PE/FE and Class B AP in home healthcare environment

[hoshinari]

Dear community,

I have read a lot of posts in the past and they have been extremely helpful. Now it is time for my first post here

We are building a desktop medical device. The current hardware has passed 60601 safety testing, as a device for a controlled medical environment (non-home use), since it was only used as a prototype in our lab and in clinical studies. It uses a 60601-certified AC/DC PSU (24V, 2x MOPP, two-prong mains plug), runs on a 24V secondary and has a type B applied part. The casing consists of a mix of sheet metal and insulated plastic, and the AP is electrically connected to the metal casing/chassis.

We have had some issues regarding electrical noise in some of our circuits (affecting our measurement performance), and noticed some HF leakage current on the AP and casing; within limits, but enough to be able to make an LED flicker by placing it between the metal casing and some grounded metal. It can also be felt when lightly brushing the device. It's a similar feeling to operating an aluminium body MacBook while using a two-prong (non-PE) power brick.

There is now a push to certify the next iteration, including for a home healthcare environment (60601-1-11), and at the same time, the desire to mitigate the above issues by grounding the casing/chassis, ideally via a three-prong variant of the same certified AC/DC PSU. The PE aspect is not relevant, since we operate on 24V, but it could act as FE.

However, 60601-1-11 seems to put two roadblocks in our way:

- Clause 6 states that home devices shall be Class I (no PE), and shall not have a functional earth terminal.
- Clause 6 also states that home devices shall only have applied parts of type BF or CF

It seems we cannot rely on PE/FE to provide a stable ground for our sensitive measurements. And even if we redesign the casing to be fully nonconductive, fully isolating the AP from the grounded chassis will be very difficult (think: optical sensor in contact with the patient, mounted on a conductive aluminum structure for stability, and a laser whose metal housing is mounted/electrically connected onto the same structure and simultaneously connected to the GND of its power supply).

Is there a way to argue for exemptions to clause 6 via risk assessment (e.g. PE not necessary for safety, only for performance improvement; no increased risk due to AP type B instead of BF*)? Does anybody have experience with a similar case?
Thank you very much!

* In the past, we were able to argue against the need for testing according to Figure 18 (external voltage on no-PE metal accessible part) by arguing that the likelyhood of exposed external voltages in the vicinity of the device is negligible.

 

[VinceTech]

Dear community,

I have read a lot of posts in the past and they have been extremely helpful. Now it is time for my first post here

We are building a desktop medical device. The current hardware has passed 60601 safety testing, as a device for a controlled medical environment (non-home use), since it was only used as a prototype in our lab and in clinical studies. It uses a 60601-certified AC/DC PSU (24V, 2x MOPP, two-prong mains plug), runs on a 24V secondary and has a type B applied part. The casing consists of a mix of sheet metal and insulated plastic, and the AP is electrically connected to the metal casing/chassis.

We have had some issues regarding electrical noise in some of our circuits (affecting our measurement performance), and noticed some HF leakage current on the AP and casing; within limits, but enough to be able to make an LED flicker by placing it between the metal casing and some grounded metal. It can also be felt when lightly brushing the device. It's a similar feeling to operating an aluminium body MacBook while using a two-prong (non-PE) power brick.

There is now a push to certify the next iteration, including for a home healthcare environment (60601-1-11), and at the same time, the desire to mitigate the above issues by grounding the casing/chassis, ideally via a three-prong variant of the same certified AC/DC PSU. The PE aspect is not relevant, since we operate on 24V, but it could act as FE.

However, 60601-1-11 seems to put two roadblocks in our way:

- Clause 6 states that home devices shall be Class I (no PE), and shall not have a functional earth terminal.
- Clause 6 also states that home devices shall only have applied parts of type BF or CF

It seems we cannot rely on PE/FE to provide a stable ground for our sensitive measurements. And even if we redesign the casing to be fully nonconductive, fully isolating the AP from the grounded chassis will be very difficult (think: optical sensor in contact with the patient, mounted on a conductive aluminum structure for stability, and a laser whose metal housing is mounted/electrically connected onto the same structure and simultaneously connected to the GND of its power supply).

Is there a way to argue for exemptions to clause 6 via risk assessment (e.g. PE not necessary for safety, only for performance improvement; no increased risk due to AP type B instead of BF*)? Does anybody have experience with a similar case?
Thank you very much!

* In the past, we were able to argue against the need for testing according to Figure 18 (external voltage on no-PE metal accessible part) by arguing that the likelyhood of exposed external voltages in the vicinity of the device is negligible.

It will be more clear if showing an insulation diagram.
Using FE in Home is possible if FE can be treated as patient accessible parts. Which means the FE must be 2MOPP isolated form live parts and leakage current of FE doesn't exceed limits of TOUCH CURRENT and PATIENT LEAKAGE CURRENT.

 

[hoshinari]

Here is a simplified insulation diagram of the current state (no PE).
Mains is converted to low-voltage 24V in an external pre-certified PSU. There are no SIP/SOP.
The applied part is a purely optical sensor, but the sensor chassis is connected to secondary ground.

https://imgur.com/a/T6pT63E

The wish is to functionally earth the secondary ground, which would include the casing and the applied part (sensor chassis) to reduce HF noise most likely injected into this secondary ground by the built-in laser.

 

[VinceTech]

I have marked the drawing. Maybe we can start discussion from here.

Having a FE which is 2MOPP isolated form mains. This FE becomes accessible part (on enclosure) which needs to pass Patient Leakage current test.
Also, IEC 60601-1-11 requires AP must be BF (or CF) type. Therefore, 1MOPP isolated from 24V is needed.
Fig 18 testing with disconnected FE is still required. IEC 60601-1-11 required no EP or BS shall relies on FE (No FE is Normal Condition).
If AP has grounded metal parts, it could be a problem as it wouldn't pass Fig 16 test. Can we cover the metal area around sensor using 1MOPP insulation? Or, is it feasible to require the patient not connect to other equipment while using this device?

 

[hoshinari]

Thank you for the detailed reply!

I understand and agree on the isolation (B) between FE and mains.

This FE becomes accessible part (on enclosure) which needs to pass Patient Leakage current test.

I assume the Patient Leakage Current test would happen with the FE disconnected from earth, i.e. as if operator is touching the "bare" FE conductor coming out of the device, while being earthed themselves. This is OK, and essentially equivalent to the current situation of touching the metal casing while earthed.

Fig 18 testing with disconnected FE is still required.

Fig 18 was also required in our previous testing (since it's not bound only to home care env), but was ruled out in our risk management as mentioned, by arguing that the likelyhood of having some external voltage on accessible parts is negligible, and this was accepted by the test house.

Or, is it feasible to require the patient not connect to other equipment while using this device?

It is absolutely feasible, and would probably require a similar line of risk management as Fig 18. But I don't know if the test house would find this line of reasoning harder to accept for a home care env.

I am trying to understand the exact reasoning behind Figs 16 and 18. Is the envisioned situation a "random live wire" that happens to be lying next to the device and touches the patient connection / metal accessible part directly? Or, as your phrasing suggests, that the external voltage comes from e.g. a second medical device, where a MOPP has failed, and is applied to "my" device through the patient?

Can we cover the metal area around sensor using 1MOPP insulation?

It seems extremely tricky, for technical reasons that I can't go into besides what I've shown in the diagram. We are evaluating options but hoping to find a risk management way out of requiring 1MOPP, and therefore of getting by with type B instead of BF.

---

In a more general sense: even if we manage to turn the applied part into type BF, is it allowed for the *accessible parts* (e.g. non-PE metal casing) to be treated as type B for testing purposes, in a home care environment? Clause 6 of -1-11 seems to require applied parts to be type F, but does not mention any special treatment of non-PE accessible parts for the purposes of leakage current.

 

[VinceTech]

Thank you for the detailed reply!

I understand and agree on the isolation (B) between FE and mains.


I assume the Patient Leakage Current test would happen with the FE disconnected from earth, i.e. as if operator is touching the "bare" FE conductor coming out of the device, while being earthed themselves. This is OK, and essentially equivalent to the current situation of touching the metal casing while earthed.

Touch current and Patient Leakage current shall be measured with FE connected and disconnected (FE connected and disconnected are treated as NC).

Fig 18 was also required in our previous testing (since it's not bound only to home care env), but was ruled out in our risk management as mentioned, by arguing that the likelyhood of having some external voltage on accessible parts is negligible, and this was accepted by the test house.

Not sure about this. Different test house may have different opinion.
It will be good to determine if AP includes grounded metal parts (if patient will contact the grounded metal parts for normal use) before we can decided if exemption for Fig 18 test is needed.

It is absolutely feasible, and would probably require a similar line of risk management as Fig 18. But I don't know if the test house would find this line of reasoning harder to accept for a home care env.

I am trying to understand the exact reasoning behind Figs 16 and 18. Is the envisioned situation a "random live wire" that happens to be lying next to the device and touches the patient connection / metal accessible part directly? Or, as your phrasing suggests, that the external voltage comes from e.g. a second medical device, where a MOPP has failed, and is applied to "my" device through the patient?

I thought the Fig 16 test is under assumption that the patient at the same time connects to a faulty device which expose the mains voltage on patient. If AP has no isolation from it's own parts which maybe connected to earth, there will be a leakage current flowing from patient to device earth.

It seems extremely tricky, for technical reasons that I can't go into besides what I've shown in the diagram. We are evaluating options but hoping to find a risk management way out of requiring 1MOPP, and therefore of getting by with type B instead of BF.

---

In a more general sense: even if we manage to turn the applied part into type BF, is it allowed for the *accessible parts* (e.g. non-PE metal casing) to be treated as type B for testing purposes, in a home care environment? Clause 6 of -1-11 seems to require applied parts to be type F, but does not mention any special treatment of non-PE accessible parts for the purposes of leakage current.

Agree it's tricky. 60601-1-11 required AP to be F type, however, the device AP designed to be earthed (which is not floating). So it looks like we either do risk management to claim Fig 16 is exempt, or claim TYPTE BF AP exempt. Both have same meaning.
I would be wondering is it possible to make a design so that AP part doesn't have accessible grounded metal part? so that we don't need to worry Fig 18 and Fig 16 tests.

 

[Peter Selvey]

Bit slow to jump in here.

First issue is the functional earth being derived from a 3 pronged plug: Obviously an accessible FE needs to be 2MOP from mains, but a close reading of Clause 8.6.9 actually requires an FE derived from a power cord must be 2MOP from accessible parts (including applied parts), which is usually a surprise and hence can easily be overlooked. There is a historical issue here in that the original version of IEC 60601-1:2005 missed some vital text, and it was only clarified in amendments. I believe there are two reasons for this: one is that you can't plausibly have 2MOP between FE and mains in the power supply and cord, extension cords and MSOs; plus there is the risk of being used with a MSO (power strip or extension board) that has provision for earth but does not actually connect the true mains earth (especially in parts of the world where earth is not provided at the wall). That could result in high currents from other devices using the same MSO cumulating and being transferred to the frame of the medical device via the common earth wire in the MSO to the FE in the medical device and ultimately the frame.

It would seem that the original issue is the pri-sec leakage in the 24V supply, which will have a mains frequency component (the bit you can feel) as well as a high frequency component (transformer switching frequency). A double insulated power supply can still have say 50µA of mains frequency leakage due to EMC caps that bridge the pri-sec insulation, and maybe 0.1mA or more at high frequency. Although this is generally safe, it can be felt at sensitive areas of the body. In particular, the open circuit voltage prior to contacting "floating" metal parts can be >>100Vp, so just prior to contact a small arc can form, which concentrates the current, and allows it to be felt. Hence lightly "brushing" a part means you can feel it, while holding on solidly there's nothing as the current density is so low. The light brushing can also create noise. Bass players know what I'm talking about.

So, the first thing might be to consider what type of noise is causing the trouble: if the patient (or test subject) has good contact with the frame is it OK? This points to the "brushing" effect (random contact with the case) as being the cause rather than stable main leakage or switching noise. It could be for example, that the a good electrical connection between the patient and the frame is more important than connecting to mains earth. Note that since this is only leakage/noise, "good electrical connection" can mean a 10MΩ/47pF type of connection to a "grounding pad", like an ESD wrist band, using safety resistors/capacitors, it doesn't mean a electrical connection solidly grounding the patient.

Separately, it's a big job to change from Type B to BF, it's not just a matter of re-labelling. I'd lean towards risk management to keep the design Type B, as the underlying reason for Type BF doesn't really apply in this case, but the business risk of rejection from a test lab could be important to consider. You might need to design an area around the applied part that is isolated enough to meet BF requirements. But, for noise this area can still be electrically connected to the frame, just choosing capacitors/resistors that meet the necessary requirements.

 

[VinceTech]

Hi Peter,
Nice to have you here.

one is that you can't plausibly have 2MOP between FE and mains in the power supply and cord, extension cords and MSOs;

Would be it better to use a CLASS II with FE power supply having IEC 60417-6092 symbol? This will be similar to using a laptop powered by this type of supply.

plus there is the risk of being used with a MSO (power strip or extension board) that has provision for earth but does not actually connect the true mains earth (especially in parts of the world where earth is not provided at the wall). That could result in high currents from other devices using the same MSO cumulating and being transferred to the frame of the medical device via the common earth wire in the MSO to the FE in the medical device and ultimately the frame.

Is this also the case for a Class I device? How would this be considered in practice?

Thanks.

 

[Peter Selvey]

I think the point is that in homes in the USA and Japan (where I live), most wall outlets only have 2 pins. Even though there are a lot of devices that are supposedly "Class I" or Class II with functional earth, sometimes 2 pins with a flying earth lead, in practice people use adaptors, break off the pin or leave the flying lead in mid air. In reality, it's not a big risk for devices designed for 230V being used in a 100-120V systems, so it kind of flies under the radar. At least the MSO typically used in homes are only 2 wires, so there is no risk of accumulation, but you can buy the 3 wire (earthed) versions, so there is a possibility of using a 3 wire MSO but then at the wall it's unconnected to the system earth.

Anyway, the medical standard is saying, quite reasonably, look, just don't rely on earth being there in a home. If you are an Australian (which I am) or European all of this might be weird as homes in modern 230V systems do have reliable earthing at the wall socket.

This also means FE used for noise reduction or EMC compliance will face the same issue in places like Japan and USA. Design and test all you like with an FE, but in reality there's a 99% chance the FE will be floating and the noise issue is going to be there. Hence, design solutions should look at not using FE or Class I for handling noise or EMC compliance. It's a little more difficult, but not impossible with some analysis.

The only case where risk management might work is if the sales are limited to areas where earthing is reliable, for example Australia or New Zealand which I know from first hand experience. I guess Europe but it might need checking on a country by country basis.

 

[hoshinari]

Thank you everyone for the helpful input!

Regarding the PE/FE, we will probably stick to the current design with a two-prong mains plug and discard the idea of a dedicated PE/FE. It was worth a thought in order to improve noise performance and reduce the "tingly" leakage current, but those are not deal breakers (noise can be dealth with in a different way) since the device already *works*. We tested a 3-prong variant of the 2xMOPP OEM PSU and it showed no difference anyway. Obviously the PE prong would not be directly wired through to the secondary (bypassing the MOPPs) but there is not even high frequency coupling, so the effect on the noise is zero.

The focus therefore will shift to the question of BF vs B. I like the wording and want to understand whether I understood it correctly:

the underlying reason for Type BF doesn't really apply in this case

Meaning, the point of BF vs B is to add MOPPs to APs indended to actually deliver some current to the patient (electrodes etc.), where as our AP just "happens" to be conductive and grounded to secondary due to the nature of the optics+laser.

We will make sure to talk to the test house beforehand, or even talk to a second (uninvolved) test house to get an opinion without a conflict of interest for the actual testing.