Hi Piotr, If you take the purpose of the *technical* definition in mind: The telecom qualification of your cable is there to decide if a conducted emissions test is to be carried out.
The conducted emission test for telecom apparatuses measures the sum of the equipment common mode current plus a value defined by the differential mode voltage te EUT presents to the ISN, attenuated by the LCL value of the ISN applied. The ISN LCL value of the ISN is to match the cable spec of the manufacturer. Anyway the max frequency tested is 30 Mhz. For the telecom cable to have substantial length for this frequency so be able to radiate, it needs to be 1/4 of a wavelength. at 30 MHz that is 2.5 meters, but to be honest, even smaller lengths do radiate at 30 MHz. So from a spectrum protection point of view a Ethernet cable is a long cable (capable of radiating it's signal contents due to asymmetry) >from approx 1.5 meters off. This calculation assumes that the DM/CM conversion takes place right at the beginning of the cable: that is not true of course. One may discuss about the actual length of twisted pair it takes to create a DM/CM conversion of "LCL" value, but imho a cable longer than 3 meters definitely is "long". Of course, for 10 MHz all values triple, but we don't know what signals the cable will conduct, do we ? Gert Gremmen Ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens Piotr Galka Verzonden: zaterdag 12 september 2009 14:14 Aan: EMC-PSTC Onderwerp: Re: EMC question I have exactly the same problem - how many meters would be appropriate ? I have device A with two RS485 interfaces. One isolated for connection up to 1200m and it is of course telecommunication port. The second not isolated to connect to the device B. Device B is provided with 3m cable (4 wires: 12V power + RS485). In most cases 3m is enough. Do I have specify the max length for this second connection to make it not being the telecommunication port ? If I have, must it be max 10m, or can it be 20m, 25m ? Best Regards Piotr Galka MicroMade ----- Original Message ----- From: "Steve O'Steen" <[email protected]> To: "John Woodgate" <[email protected]>; <[email protected]> Sent: Thursday, September 10, 2009 6:42 PM Subject: RE: EMC question > What maximum length would be appropriate? Consider if the manufacturer > provided the cable for that particular port but did not limit the cable > length for that port in the Installation Instructions. I would think > this would also satisfy the criteria as long as the provided cable is no > longer than the length identified in the answer to the above question. > > Thanks, > > Steve O'Steen > Director, EMC > Advanced Compliance Solutions, Inc. > [email protected] > 770-831-8048 ext. 210 > www.acstestlab.com > > > - 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]> - 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]>
