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. 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. Isolation of signal circuits from mains must be done by insulation and separation, even when worried about rodent damage and other misuse. Expecting earthing to protect from mains shorts for any long light gauge wire is a mistaken concept.
Bob Johnson ITE Safety <http://www.itesafety.com> John Woodgate wrote: In message <[email protected]> <mailto:[email protected]> , dated Fri, 27 Mar 2009, Joe Randolph <[email protected]> <mailto:[email protected]> writes: A Class 2 supply is the most common type of supply that I have seen for POE injection. And yes, the output should be isolated from earth ground to comply with IEEE 802.3af. Can somebody explain to me why an isolated power supply is 'safer' than an earthed one? I am particularly concerned that this applies to IT networks and anything else that may run through, for example, roof voids and similar spaces. Within one room, as for Class II consumer products, the techniques is probably acceptable. I know there are 'segregation' requirements, but cable runs can be disturbed by non-electrical people during maintenance. The point is that cable damage (by humans, rodents or building movements) can create a connection between live mains and the 'SELV' circuit. Since the latter is not earthed, the fault can persist indefinitely, until someone contacts the SELV circuit. The hazard is great, the probability is low, and to me that results in a tick in the 'unacceptable risk' box. We now have the concept of 'PELV', protected extra-low voltage, which has reinforced insulation from mains voltage AND is earthed (preferably at one point only, to avoid currents due to earth potential differences. I think that all 'SELV' requirements must be re-examined to see if they should be changed to 'PELV'. In hazard-based terms, there is no contest. SELV has two safeguards, PELV has three. Personally, I do not see any safety compliance problems with this construction when used as intended (inside lines). By the way, one problem that I *have* seen with this construction is performance related. Most of these POE injection power supplies are switching converters, and the Y-caps used on the input for EMI suppression create a voltage divider that places a very high 50/60 Hz common mode noise signal (typically one half of the AC mains voltage) on the Ethernet outputs. How can Y-caps do that? Y caps go from both mains poles to earth. This means that an IP telephone powered by the POE injector will be riding on the 50/60 Hz common mode noise. The analog audio circuits in the IP phone, such as the microphone for a hands-free mode, can be sensitive to this common mode noise and produce audible hum. Grounding the chassis of the POE injector fixes the problem, but most of these devices are ungrounded. Then they cannot possibly have Y-caps. So, the IP telephone has to be designed with an extraordinary level of 50/60 Hz common mode immunity in order to avoid hum in the analog audio path. It sounds as though people are making some fundamental error that shows up as this problem. - 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]>
