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Safety Concept Description: What a Concept!

October 20, 2020

One of the toughest challenges in submitting a product to a certification agency and the subsequent certification project is getting that agency to see things your way when it comes to the compliance design details. Assuming you have done your homework, thoroughly educating yourself and your design team in designing to compliance, understanding the applicable standards, and you've done a lot of hard work marrying up the certification and safety requirements with actual functional requirements. That latter part is a real challenge, very costly in labor and high in schedule risk - conflicting requirements that result in a lot of give and take, with very little give on the compliance side and a lot of give on function, in many cases.


Now that you have a product design that you are highly confident is safe and compliant to the applicable standards, and maybe you've even worked through a Preliminary Review with a consulting house, or even the certifying body themselves, you think you are good to go. And yet once submitted, the certification agency's project engineer shoots your design full of holes.


So what happened? In many cases, it turns out to be a difference of standards interpretation, and how you present and explain your design.


I've seen this situation many, many times, and often the design is fine, but requires meetings, discussion, in some cases with the certification agency project engineer's reviewer as a referee. Certification agencies often hire new engineers as project engineers and train them in their own ways, but interpretation takes time and experience to develop maturity in it, which can't be just a matter of training. That's the purpose of the oversight of a reviewer. If you've really done your homework, and you fully understand the applicable standards, you very well may be in the right in a disagreement over compliance of design details, but it takes a reviewer to intercede and show the project engineer how you are right. However, many companies don't understand that they can defend their design, and end up changing a perfectly good design.


The result is costly project delay.


These challenges and costly delays can be avoided by improved up-front communication with the certification agency, with good documentation, a specific kind of document called a Safety Concept Description:


Safety Concept Description document


A Safety Concept Description is essentially a Theory of Operation with a focus on safety-related design details of your product. Each detail is explained, describing why it's there, what it does, and what standard and clause that detail is designed to meet.


For instance, say your product includes compliance to intrinsic safety, you may describe a circuit that is two or three parallel shunt diodes with "infallible" traces between them, to limit voltage to a particular section of circuitry - you explain that what this circuit is, describe the diodes and refer to datasheets or other documentation, and state that this circuit is designed per IEC 60069-11 (or UL 60079-11 or other), and you also reference the applicable clauses within the -11 standard.


It is also wise to refer to your applicable schedule drawings. For example, in the instance described above, say you have that circuitry detailed on a schematic or Construction Control drawing*. You would include in that circuit's description a reference - maybe the coordinates in the drawing, and a reference to the flag note in that drawing describing the requirement for the infallible trace.


Circuitry isn't the only type of detail that you would want to include in a Safety Concept Description. Examples of other items would include:

  • PCB layout details
  • PCB layer stack-up, spacings affected by the same
  • Insulation materials & dimensions
  • Potting (encapsulation) details - materials & dimensions
  • Wiring, wire routing
  • Mechanical details - materials & dimensions, surface areas of exposed non-metallic materials (as defined in IEC 60079-0)

Any design detail that exists for the product to be compliant to a specific safety standard should be included.  If you have to make some aspect of the product a certain way due to a clause in a standard, include it. If you find you had to refer to the standard to tweak any aspect of the design, that should be included.   This would include non-haz/loc (ordinary location) standards such as IEC/EN 61010.


The Safety Concept Description should be a companion document to your schedule drawings you submit, to explain why your design is why it is.

The advantage of this approach is to defend your design UP FRONT. This avoids misunderstandings on the certification agency's part.


Also, it shows the certification agency that you are highly prepared, serious about certification, care about safety and your design, and educated in the standards. This goes a long way in developing a positive relationship with the agency.


For detailed teaching on how to do a Safety Concept Description document effectively, reach out through my contact page.  I'd be glad to offer my services to teach this highly valuable concept, and help you get to market faster!


*A Construction Control Drawing is type of drawing, that defines the safety related details of your design that must be governed by your certification, and would be the Schedule Drawing that governs your design, rather than your manufacturing drawings & bills of material themselves.  I'll likely write a Blog entry on this in the future.  For more detail, please contact me using the contact page.  I'd be glad to describe the value of this drawing, and offer my services to show you how to do this type of drawing effectively.

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