) adding separation devices to ensure the requirements are met or b) recommend keeping the cover that requires a tool to be removed, maybe redesigning it for better usability but still requiring a tool (e.g. using a coin instead of a screwdriver). Regarding the USB, it seems to me that a separation device (USB isolator) will be required anyway.

So, my question is if I am misinterpreting the standard or missing some other subclause that would avoid complicating the design. Could be the fact the AC/DC meets 2xMOPP/BF AP requirements be used as a mitigating factor?

Thank you very much for any advice.
 

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Peter Selvey

Leader
Super Moderator
For accessible connectors that are at safe voltages for the operator (i.e. <42Vac(peak) or <60Vdc, separated from mains by 2MOP, Clause 8.4.2c can be used.

According to this clause, the risk of the operator contacting the connector and the patient should be assessed. In the rationale, the focus is on probability of contact, however the the actual (normative) clause refers only to the risk, which is based on the probability of harm and also severity. In most situations the overall probability of harm is extremely low and the severity minor (minor tissue necrosis), and would require hours to arise, even if the operator contacts the port and the patient at the same time, especially for 5V signal levels, with small pins and contact areas.

For area C, the working voltage should be 12.6V, not 240V. However, it is arguable whether area C is needed. Normally if the enclosure is metal it will be connected together (and typically directly to the zero volt of the 12.6Vdc supply), which means it acts as a faraday cage around the 12V circuit and no insulation is needed. For any MOP to be required, there has to be a plausible current path in the first place. Moreover, if the standard allows direct contact with the pins of secondary circuits under 8.4.2c, it does not make sense to require 2 MOPP to the same circuit.

Although some test labs will insist Area C needs to be 2MOPP and will actually test (e.g. 500V, 3.4mm creepage) this is not practical in the broader context. A type test is only the start of the bigger picture which is regular production. If area C needs to be 2MOPP then all the parts involved need to be critically controlled in production and design changes, and this may include PCBs, switches, connectors, wiring, touch screens, keypads , indicator lights and so on. Many of these parts will be prohibited by the part manufacturer to be used in this way, even if they pass the 500V test. (use outside of specification, which conflicts with Clause 4.8). Moreover, test labs tend to assess this for smaller devices where it is practical for the testing side, while happily and inconsistently ignoring it for larger devices like patient monitors, dialysis machines etc where it is impractical even at the testing stage, due to the large physical volume of secondary circuits and user interfaces.

:2cents:
 

eldercosta

Involved In Discussions
Hi, Peter, thanks for your quick response.

The ENCLOSURE is plastic, with the exception of some screws (not accessible in the sense of the standard) and the aforementioned mounting adapter, which is screwed on some inserts in the plastic, so there is insulating material between the secondary circuits and the adapter.

I set the WV to 240Vac based on my understanding of that paragraph of the subclause along with some comments in posts of this forum regarding the Class II lack of protecting earth requiring the worst case to be considered, in this case, failure of the insulation of the AC/DC converter causing a high leakage or worse. It is something extremely unlikely but not impossible as far as risk analysis is concerned.

The contact between operator and patient while handling the connectors (unlikely in practice anyway) is manageable (warnings etc.). But what about the safety of the operator themselves, if by any chance there is a significant leakage from mains to the secondary causing electrical shock to them? Air clearance/creepage distances are defined by the standard thinking of the worst case to minimize risk. In many cases they are overkill; but aren't we obliged to comply with them to be certified? Is there a way around it?

I am playing the devils advocate here as I fear some laboratory may interpret the standard in this more stringent way and declare our device not compliant, thus not certifiable. I would like to have a good argument to refuting this NC if it occurs.

BR.
 

Peter Selvey

Leader
Super Moderator
8.9.1.12 is intended to decide which table applies, not the working voltage. The working voltage should still be based on 3.129 and 8.5.4, and would normally be based on 12.6V in this case.

Since the power supply is certified for 2MOPP there is no need to worry about mains related voltages or leakage currents for the secondary. The operator can directly touch the output of the power supply without any concern for mains shock, both in normal and fault condition. Therefore, the only "concern" is 12.6Vdc in the secondary itself. Most standards consider 12Vdc as safe for the operator, and there is no evidence to the contrary. 12Vdc can be unsafe for patients but even then it requires highly special situations and generally involves low severity harm.

IEC 60601-1 has a basic error in that accessible 12Vdc fails the first line of 8.1 since it exceeds touch current limits even for the operator. The standard then has various (hidden) exclusions or relaxations for operator protection such as 8.4.2c. The correct approach is to separate out operator and patient at the start (8.1), and for the operator exclude parts separated from mains 2MOOP and less than 42.4Vp(ac)/60Vdc from touch current measurement and hence any MOP. Under risk management, the risk of these parts having low impedance contact with the patient should be considered. However, risk control methods such as using female pins on connectors, minimizing the accessible parts at voltage, locating connectors at the rear of the device, under covers etc, should be acceptable. Risk control measures should rarely involve MOP according to 8.8, 8.9, as this is not practical for both design and production.

In your case, for a plastic enclosed device, the solid plastic part is normally OK regardless even for the extreme interpretation of needing 3kVrms. But most devices will have user interface items like switches, screens, indicator lights, softkeys and so on. These are the weak points. Consider for example, a switch rated for 30Vdc, not designed, specified or marketed for mains related safety insulation since it is intended for 30Vdc only. But the test lab or designer insists on 3kVrms test anyway, and it happens to pass. But ... this is just the type test. The pass result depends on switch construction which depends on the switch manufacturer. If the part is relied on for mains isolation but not specified or certified for mains isolation by the part manufacturer, the end product manufacturer must engage in special controls for that part that should involve contracts and agreements with the part manufacturer, as well as special inspection and tests, and procedures to notify and handle design changes inside the switch. Clearly, that is overkill and crazy. But the overkill is not the production controls. The overkill was that the 3kVrms was never reasonable in the first place.

IEC 60601 series has evolved to the point that it has some clear errors and overkill, which undoubtedly will get fixed in the future. In the meantime, a common workaround is to get a test lab to write "Pass" for prototype sample then quietly forget about production controls. While this is seems OK, this approach can bleed into critical aspects: a big focus on the type test and less on production where the real regulations apply. For now, regulators, test labs and manufacturers seem to be happy with type test delusion, but at some point it's going to blow up and questions should get asked. The root cause in my opinion is overkill in standards, and treating 12Vdc as dangerous for the operator is a classic example of this.
 

eldercosta

Involved In Discussions
Hello, Peter.

First off, thank you very much for your comment. It was helpful, providing some directions I had overlooked.

8.9.1.12 is intended to decide which table applies, not the working voltage. The working voltage should still be based on 3.129 and 8.5.4, and would normally be based on 12.6V in this case.
It makes sense. I mixed things up. I agree the working voltage is 12.6Vdc in my design's case. Table 13 and 14 apply. Still, air clearance for 2xMOOP is the same as with WV=240Vac or 4.0mm (times 1.48 for operation up to 5000m).

I find this intriguing: why would one need such a clearance for such a low secondary WV? I think insulation of non-PE ACCESSIBLE PARTS from MAINS (WV=240Vac) is way more critical and it is covered the item B of the Insulation Diagram, requiring 2xMOOP. Why 2xMOOP from the secondary to the ACCESSIBLE PARTS and with such a long clearance? It seems to me that using IEC 62368-1:2018 - which I understand the general standard allows for MOOP - would lead to shorter distances.

About the SIP/SOPs, they easily meet the 2xMOOP from MAINS (item D) and the 3kVac test because of the AC/DC specs. BTW, I haven't talked about the PATIENT side because existing insulation/separation already satisfies requirements. Risk analysis indicates the probability of the operator handling the patient and making contact with those SIP/SOP contacts is practically zero, in addition to low voltages and currents that do not cause any real harm.

I still have the concern about air clearance/creepage distances of the connectors, even if they have recessed contacts, based on the clearance requirements of Tables 13/14 commented above. For example, taking the RJ45 connector which is a plastic version in the design, the contacts will not meet the 6mm (4mm x 1,48) distance when the male is inserted (this is the normal use). The contacts are reachable by the test finger when no male is connected (one could think of it as foreseeable misuse by a very foolish operator ;)).

I understood from your previous post that 8.4.2 c) could provide some way out of the clearance issue above. I suppose you mean the first dash:
c) * The limits specified in b) above do not apply to the following parts if the probability of a connection to a PATIENT, either directly or through the body of the OPERATOR, through which a current exceeding the allowable TOUCH CURRENT could flow, is negligible in NORMAL USE, and the instructions for use instruct the OPERATOR not to touch the relevant part and the PATIENT simultaneously:
– accessible contacts of connectors;
Is that so? Am I in the right direction?

BTW, the AMD2:2020 added extra text to 8.4.2 c), prescribing a test to determine the current between the SIP/SOP contacts and earth through a 10k resistor (42.4V peak ac or 60Vdc maximum) in NORMAL and SINGLE FAULT CONDITIONS; in my design's case, because of the AC/DC converter, the voltage on the resistor will be very low even with DOUBLE FAULT (neutral and functional earth open) so it could provide an extra argument to exempt these connectors. Does this reasoning make sense?

Once more, I thank you for your help and appreciate any further comment that could help me to solve this puzzle. :)

BR
 

Peter Selvey

Leader
Super Moderator
Clearances are based on "overvoltage transients" which are spikes on the mains from switching and lightning that commonly reach 500V and can reach a few thousand volts. Depending on the case they can also flow through to the secondary although they normally get reduced. In IEC 60601 they are assumed to reduce if the secondary is earthed (Table 15), if the secondary is floating they are assumed not to be reduced (Table 13/14, which has higher values than Table 15). The transients are very short (µs) so they are not a factor in electric shock per se; an operator touching a double insulated secondary might get subjected to these spikes without any concern. However they are a factor to be considered for insulation integrity over the long term. Thus if (and only if) insulation is required, the overvoltage transient should be considered, and this can lead to higher than expected values even for lower voltage secondaries.

I think it makes sense to consider a real case where Tables 13-16 should apply.

Consider for example a 2MOOP secondary which then has a non-isolated dc/dc converter for driving a transducer, which has a peak of 150Vp but a low rms of 5Vrms. This exceeds the 42.4Vp so has to be treated as dangerous, which it is, because 150Vp can punch through the skin impedance and reach the body blood circuit ~1kΩ. If this circuit is earthed, then the standard makes the reasonable assumption that overvoltage transients will reduce and table 15 applies, with a result of 2.0mm for 2MOOP (assume mains supply 230V, Pollution degree 2). For creepage, the low rms means only 1.0mm, but the rules state that creepage ≥ clearance so 2.0mm also applies.

This is the kind of situation these tables were intended for and they make sense in that context. They were taken from standards like IEC 60950 (now IEC 62368). The difference is that those standards explicitly exclude double insulated secondaries with ≤42.4Vp/60Vdc (these used to be called SELV, safety extra low voltage). Thus, all the typical 5V, 12V, 24V, 30Vdc secondaries found in normal electrical devices are considered SELV and completely excluded and nobody would look at these tables at all and try to figure out clearances and creepage. These voltages are totally benign, because they are unable to break through skin impedance.

The mess occurs because IEC 60601-1 deleted the exclusion for SELV for operators and thus we find ourselves in this mess, using tables that were never intended for these kind of circuits.

Think about it: in IEC 60601-1, even a 1.5V AA battery is dangerous! That's crazy. No other standard considers a 1.5V battery dangerous, but IEC 60601-1 does. This is why we get test labs applying 500Vdc tests and 3.4mm creepage to a coin cell powered electronic thermometer.
 

eldercosta

Involved In Discussions
Hello, Peter.

Thanks for the explanation, it makes more sense to me now. I always worked with Class I devices, the secondary connected to the PE so some things I took for granted and it was "easier" to handle these clearance/creepage requirements. Working with Class II is uncharted territorry to me.

One information missing from my previous posts: though the device is Class II because of some product related requirements, the intended environment is the Professional Healthcare Facility Environment. The AC/DC converter is Class I and it must be connected to an PE outlet with the PE working as a FE as required by the standard. The Secondary Part of the AC/DC converter has a Y1 capacitor between the 0V terminal and the PE; the input filter of the converter has a filter with X1/Y1 capacitors + chokes. Woudn' those make transients at the MAINS side to be reduced on the SECONDARY side of the converter, in which case the second part of the fourth paragraph of 8.9.1.12 would allow me to use Table 15?
 

eldercosta

Involved In Discussions
Peter,

I have one additional question: you mentioned a test with 500V a few times. Is this based on the 601-1 or some other standard at the lab's discretion? According to Table 6, for 12V WV one would not need testing.
 

Peter Selvey

Leader
Super Moderator
The 500V is for patient (1MOPP, 2MOPP would be 1000V ....). It's just to point out that values in IEC 60601-1 don't make sense. For operator (MOOP) there is no test for 12V.
 

eldercosta

Involved In Discussions
Hello, Peter.

I realized it was related to the patient a while after posting. Unfortunately the forum does not allow editing or deleting posts.

Once again I want to thank you for your comments above and for pointing directions on this subject.

BR.
 
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