Battery powered device - electrical protection requirement

sreibs

Starting to get Involved
#11
Why do you need 2 MOPP against the 5V? - Of yourse a current can flow in a battery ooperated device, that's actually what the battery is there for ;-) You have to make sure, that the patient cannnot connect the (-) and (+) of the battery at the same time, eg. with two fingers. In this case a patient leakage current of 5mA would flow which exceeds tha maximum allowed values. Sometimes there are mounting screws or other conductive parts which are accessible to the patient and connected to battery (-) at the same time, e.g. via the mounting hole on the PCB and a common ground. In this case you have to assure, that the patient is separated from the battery (+) with 2 MOPP.

For a working voltage of 5Vdc the corresponding dielectric strength test voltage is 1000V for 2 MOPP, that is correct. But maybe it is possible to apply two independent 1 MOPP with 500V each only? E.g. if you have areas, where you need to protect from the battery (+) or (-) only?

Regarding the creepage and clearance for 2 MOPP. I think the required creepage of 3,4mm is to large. Creepages can be interpolated. In my oppinion it is possibel to interpolate between 0mm for 0V (which is not in table 12) and 3,4mm for 17V (first line of table 12. For 5V you would end up with 1,0mm. BUT: The creepage cannot be less than the clearance, which is 1,6mm (no interpolation possible).

Again it might be possible to seperate the required creepage/clearance into two single 1 MOPP with 0,8mm each?
Hi Benjamin,

I think this section "8.9.1.6 * Interpolation" says that we cannot interpolate between 0 and 17V :(. Too bad
 
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sreibs

Starting to get Involved
#12
My Twopence worth...
Given the geometry you have outlined, and the test house comments, I believe they are looking for 2 MOPP because they consider the camera and SPO2 to be separate (type BF) applied parts. This is to limit the potential for auxiliary currents between each of the applied parts (leakage currents are not possible because the charger cannot be connected while the device is in use).

Given the small size of the device, and only 10cm length on the camera, and that the camera is not detachable, I would consider the SPO2 and camera as the same TYPE BF APPLIED PART, (i.e. the whole device, excluding the charger, is a single type BF applied part) and this will greatly reduce the testing burden.
Hi Jordan.

We were in touch with the labs again. They are ok with considering the whole product as a single BF applied part. Also, we decided to add a cover glass on top of the cover glass of the SPO2 sensor (ridiculous yes), in order to implement 2MOPP (creepage distance).

However, they still want 2MOPP on the camera side. They say it is to avoid current flowing from the battery (minus or plus) to the earth, through the patient. They say that the standard says in 8.5.1.1, that it must be compliant with 8.4.2 which point to 8.7.4 and the creepage distance we are talking about.

I totally don't understand why I need to put 3.4mm between battery poles and the earth. Is it really the correct interpretation of the standard ? Thank you for any help !
 

jdoran

Starting to get Involved
#13
Hi Sreibs,
That is good progress that your test lab will accept the device as a single type BF applied part!

I can understand the need for some means of protection given that the device is 30cm long (i.e. reducing risk of auxiliary currents), but I don't think it makes sense from the perspective of preventing currents flowing from the battery to earth through the patient unless there is a path for the return current to flow from the battery to earth. Is there a return path? i.e., could the USB connector be exposed that could contact an earthed part such as a part of a hospital bed?

One other thing to consider - is the device only intended to be used by a patient that is also the operator?, i.e. is the patient always conscious, and in for example a home environment? If that is the case then the test lab may accept the use of MOOP (means of operator protection) instead of MOPP which would significantly reduce any insultation requirements.

J Doran
 

sreibs

Starting to get Involved
#14
Hi Jdoran,

Thank you for your reply. Well, the MD can be used by an "assistant" on non autonomous patient (old people for example).

To me there is no return path possible. There is no metal parts accessible, they all have solid insulation. Hence, even if the patient touch the metal on the tip of the camera (which is, by the way, the battery minus pole), there is no way the current can come back. The USB connector is behind a waterproof silicon cap, so there is no way someone can touch the metal part of the connector. Even if the silicon cap is away, the connector is recessed in the plastic enclosure which make it almost impossible to contact with an earthed part unless it is sharp.

I will try to draw an insulation diagram to make it clearer.

Thank you for your help,

Best regards
 

sreibs

Starting to get Involved
#15
Hello,

Here are the news. It is actually ok from the "battery to earth current flow" perspective. However, we need to make sure there is no way the patient can access two opposite pole of the battery (so GND and any other V+ in the system), which is ok to us. However two questions about this:
- I cannot find in the standard the reference paragraph of this requirement ?
- The only site this could happen in our product, is an opening of 3.5mm diameter. In my opinion, if the user touch this part and (unlikely) is in contact with both GND and 5V, I guess the current won't flow into his whole body to come back in this opening.. but I cannot find proper justification for this.

Thank you for your help
 

jdoran

Starting to get Involved
#16
@sreibs ,


Categorizing the potential electrical hazardous situations may help the discussion.

1) Leakage current - not possible with your device if the USB connector cannot connect to other equipment during use (you can address that in your RM file)

2) Mains on patient (from another device failure) - as 1), low risk if there are no current paths back to earth

3) Auxiliary currents -

a) between the main enclosure and camera - the risk of AC currents above the acceptable limits (100uA/500uA) is very low for your equipment. The risk of DC currents exceeding 10uA is probably higher though, especially if your assertion that the camera tip is connected to the GND net (0V).

b) between the main enclosure and SPO2 sensor - as above, but lower risk if there is no direct conductive path to the SPO2 PCB

4) Touch currents between opposite poles of the battery inside the enclosure - the risk is negligible, and you can address it as such in the RM file. You can easily demonstrate that the severity of harm AND the probability of occurrence are negligible if the surface areas over which the poles of the battery can be touched are small, because the voltage is low and the impedance of breached or unbreeched skin is relatively high compared to the MEASURING DEVICE in 60601-1. For example, in reference [1] Reilly, pp 36, Figure 2.17, the impedance could be hundreds of times higher than that assumed by the safety limits in 60601-1 (e.g. 100kOhm instead of 1kOhm), and the auxiliary currents correspondingly could be 100's of times lower. I have used this to minimise the MOP required in previous designs wherein the geometries of signal nets are very small compared to the 82 cm2 that is typically considered for the full hand.

One method to assess the risk of 3a is to measure the auxiliary current between the main device and the camera while in use, e.g. by submerging both ends in separate saline baths and using the measuring device is clause 8.7.4.4. to compare against the limits 100uA AC, 10uA DC NC, or 500uA AC , 50uA DC for SFC. The results of this test will indicate whether you need to rely on the enclosures as a means of protection.

The only slight concern I would have for you current design (as I understand it, and without having seen any drawings or an insulation diagram) is that the camera patient connection is connected to your 0V net, which could allow DC auxiliary currents a path to flow under a fault condition of e.g. the main enclosure getting wet or the main enclosure plastic insulation breaking. If the camera patient connection must be connected to 0V for some functional reason (or for EMC), is there a possibility that it could be connected via a capacitor, which would serve to block the path for DC auxiliary currents to flow?


J. Doran

References:

[1] Reilly, J. Patrick (1998). Applied Bioelectricity: From Electrical Stimulation to Electropathology (2nd ed.). Springer. ISBN 978-0-387-98407-0. LCCN 97048860. OCLC 38067651.
Picture1.png
 

jdoran

Starting to get Involved
#19
@sreibs
If there is a small creepage, and not solid insulation between GND and camera surface, is there a reasonable probability that moisture / fluid can bridge GND and camera surface during normal use?
 
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