Power over Ethernet (PoE) basics and beyond

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?

What every engineer really should know!

Always buy standards based equipment. 802.3af, 802.3at, 802.3bt. You can label cables and jacks with red lettering (“Passive PoE. will fry your laptop port. Really!”) but it only takes one mistake to let the magic smoke out.

I completely avoid passive PoE. Not worth the risk. On the standardized active stuff I’ve never had any issues even when I’ve plugged it broken cables to unpowered devices.

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.

Well the entire point of standards is so things work exactly as they should every time. I haven’t seen any issues with standards based poe

All 21,000 ports I administer have 802.3 standard PoE enabled at all times. Incidents of inadvertent powering are at zero. I think this is just a non problem.

From what I have seen, Ethernet ports always have a small isolation transformer for each twisted pair, between the connector and the PHY. Usually four of such transformers are combined in one small magnetics package. The insulation in the transformer is specified to withstand over a kilovolt of lightning induced voltage -- that's one of the purposes of such galvanic isolation.

The data travels as the differential voltage in each of the twisted pairs, and is transmitted magnetically by the transformer to the secondary winding. The power is applied between different pairs, and in each pair appears as a common mode voltage. This is all stopped by the transformer, and in devices designed to support PoE, the PoE circuits tap the mid-point of the primary windings to access the supplied voltage.

So at a first glance, it seems that if 48 volts is applied between the twisted pairs to a non-PoE device, this voltage would simply be blocked by the transformer. But since there is a widespread concern about this, there must be more to the story -- maybe somebody who actually worked with these circuits can explain why this is more complicated than it seems at first?

Edit: Found an answer. It seems that at least some of the designs of non-PoE Ethernet jacks terminate the common mode signals to a common ground though 75 Ohm resistors. In this case, if the voltage were applied between the twisted pairs, the resistors would dissipate far too much power and would burn out. So there is definitely a concern with the dumb PoE injectors and at least some non-PoE devices. https://electronics.stackexchange.com/questions/459169/how-c...

With modern 802.3 spec compliant PoE: I don't worry about it. At least with all the switches I've used. Never ever had it send power to some device that wasn't expecting it.

This is a bit analogous to USB-C PD power supplies, which can supply 12V/24V, but only do this when devices ask for it. I don't worry that my laptop's USB-C power supply will go rogue and send 24V to my earbuds.

Many years ago I was using 12v passive homemade PoE at my house, manually had injection wires in on switch side of cable and manual break out on far end into barrel connector plugged into an AP.

Once I accidentally plugged the cable into a laptop and the port didn’t work until I powered the laptop off and on again, but no lasting ill effects on laptop at all.

Network engineer here: It's not an issue. If the device isn't expecting PoE, it doesn't negotiate PoE, and no power is transferred.

Because of how ethernet works (differential signaling + signal transformers), PoE is effectively a wire at 48v connected to nothing if the device doesn't support it.

The only issue arises if somebody wires a patch cable completely wrong (neither A nor B), and manages to put one leg of passive PoE's +24v pair matched to one leg of the 0v pair. Which, will promptly smoke the signal transformer... assuming short circuit protection doesn't cut power first. This is why we killed passive PoE.

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.

Never noticed the 75ohms before- but it'd be 150ohms for passive PoE. (Through one pair and to another, two 75's)

Theres fixes, but passive PoE was a pretty dirty hack- so negotiation got added.