Dave, You pretty well outlined the main concerns for safety critical components.
Identifying all the hazards and risks is the first step and then identifying the components relied upon to mitigate those issues is next. As product testing proceeds there may be additional components added. I have certified products with only a handful of critical parts and then others with a list of several dozen. Some may be component parts and others may be outsourced sub-assemblies or purchased parts. There are some obvious choices such as enclosure (material, thickness, etc.), power inlet/outlet modules, brick power supply, fusing, etc. Others may not be so obvious. Outdoor equipment may have thermal performance tied enclosure paint color, high leakage current equipment may have a particular hardware stackup and paint mask for grounding, high voltage resistor feedback strings may have components that are tested for high reliability, you may use conformal coatings for pollution/corrosion control, custom isolation transformers should be qualified and controlled for each manufacturer, circuit networks relied upon for protective impedance must have a controlled schematics and BOMs, and the list goes on. For externally sourced modules and assemblies, it is always best to try for certified materials, appropriate to your application. For example, brick power supply certified to 60950 for OV Cat II may not be appropriate in a product being designed for use in OV Cat III environments. Conversely, I was once involved in a product design which had a large DC rated fuse at a particular point deep inside. It was not relied upon for mains protection but the design engineer felt it would be prudent to include for minimizing damage in the event of an IGBT failure. The fuse cost $70. After completing all of my review and testing for performance, foreseeable faults and abnormal operations, that fuse never opened. Not only did I remove it from the list of critical parts, I gave the engineer the option to remove that fuse from the product. It served no useful purpose. One last point, any components included on your list should also pass through incoming inspection and a minimum list of attributes verified on each part before incorporating them in a product build. This alone is a reason to keep your critical components list as small as possible. But no smaller. Hope this helps. In the end, you will find selecting safety critical parts is a bit of an art. And it is likely that no two safety engineers will produce exactly the same list. Finally, it must be received and approved by your certifying agency. Thanks, - doug Douglas Powell http://www.linkedin.com/in/dougp01 Original Message From: Nyffenegger, Dave Sent: Saturday, February 22, 2014 7:41 AM To: [email protected] Reply To: Nyffenegger, Dave Subject: [PSES] Critical Components definitions Hi Folks, I'm looking for some more recent definitions/guidelines/criteria for determining what constitutes critical components for the critical component lists in the product technical construction files, in particular the files to be maintained as part of CE compliance. I am not un-familiar with the topic but would like to see what's out there. There are some very old threads regarding this topic in the forum archives so I'm thinking there may be more recent material on this somewhere. I'm primarily looking for the safety related components. There's a lot of threads on EMC critical components. Certainly any components which improper operation or failure could cause unsafe operation of the product would be critical. Then there is the question of the components that could result in other safety concerns such has fire or stored energy release. In particular would we expect to include individual power supplies of commercial equipment (which typically are either integral with the unit or come with it as an external plug in "brick") that are incorporated into the product in the critical components list for example PC power supplies, display monitor power supplies, printer power supplies. I realize this is a subjective issue so any specific citings, white papers, published guidelines from NRTLs or industry experts, etc. would be more useful than opinion. Thanks David P. Nyffenegger, PMP, SM-IEEE Product Development Manager Bell and Howell 3791 South Alston Avenue Durham, NC 27713 Phone: 919.767.6419 Web: www.bellhowell.net - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
