Creepage and Clearance for CR2032 powered fob for a Medical Device

C

CaliforniaMike

#1
Hello,
I am working on a small CR2032 powered fob for a medical device and my interpretation of the necessary creepage and clearance seems silly.

Basically, the lowest values that I see are 3.4mm / 1.6mm. My interpretation is that this means that I need a fuse with 3.4mm of clearance between terminals and 3.4mm all the way around the coin cell holder. This is a large portion of my available board space.

The nominal internal resistance of a CR2032 is 10 ohms so the most power that I am going to dissipate in a full short is 0.9W which is well below the 15W where this would be considered a fire hazard. Also, the entire device is enclosed in a small polycarbonate enclosure so I dont see a real hazard here.

Is there some consideration that I am missing to get me back some of the 3.4mm or am I stuck with this? Can I ask my test house to short out a few batteries from a few manfacturers and show that the enclosure does not lose integrity or become too hot and consider this a pass by testing?

Thanks,

Mike
 
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Peter Selvey

Staff member
Super Moderator
#2
Re: Creepage and clearance for CR2032 powered fob

You are right, the creepage limits for <50V region are silly. During a training session on the 3rd edition a few years ago I quizzed the trainer who is on the standards commitee why these large limits remained (in Table 12), but he could not answer.

It is reasonable to have creepage distance around parts to prevent failure in normal condition, however, at 3V an appropriate limit on a printed circuit board based is just 0.04mm (based on IEC 60664-1), not the whopping 3.4mm.

But, under the 3rd edition the situation is still greatly improved. First, limits for spacing (creepage distance, air clearance) are only applied for protection against electric shock. So, for fire, battery explosion and other hazards spacings do not need to be measured. Even if was applicable, the limit for a battery operated circuit has been effectively reduced to 0.4mm.

In any case, most test labs ignore creepage distance measurements in secondary circuits (regardless of 2nd or 3rd edition).

However, if you are working under the 2nd edition, I would be careful about asking a test lab for an opinion. In that case there is a risk they might follow the standard and ask for 3.4mm!
 
#4
fob = small pocket for watch etc. formerly made in waistband of breeches. (OED).

Except here it pertains to the sort of lump on your key chain that you might then put in such a pocket.
 
C

CaliforniaMike

#5
Re: Creepage and clearance for CR2032 powered fob

Peter,
Thank you for the detailed help on this. Knowing that I can go to 0.4mm is an immense help. Can you tell me where in the standard to find the battery operated exclusion. I just started parsing the 3rd edition and none of my old notes apply to the new organization.

For reference, I just ran a shorted (0.15 ohm load resistor) CR2032 to empty and though there was a brief (10 seconds or so) period of current on the range of 400ma, after that it quickly dropped to 100ma and tapered off from there. The cell was at its worst lukewarm to the touch. Pretty tame. I will document this more carefully for our file and post if I see anything that deviates from this.

For the record, fob = small keychain remote control.

Thanks all. Greatly appreciated,

Mike
 

Peter Selvey

Staff member
Super Moderator
#6
I'm not sure if it is intentional by the committee, but the exclusion of creepage and clearance in your case is based on the widespread use of the phrase "means of protection" (MOP) throughout Clause 8, and a MOP is defined as something that protects against electric shock.

I'm sure some will disagree, and since shorting a component is still considered a fault condition, it is a moot point anyway. If your battery connects to an IC, it is allowable to short the IC and you end up with the same result.

Shorting a CR2032 is probably safe not because it does not have much energy but because it is so dangerous that manufacturers have to fit up to three types of protection systems inside the cell to prevent explosion. Without this protection, under short circuit Li-ion converts to Li which then explodes in a spectacular fashion.

But this protection is not always reliable thus it is a good idea to avoid shorting cells through good design. Normally this would include:

- if possible a series resistor which limits fault current, value selected to be the highest that does not impact normal operation or significantly reduce battery life. Reliable low value fuses are difficult to find and often resistive anyway, so may as well go with a known resistor.
- good mechanical restraint, don't use cheap battery holders
- reasonable spacing of metal parts around the battery, accounting for possible movement
- vibration/normal use testing, check for movement of the battery
- if replaceable by the user, make sure it is difficult to short (usability testing)

etc
 
C

CaliforniaMike

#7
Peter,
Thanks again for the feedback. It is greatly appreciated. I am the lone electron guy at this company and it helps a lot to get some input from outside of the bubble.

I tend to tryto find the worst case scenario and then work back from there. The short test was performed under controlled conditions (inside a 1/4” thick aluminum chamber.) to get an idea of this worst case scenario. We use Li ion packs for other products and have integrated all of the over current/ overvoltage / over temperature protections for these. Coin cells are a new one for us. After looking at the information on the web, these do not seem to have any of these.

I suspect (dangerous word) that the worst case in these cells is limited by internal resistance. At the stated initial 10 ohms, perhaps intrinsic or perhaps put there as a protection, the maximum current available is 300 mA. (I saw ~380 mA for about 100 ms and then it dropped to 100 mA at 7 seconds.) At this current, the maximum power being dissipated is <1 watt which in something as large as a 20mm disk is not going to get that hot in a few seconds. After the test, the internal resistance was up to close to 30 ohms.
I suspect (there is that word again) that the short duration of 300 mA at the start and the gradual increase in impedance over discharge points to the current limiting being a reaction rate or surface area based phenomena and hence an intrinsic limit for the CR chemistry.

Our peak current draw, for a few ms, is about 30 ma. I have a 100ma fuse in line resulting in a peak power of 0.3 watts. (Though I acknowledge the issue with lower current fuse accuracies, I cannot afford to throw away any more battery drop when I need 2.0 V ot operate and already have 300 - 900 mV going into internal resistance over the life of the cell. Also, if the fuse is as far off as 200ma, we are still limited to 600 mW for a few seconds. I also have ~ 1/2 mm clearance to all parts prior to the fuse as well, a polarity protection FET. I have based clearances to the battery on worst case clip placements and battery insertion geometries.

All of our board designs go through HASS so they will get the heck shaken out of them and any mechanical weakness should show up quickly.

I think we are pretty safe.

This said, with a standard PCB coin cell clip, with the positive cage extending around the top and down the sides of the cell to the PCB, if the cell is placed in upside down, it is likely to short through the battery clip to the side of the cell and create a current path back to the top of the cell. In fact, most designs virtually guarantee a short if you put the battery in firmly. I do not see any way to protect against this with any of the PCB clips. (Check the BK 912 from memory protection devices for a reference.)

If there was a problem, there should have been a number of documented failures and that this would not be the standard PCB clip design for nearly all of the battery clip companies.

I have searched the web and in the tech journals and while I find numerous warnings about small coin cells, I cannot find a single report of them loosing integrity under shorted discharge conditions or from being inserted upside down in a clip, or carried in a pocket with change or the like. (I have seen some videos of them exploding, but these were the result of running 240 VAC through them. Somewhat fun, but not very helpful.)

So…
I currently plan to move ahead with the standard protections (fuse, spacing, polarity protection , mechanical /vibration testing) , performing and documenting short tests on the commercially available CR2032s, and at this point trusting that the industry would have done something by now if shorting a cell out by placing it into a PCB clip upside down was really a risk.

Am I missing anything here? Anyone have a documented case of a serious failure caused by a shorted coin cell?

Thanks again,

Mike
 

Peter Selvey

Staff member
Super Moderator
#8
Maybe the same here, most of my research is based on larger packs which are of course more of a concern.

From previous research on larger Li-ion cells (2400mA types), I got the impression that the failure rate of internal cell protection when shorted is around 0.001; i.e. one in a thousand will still explode when shorted.

I found at least one reference showing that CR2032 do have some safety features, and a PTC device could be the cause of the increase in impedance (drop in current) when shorted.

But, no references of any failure of CR2032 protection. So I guess the failure rate of the protection for coin cells is much lower, maybe 0.0001 or less. Combine this with good design practice and the probability of serious harm is likely to be well below the proverbial one in a million.
 
C

CaliforniaMike

#9
All,
Just a quick followup and thanks. We finally made it through our year of patient testing, compliance, and have been out on the market long enough to see that we did not miss anything.

Thanks to the help here, we sailed through 60601 without a need for any revisions. Thanks. Nice to have a bit of backup when things get odd.


Mike
 
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