IEC & UL 60601-1 Plastics Flammability - Guidance on flame-rated plastics usage

E

epierce

#1
Hi all,

We're looking for guidance on flame-rated plastics usage in our power wheelchairs. RESNA standards are our typical go-to, but unfortunately they only address upholstery flammability. I've tried to dig through these two standards for guidance, and I wonder if anyone could just verify that I'm interpreting them somewhere near correctly..

IEC-60601:2005 seems to only dictate material flammability (among other things) for "fire enclosures," which are intended to mitigate the risk associated with circuitry where a single fault test has revealed a heat or ignition hazard.

UL-60601:2003, on the other hand, says that all polymeric external surfaces on a device need to be UL94-V2 rated (in the case of a mobile device).

To apply this to a typical situation for us.. Let's say I have a drive motor that's well over 15W, with a plastic shroud that covers it. My read is that as long as the single fault tests (capacitor open/short and max voltage w/ locked rotor) don't result in excess temperature or emission of flames, then IEC says that the shroud doesn't need to conform to fire enclosure requirements. Beyond that, it has no requirements. UL would count the shroud as an enclosure, and therefore would need V2 rating.

Am I way off??

Thank you!
Eldon
 
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Peter Selvey

Staff member
Moderator
#3
Basically your reasoning is correct, but you have to be careful to cover all component fault tests not just the motor (every resistor, PCB track, connector, wire etc becomes a potential source of fire, at least in theory).

If you choose the fire enclosure option, it means you don't need to worry about this detailed component fault analysis.

Amendment 1 to IEC 60601-1:2005 has increased the limit for secondary circuits from 15W to 100VA provided the PCB and wiring is flame proof. This makes sense as it is usually much harder to start a fire from secondary circuits, especially if the PCB and wiring are flame proof.
 

Ronen E

Problem Solver
Staff member
Moderator
#4
Hi Eldon and welcome to the Cove.

Why not specify UL94-V2 conforming plastics and be done with it? V2 is pretty mild and it shouldn't be a problem identifying conforming resins.

Cheers,
Ronen.
 
E

epierce

#5
Thanks all for your responses, and over the Easter weekend no less!


Peter: Our motors/controllers are generally just over that limit, unfortunately. Due to complications you describe (and common sense), we'll surround our controller circuitry with fire enclosures, of course. I need to look into it in more detail, but our motors might actually be classified as being "enclosed" as well, as the motor housings are generally of a vent-less design, though there are some small gaps in the sheet metal.


Assuming that our controllers and motors are fire enclosed, that generally leaves the wires going between them. Enclosing them is not practical in our application, as well as may others, so you must just confirm that the wire can handle any single fault current (e.g. fuse failure) without lighting on fire? Maybe I missed it, but I didn't find any specific tests for wire flammability.




Ronan: That has been our approach up until now. You're right that the difference in cost and effort is generally pretty minimal. However, there have been a few instances where it has impacted function and/or aesthetics. Adhering 100% to the V2 blanket rule also doesn't make much sense when you start looking at small off the shelf parts like nylon washers that are nowhere near a power source, but technically "external." Seems like there will always be exceptions, but we would like a consistent rationale for our design control process.
 

Peter Selvey

Staff member
Moderator
#6
It is not difficult to find off the shelf wiring which is flame proof. But if not, you could select the worst case fault current and prove that interconnecting cable is not capable of causing fire.

The catch is flexibility: a design change around that area means you need to repeat the tests or at least do a paper review. Using flame proof materials exempts you from worrying about this (there are of course concerns for the environment ...)
 

Ronen E

Problem Solver
Staff member
Moderator
#8
Ronan: That has been our approach up until now. You're right that the difference in cost and effort is generally pretty minimal. However, there have been a few instances where it has impacted function and/or aesthetics.
This is what plastics engineers are around for. I'm 99% sure that a V2 grade can be found for 99% of applications, with minimal impact on cost, function and/or aesthetics. Most real-world problems have more than 1 viable solution, and it's basically a matter of choice (or bias) which one to pursue.

Adhering 100% to the V2 blanket rule also doesn't make much sense when you start looking at small off the shelf parts like nylon washers that are nowhere near a power source, but technically "external."
Is there anything considered "internal"?... Have you fully sorted out what does "external" mean in that context? I'm not challenging your judgement, I just don't have that standard in fornt of me. Either way, there are V2 Polyamide grades available, and anyway, why does a washer have to be of PA? I understand that this was just an example, but my point is that IF you had an embedded V2 requirement I guess you could have sourced all off-the-shelf components so that it was met. I always prefer upfront compliant design over testing.

As a generalization, the earlier you deal with issues (along the design progress axis), the cheaper, quicker and easier it is.

Cheers,
Ronen.
 
E

epierce

#9
Thanks again everyone, great discussion here.


Peter: maybe I'm misunderstanding what you mean by flame proof, but we do use wiring with flame retardant insulation (VW-1). I'm not convinced that this would be enough to eliminate fire risk. Maybe you aren't suggesting this, but I would think that if the surrounding materials (e.g. shrouds) were not flame retardant as well, then you'd have to test the entire assembly anyways.


Ronan: I haven't found much in Ul-60601-1 by way of interpretation/exception for "external."
Their definition is "exterior surface of equipment including:
-all accessible metal parts, knobs, grips, and the like;
-all accessible shafts;
-for the purpose of tests, metal foil, with specified dimensions, applied in contact with parts of the exterior surface [...]"


Perhaps they have devices in mind with a well-defined housing (?). The definition is more clear in this case.


Cheers,
Eldon
 
#10
Hei Eldon,

I have one more comment to this post:
Why you want to fullfill UL 60601-1:2003, as ANSI/AAMI 60601-1:2012 is already announced as state of the art? At the end of the day the requirement UL94-V2 for a housing of transportable equipment is obsolete as UL 60601-1:2003 has been superseeded by ANSI/AAMI 60601-1:2012.

Furthermore in respect to fire enclosure my opinion is, that a fire enclosure is the last possibility for a product to comply with the standard. Always try to reduce the risk of fire in a product by protective measures which are based on protecting and limiting elements (like fuses, current limiters, etc.) in your electronics so that any failure in your device does not end in a hazardous situation. Only use fire enclosures when there is no other possibility to reduce this risk.

Cheers
Chris
 
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