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257 points voxadam | 2 comments | | HN request time: 0.417s | source
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rmunn ◴[] No.45663921[source]
Practical question for HN: How do you all label your PoE cables so that you don't accidentally plug the powered cable into a socket that wasn't expecting 48 volts on those pins and fry something? (I know the power injector is supposed to only deliver power when it's safe, but if all your devices work exactly as they should all the time, then I'd like to buy that bridge in Manhattan you're selling).

Do you buy Ethernet cables of different colors and say "Yellow is reserved for PoE, all yellow cables should be assumed to have power on them"? Or do you slap a "48V" label on both ends of the cables you're going to use for PoE and the label is what warns you that this cable should only go into the PoE receiver, and not into an unpowered device? Or do you just not label your PoE cables any differently, and trust that the injector will never malfunction at the same time that you plug the PoE cable into the wrong device?

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1. Kirby64 ◴[] No.45663995[source]
Unless you’re using the “passive” PoE variants (ubiquiti sold these for awhile, for instance) that always has voltage on the pins, there is no risk. Negotiation is mandatory for the actual IEEE variants. Just use those and don’t worry about it.
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2. sodaclean ◴[] No.45672710[source]
Even with passive PoE you're fine as long as everything is A, B, or any mix thereof.

Apparently, some mag-jacks have the center taps for each pair commoned via 75ohms to ground through a capacitor... so I could be wrong.