Hi Piotr, John is correct, PTCs are commonly used to limit energy to the LPS limits, and are allowed by the standard. One thing to keep in mind if you were to go this route would be that PTCs have a max. current limit - in other words, the maximum output current of the power supply that provides power to the PTC would need to be limited to within the max. current rating of the PTC. Of course to do this would require another instruction in the manual and/or marking.
Regarding the issue of the fuse itself on the supply-side not being limited by LPS (as you stated): "Trying to be in accordance with 60950-1, I have redesigned our next product (we just did 3 prototypes with connector for cables) to connect cable directly to PCB (connecting points in my opinion are not elements) and used fuse as first element (my contract manufacturer told me that SMD fuses are no popular, all others use PTCs -> they all must be wrong ;-) ). I suppose that way all elements at my PCB are powered by LPS, but what with this fuse? Is it powered by LPS or no? Is it limiting power for itself or not ? If it happens that its first pin will be shortened to ground there will be nothing (except cable resistance) to limit the power. I am now starting to write the instruction to it. Should I write there to use only LPS power supply ? Or should I look for cable mounted fuse to add to my device." You have a very limited number of components here with very specific fault conditions. Why don't you use fault testing to determine acceptability of that portion of the circuit that is not LPS? Method 2 of clause 4.7.1 allows this. Clause 4.7.2 does not require a fire enclosure for nonLPS circuits where fault testing shows that temperatures sufficient for ignition are not present (see also 4.7.2.2). Pick the worst case/highest output current power supply your product could be connected to for your test. Once again, this requires an instruction in the manual that the supply used cannot be cable of providing more current than the supply used for the test. Perhaps you have already considered this possibility, but I thought I'd mention it just in case. Scott Aldous Compliance Engineer Advanced Energy Tel: 970-407-6872 Fax: 970-407-5872 From: [email protected] [mailto:[email protected]] On Behalf Of Piotr Galka Sent: Friday, September 18, 2009 7:54 AM To: EMC-PSTC; John Woodgate Subject: Re: Looking for Source of Requirement John, ----- Original Message ----- From: "John Woodgate" <[email protected]> >>I don't know. I'm asking you, experts here. > > I know that you don't, but some of the guys here probably had a hand in > writing that text, so know what they meant it to mean. > That is why I have written experts and not expert ;-) > The advice you were given about not using a PTC thermistor seems to be > wrong. Clause 2.5, item b) of IEC/EN 60950-1:2006 allows the use of a PTC > complying with selected requirements of IEC 60730-1. It is easier for me to find the fuse with approvals I think I understand than PTC which is specified to comply with this IEC (not sure but probably I have never seen in PTC datasheet such information). > The simplest practical solution for your 12 V 0.3 A product is to sell the > power supply with it. That way, you have provided an LPS. There are many power supplies for use with our devices. It depends on how many devices are in specific installation the smaller or bigger supply is used by the installer. It depends on how long it is expected to work when power is off the supply is selected with smaller or bigger accu in it. There is no sense to transport (CO2 emission) all these kg to us and then to installer who probably can buy it in the next street shop. As I understand it the other problem is that if you just have in your offer some power supply this not solves the LPS problem, but when you sell your device as a set with somebody's power supply you become responsible for the whole set, so ..... > If you can't do that, for some reason, then specify an LPS in the > instructions and, if possible, label the power connector '12 V 0.3 A LPS > source'. And we came back to this thread beginning ;-) Best Regards Piotr Galka - 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. 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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]>
