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Just got assigned (sole) responsibility of a major product group in my company. I have discovered that we regularly use components outside of their specifications, and justify that by testing the snot out of them.

Specific example --- We have a switch rated at 10A 240V. But we're switching maybe nominal 20A @ 12V load which is a motor and thus can ALSO suffer massive surge (startup/stall) currents far larger than 20A and of course also experience inductive arcing.

Been doing it for years apparently. They just test a bunch of samples of the the switch to failure and get the vendors to promise if they change the switch in any way, they'll contact us.

So am I nuts to think that's nuts? That we shouldn't just seek out switches that are actually rated for the loads we're putting on them???

Kyle B
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    I mean...I suppose on some level it makes sense since some human, somewhere is going to end up having to test it and decide its rating anyways. Whether it is at the manufacturer or not doesn't make too big a difference as long as the quality of testing is the same. And just because something is not designed with a particular use in mind doesn't necessarily mean it can't fullfill that use. – DKNguyen May 20 '20 at 18:52
  • Manufacturers will only guarantee operation within the specifications however much you test. It might test ok and fail as soon as you use it - or never. – Finbarr May 20 '20 at 18:53
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    This may be getting into the realm of opinion, but I don't think it is nuts at all. Relying on results from empirical data you yourself collected as a company is the most solid way to validate or verify almost anything. As long as the test suite is good and adequate. It only requires you to trust the vendor to be responsible about design changes. If there is safety involved, it would be a good idea to add an incoming quality inspection where you test a few samples from every shipment to make sure the vendor didn't accidentally forget to tell you about a design change. – user57037 May 20 '20 at 18:55
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    What if a component does fail (for whatever reason) and someone replaces one with the same "ratings" but that hasn't been tested with the same rigor out of its specs? Also it just kinda makes things hard to track, no? I guess if it works it works, but a lot of "what ifs" are involved. (I dont have experience with mass manufacture but.. thats how I see it) – Wesley Lee May 20 '20 at 18:58
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    "We have a switch rated at 10A 240V. But we're switching maybe nominal 20A" if you have 20A RMS going through the switch your are way overboard, you are dissipating 4x more heat than if you had 10A RMS, do they check for temperature on those switches? Also, I don think arcing will be a problem for those values, do they specify what is the interrupting current (it is usually way higher than the nominal one)? – jDAQ May 20 '20 at 18:59
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    Even the UL will sometimes let you use an unlisted component if you verify its function with test. Of course if you use a listed component they may not require you to test so sometimes that is easier/better/cheaper. – user57037 May 20 '20 at 19:00
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    @Wesley Lee Well, I believe that would fall under "this equipment has no user serviceable parts inside". – DKNguyen May 20 '20 at 19:01
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    @jDAQ you are assuming the limitation on the switch is thermal. But you don't know that. – user57037 May 20 '20 at 19:01
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    Here is a question. Don't answer here just think about it. Has the company you work for been in business for a long time and are their products reliable? If so, be humble and learn. If not, maybe you can employ more discipline and turn it around (or find another job). – user57037 May 20 '20 at 19:16
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    @mkeith I am assuming the limitation on switches can be temperature from professional experience, at 12V they are very far from having dielectric breakdowns. Also, I did also mention checking for the current interruption limitation (other source of sticking contacts). – jDAQ May 20 '20 at 19:17
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    @jDAQ I bow to your professional experience. Also, after I typed my comment I tried to think of what the limit would be if not thermal and I didn't have any good ideas. The OP also mentioned the arcing. In any event, though, a testing program will uncover all these problems. – user57037 May 20 '20 at 19:41

3 Answers3

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I am going to defend the practice. As long as your company is willing and able to do the necessary testing to qualify the components for use in your company's products there is nothing wrong with violating data sheet parameters for the component. This does require some knowledge and skill and experience because the test suite needs to be well designed and properly executed. It may be easier to find components that are rated for your intended use because intense component qualification programs do require the company to commit a lot of resources.

Obviously, it is important that the supplier have good process controls in place so that you will not encounter unexpected variations. This is definitely something that needs to be discussed with the supplier so they understand that you need to be notified of any design or process changes for the component. Many suppliers are accustomed to operating that way already so this is not necessarily a new burden for the supplier. There is the question of recourse in the event of failure. If you have problems with a batch of parts, the supplier may not take them back if they meet datasheet parameters. So dialog with the supplier ahead of time is necessary.

Ultimately, products made on assembly lines are generally very similar to each other. That is the whole point of the assembly line. It is reasonable to assume that products made tomorrow will behave the same way as products made yesterday on the same line. When clusters of defective products are found, they can usually be traced back to a change of some kind in process or testing or a supplier who made a change without telling the customer.

Once during product qualification for a consumer product I found that there was a bus timing violation. We were not meeting the setup time. My boss told me to talk to the supplier and find out what the real setup time was, as opposed to the one in the datasheet. What? WTF is he talking about? But he was right. The datasheet specification was an exact, parameter-by-parameter copy of a part they were competing against. The real setup time was much, much more permissive and they performed 100% internal testing to verify it and showed me test reports. They would not change the datasheet because the parameter match parity was too important. Sometimes component engineers might reject the part as a drop-in replacement if any parameter is different.

So, sometimes the motivation behind specifications in a datasheet is not what you think.

I am very skeptical, honestly, that a 10A switch could handle 20A. My comments are more on the concept of qualifying by test for things that are outside the datasheet specification. Not necessarily in direct excess, but just outside the scope. I got the impression from the OP's post that the question was about the general concept and not about the specific switch (which does sound dubious). But if someone asked me to approve using a 10A switch with a 20A load I would start by doing a heat rise test and then try to weld the contacts by interrupting an inductive load like a motor.

But also, it is a mistake to ignore history. If the switch has been in use for mass production for a long time and it is well known that it has not caused reliability problems, then for goodness sake don't change it now.

user57037
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  • If a company is going to "cheat" on the ratings for a part like the switch in question, and use it outside of it's ratings, then I doubt if that company has the where-withal and organization in place to verify " that the supplier has good process controls in place." – SteveSh May 21 '20 at 10:51
  • @SteveSh it seems like your applying moralistic judgement to an engineering and manufacturing scenario. – user57037 May 21 '20 at 19:00
  • It seems more like you're trying to rationalize a hack approach to designing a product. Did you read my answer below? – SteveSh May 21 '20 at 19:17
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So am I nuts to think that's nuts? That we shouldn't just seek out switches that are actually rated for the loads we're putting on them

I also think it's nuts - find a switch that can handle the current or get the switch supplier to justify that the current and voltage profiles seen in your application are acceptable (despite their data sheet saying otherwise). Get them to underwrite your application.

Get it in writing and get them to think seriously about adjusting their data sheet to cover your application. Of course if the switch supplier has a dubious quality system, you can't rely on them so find an alternative is my strong advice.

I would not be happy about using components outside a specification provided by the supplier.

Andy aka
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To a large extent, the answer depends on what are the consequences of a failure of that part (a switch in this case). If the failure is that the motor doesn't turn on and you can replace the switch relatively easily, that's one thing. If the problem is that the switch doesn't open up (because of welded contacts) and the motor continues to run, that may present a different problem, and maybe create a dangerous situation.

We create special drawings to cover use of a part at points in its operating region that may not be explicitly called out on the data sheet. For example, a device may be specified as being able to source 20 mA with a 100 ohm load on it, and has a max output current rating of 50 mA. But if we want to use it with a load of 30 mA and still need 2V across the load, then we may have a test for that condition in the part drawing. Also, the drawing would call out testing the part across temperature range that is larger than what the part would see in operation. If the expected operating temp range is 0C to 40C, we may test the part from -20C to +60C, in order to weed out marginal parts.

But, and this may be unique to the mil-aero industry, we would never use a part beyond the limits specified in it's absolute maximum rating table of the data sheet. Never.

SteveSh
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