# # PoE (Power over Ethernet)
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2005/05/11 PoE (Power over Ethernet)

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© May 2005 Tony Lawrence

Sounds like a horrible idea to me, but I'm not an Electrical Engineer, so don't put any weight on that opinion. The basic idea is that there are unused pairs in Cat 5 that can be put to use carrying current. Of course you can't push a lot of current, but computer devices are becoming more and more low power and there are even POE capable computers

Well, it sounds scary to me. Just because wires are unused by IP doesn't mean that some piece of equipment took pains to avoid them. The unused pairs could be carelessly connected where they shouldn't be, not causing immediate damage, but the the introduction of power there might cause general weirdness or worse - but again, what do I know?

Here's somebody's HOWTO if you are curious and more brave than I.


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Wed May 11 11:29:41 2005: 488   anonymous


I for one will be checking and double checking my wiring jobs from now on - 48v on non-poe-compliant equipment can fry them and put a dent in your pocket!



Wed May 11 14:06:26 2005: 490   BigDumbDinosaur


The basic idea is that there are unused pairs in Cat 5 that can be put to use carrying current.

NO, NO, NO!!! Absolutely not!!! Very dumb idea!!! If used to transport power (or phone signals, a setup I fixed at one location), those "unused pairs" will induce noise into the signal pairs (two of 'em, coming and going) and the result will be a lot of packet errors. Whomever thought up this idea is a blithering idiot! Are they that hard up for cable that they have to usurp pairs for non-network purposes? Besides, there are instances where the unused pairs are internally grounded in some network gear. Put voltage to that and something's gonna go up in smoke. At the risk of being repetitious, really lame idea!



Sat Jun 18 00:43:58 2005: 664   anonymous


A lot of the point of PoE (Power over Ethernet) is for wiring things that are mounted outdoors so you only have to run one wire to them instead of having to run an ethernet cable and a power cable to them. As long as you know you are using PoE and you mark the cables so anyone who comes in from the outside will know what is going on they are fine.



Sat Jun 18 13:45:01 2005: 666   BigDumbDinosaur


A lot of the point of PoE (Power over Ethernet) is for wiring things that are mounted outdoors so you only have to run one wire to them instead of having to run an ethernet cable and a power cable to them. As long as you know you are using PoE and you mark the cables so anyone who comes in from the outside will know what is going on they are fine.

Dear reader: please read the comment immediately above yours again. Perhaps the fallacy of using "unused pairs" in UTP supporting Ethernet didn't fully sink in. In any case, if you are able to run one cable to connect your network device, how much harder can it be to run a low voltage power cable???

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