Convert Class B Medical Device to Class A

W

wally4u

#1
Hi everybody,

We have a simple medical device where small electric currents are generated.
Due to the nature of the device, delivering electric currents to a user, it's possible to cause skin irritation when used incorrectly. The software embedded in the device actively prevents this.

The way I read the IEC 62304 software classifications ANY injury (irritation/brusing/burns/etc) will cause the system be class B. So when the software has a runaway process the safety features could not function properly. I prefer not to have to write all the class B documentation. So I would like to classify the device as Class A.

I was wondering if a hardware watchdog (either internal to the MCU or external hardware) would be sufficient to guarantee that the software is properly running with the software safety measure active (not a runaway process), and thus allowing for Class A.

Any ideas if this would be acceptable?
 
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glork98

Involved In Discussions
#2
I gotta say "no."

How about the software is just wrong? Sounds to me like you've got a Class B product and need to show that it won't hurt people and that the safety is proven to a high level of confidence.
 
W

wally4u

#3
Hi Glork98,
So basically what your saying that using a watchdog that trigger the device to go into a "safe state" when it isn't activated once in a while (only possible when the software has gone bad) isn't sufficient?

My main issue is that one of the safety features is implemented in software, and I need to make sure that is still active. Section 4.3 notes:
If the HAZARD could arise from a failure of the SOFTWARE SYSTEM to behave as specified, the probability of such failure shall be assumed to be 100 percent.
If the RISKof death or SERIOUS INJURYarising from a software failure is subsequently
reduced to an acceptable level (as defined by ISO 14971) by a hardware RISK CONTROL measure, either by reducing the consequences of the failure or by reducing the probability of death or SERIOUS INJURYarising from that failure, the software safety classification may be reduced from C to B; and if the RISKof non-SERIOUS INJURYarising from a software failure is similarly reduced to an acceptable level by a hardware RISK CONTROL measure, the software safety classification may be reduced from B to A.
So wouldn't a hardware watchdog count as a risk control measure? Since it will guarantee that either the safety feature is active or the device is set to save mode by hardware.

On a side note, do you agree with me on the assumption that ANY injury (irritation/brusing/burns/etc) will cause the system be class B?

Or should I just have to document the safety features as class B and the rest of the device as class B (or does that depend on how good I can partion the device in software items?)
 

glork98

Involved In Discussions
#4
So basically what your saying that using a watchdog that trigger the device to go into a "safe state" when it isn't activated once in a while (only possible when the software has gone bad) isn't sufficient?
You need to show that it is mitigating the only possible failure mode. My experience is that software can fail to operate correctly in many (many, many) ways. This includes a failure to detect and mitigate hardware faults.

On a side note, do you agree with me on the assumption that ANY injury (irritation/brusing/burns/etc) will cause the system be class B?

Or should I just have to document the safety features as class B and the rest of the device as class B (or does that depend on how good I can partion the device in software items?)
That's what B is: ability to injure.

If you're getting a splitting the code into A and B then, yes, that's something that's done a lot. The critical control code can be B while file output or UI display stuff is A. If the operator can enter settings or configuration data is read then that becomes B, too, as a mistake will propagate into the control code. Also, bad pointers or stack overruns and other failures that have unintentional side effects push code on shared processors into B.

It's hard to quantify all the situations and cases. That's what 62304 calls for. Do an architecture and then do a hazard analysis and see what comes out of it. Since your product can produce an injury you need to follow that path.
 
W

wally4u

#5
Thanks for the info. So basically section 4.3 doesn't apply since the device itself is a Class B system.
 
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