In message <[email protected]>, dated Fri, 27 Mar 2009, Robert Johnson <[email protected]> writes:
>Earthing the output of a power source is one way of preventing faults >within the source between mains to output from energizing the output. >It quickly overloads and opens the mains circuit. Indeed. >Relying on an earthing for protection of a signal circuit which runs >any distance from the source to protect it from mains short is unwise. >It is unlikely to operate overcurrent protection and may cause fires. >For example typical cat 5 wire (24AWG) is 0.088 ohms/meter which means >lengths approaching 100m would not operate branch circuit protection >but simply overheat. I can't visualize a scenario in which that would occur. I'm certainly not suggesting grounding the POE at a distance from the sender. >Isolation of signal circuits from mains must be done by insulation and >separation, even when worried about rodent damage and other misuse. Those measures are necessary, yes. >Expecting earthing to protect from mains shorts for any long light >gauge wire is a mistaken concept. I don't think I came even close to making that mistake. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk Things can always get better. But that's not the only option. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - 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]>
