Hello Mark,
In general, I would recommend eliminating the situation. However, I’m going to go out on a limb here. If the plastic is rated to support the voltage, and if all necessary spacings are met, and if the plastic is thick enough to provide insulation where necessary, then it seems possible that the construction would acceptable. Let’s look at an analogous situation. For example, if you have a bussbar at hazardous voltage, it will likely be supported by a screw or bolt through some type of insulating material. That bolt will probably be energized to the voltage of the bussbar, but it is accepted as long as its support meets the necessary isolation requirements. Why should your situation be different? I am presuming that the product is not intended to be serviced live. If it were, I am not even sure you would have a problem. Would the service personnel be expected to remove the circuit board in question while it was live? In that case, the screw could present a hazard, but it probably wouldn’t be the only hazard in removing a live board. If the product is serviceable, and the service instructions require removing power first, then there should be no service issue. There are questions that come to mind. If the screw is over tightened, will it cause problems? Could it cut through the trace? Could it split the standoff? Are you going to now control purchasing of the screw? What is the risk that somebody decides to use a different screw with a bigger head that violates creepage or clearance requirements? Eliminating the issue would be advantageous if only because it is one less thing you need to control during the life of the product. You would hate to have the product become unsafe just because nobody realized that changing a screw would cause a problem. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] <mailto:[email protected]> The opinions expressed are my own and do not necessarily reflect those of my employer. From: Mark Gandler [mailto:[email protected]] Sent: Wednesday, October 06, 2010 6:01 PM To: [email protected] Subject: Re: concern for metal screw touching AC trace on the PCB Fred, as I mentioned in my e-mail, screw is covered by enclosure, same as any other primary AC component, it is inside the box, nothing exposed on the outside. It is still in development so if there a real problem, it will be fixed. Mark ________________________________ From: [email protected] To: [email protected]; [email protected] Subject: RE: concern for metal screw touching AC trace on the PCB List-Post: [email protected] Date: Wed, 6 Oct 2010 17:52:34 -0700 Mark: >From an engineering standpoint I assume this is a screw-up that management doesn’t want to spend the money to fix. From a legal standpoint I assume this is a known defect. Are there adequate warning labels to protect both the user and service personnel? I have a hunch, not to be confused with a WAG, that it would be cheaper to fix the problem than apply warning labels. Fred Townsend DC to Light From: [email protected] [mailto:[email protected]] On Behalf Of Mark Gandler Sent: Wednesday, October 06, 2010 4:42 PM To: [email protected] Subject: concern for metal screw touching AC trace on the PCB Hi, let's assume there are 120VAC traces on the top layer of the PCB (AC comes in >from a detachable power cord) Right next to the trace (<1mm) there is a plastic stand with metal screw (washer build-in) in it. metal washer lays on top of the AC trace, separated only by the mask. Screw is not connected to any grounds or circuits or any other conducted part. It is untouchable to the user side. Is where any safety concern? Product is evaluated under 60950. One thing came to my mind, is a plastic standoff, probably should be able to support high voltage? is it valid concern? any others? Thanks, Mark - ---------------------------------------------------------------- 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.ieeecommunities.org/emc-pstc <http://www.ieeecommunities.org/emc-pstc> Graphics (in well-used formats), large files, etc. can be posted to that URL. 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Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html 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]>
