5 ms·

Having inadvertently plugged a 24v passive PoE live connection into an old 2012 Mac Mini and immediately frying it, this is welcome news!

mmastrac 3y ago
I thought most modern devices had isolation!
- progbits 3y ago
  Indeed, surprising. From IEEE 802.3 (revision 2012), section "32.6 PMA electrical specifications": > The PHY shall provide electrical isolation between the DTE or repeater circuits, including frame ground and all MDI leads. > This electrical separation shall withstand at least one of the following electrical strength tests: > [...] > b) 2250 Vdc for 60 s, [...] Non-compliant Ethernet PHY?
  - evadne 3y ago
    Passive PoE is always on Active PoE is negotiated with handshake
    - jcrawfordor 3y ago
      Yes, but passive PoE is almost universally at 24v, and per the Ethernet spec (as quoted above) an Ethernet PHY should tolerate 24v fine. This is important as transients from nearby lightning or occasionally even coupling to power cables can produce this kind of voltage. Ethernet connectors are magnetically coupled for protection from these transients. The problem with PPoE in these cases is, I think, not the voltage so much as the current. The continuous 24v supply may overheat the magnetic coupling transformer and cause it to fail. Some Ethernet interfaces, usually on telecom equipment and quality switches, have over current protection to prevent this. Unfortunately consumer devices usually don't. It's important to understand this because 802.3af etc. does provide power without being asked - as a rest for a characteristic resistance on the receiver. Otherwise it wouldn't know if a PoE-capable device was connected. Up to 20v can be applied during this process but it is time limited. In general, 802.3 PoE supplies must monitor the current usage of the powered device and cut off power if it is too high or even too low for more than a short period of time. This is in part to prevent this overheating problem on devices that might, for some coincidental reason, fall into the appropriate resistance range to activate PoE. In other words, 24v or even hundreds of volts for a few seconds is perfectly safe. 24v for minutes is likely to cause damage to devices without better protection than the spec requires. Old Ethernet equipment used to make the non-isolated components relatively easy to replace so that repairs after a problem like this were easier but now the isolation is a tiny surface mount part and replacing it will require tools and skill.
      - sacnoradhq 3y ago
        Most people don't realize the huge transient voltages in wires of any significant length that can be induced by nearby electrical storms. Granted, CAT 5-8 cables are supposed to be twisted pairs and sometimes shielded, but it's imperfect. PS TIL: https://en.wikipedia.org/wiki/Litz_wire https://en.wikipedia.org/wiki/Litz_wire
      - numpad0 3y ago
        Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE. On ISDN, pinout could vary but one I could find said 1-2, 4-5 for V+ and 3-6, 7-8 for V-. In either cases, both sides of isolation magnetics are connected to pins of same potential and current is only proportionate to voltage imbalance within each pair which should be minuscule. I wonder if the problem is that 1:1 signal transformers for decoupling are being replaced with simple DC blocking circuit. That can be most simply done with a spare 0.1uF and an R10k per pin, which by the way generate load of from V/R=I 48V/10kohm = 5mA and 48V * 5mA = 230mW > 1/6W. That could cause resistor to burn off if phone-sized components would be used. Or if the cap might only be rated for 10V, it could burn open. I have nothing to support these hypothesis though. I could be entirely off. Also I suspect the "passive PoE" mentioned above could be cheap injectors with very rounded corners and always active 48V. Those are widespread for surveillance cameras and other neckbeardy applications.
        toast0 3y ago
        > Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE. There's three common pin outs. You've described 'alternate B' -- the unused pairs of 10/100; you can also use 'alternate A', using the pairs used for 10/100. Or 4-pair would have both.
- somat 3y ago
  Note that there are real 48v 802.3af/at power over ethernet devices where the power source has to assert that the power drain wants the power before it powers up and fake "passive" 24v systems where the line is always energized. probably where the GP got the injector from. My personal involvement on this was several years ago when I was going to buy several unifi access points. It turns out you have to be careful because many of the models advertise as being POE but in reality are jank 24v passive systems. I have not kept up with the current unifi lineup but at the time you had to make sure to get the "AC Pro" to have real 802.3af compatibility.
  - mmastrac 3y ago
    I have a load of Unifi stuff here and it all supports active PoE, but works just fine with passive. The PoE outputs all autonegotiate.
    - somat 3y ago
      Eh, it is not like they are outright lying. when you dig into the specs they do say 24v passive. but when someone says poe I expect 48v 802.3af compatible poe. I don't know how true it is but I heard horror stories where ubiquiti 24v gear would activate, then fry when hooked to a real poe source. personally I suspect it was a 48v passive source. I did a quick informal survey of the ui store and it looks like the newest generation(U6) all handles 48v while the prior generation(AC) the pro models were 48v and the lite and long range models were 24v passive.
    - youngtaff 3y ago
      Some of the older Unifi kit was non standard By older I mean you could still buy it three / four (?) years ago
- monocasa 3y ago
  It should be that all devices have isolation. Ethernet is capacitively coupled which keeps you from having to coordinate your power/ground between devices. PoE is the recognition that if the DC levels don't matter because of that required isolation, we might as well not isolate in a power stage before the magnetics and deliver some DC power.
  - NavinF 3y ago
    > Ethernet is capacitively coupled It's usually magnetically coupled. I've only seen capacitive coupling when both devices are on the same PCB

mmastrac 3y ago

I thought most modern devices had isolation!

progbits 3y ago
Indeed, surprising. From IEEE 802.3 (revision 2012), section "32.6 PMA electrical specifications": > The PHY shall provide electrical isolation between the DTE or repeater circuits, including frame ground and all MDI leads. > This electrical separation shall withstand at least one of the following electrical strength tests: > [...] > b) 2250 Vdc for 60 s, [...] Non-compliant Ethernet PHY?
- evadne 3y ago
  Passive PoE is always on Active PoE is negotiated with handshake
  - jcrawfordor 3y ago
    Yes, but passive PoE is almost universally at 24v, and per the Ethernet spec (as quoted above) an Ethernet PHY should tolerate 24v fine. This is important as transients from nearby lightning or occasionally even coupling to power cables can produce this kind of voltage. Ethernet connectors are magnetically coupled for protection from these transients. The problem with PPoE in these cases is, I think, not the voltage so much as the current. The continuous 24v supply may overheat the magnetic coupling transformer and cause it to fail. Some Ethernet interfaces, usually on telecom equipment and quality switches, have over current protection to prevent this. Unfortunately consumer devices usually don't. It's important to understand this because 802.3af etc. does provide power without being asked - as a rest for a characteristic resistance on the receiver. Otherwise it wouldn't know if a PoE-capable device was connected. Up to 20v can be applied during this process but it is time limited. In general, 802.3 PoE supplies must monitor the current usage of the powered device and cut off power if it is too high or even too low for more than a short period of time. This is in part to prevent this overheating problem on devices that might, for some coincidental reason, fall into the appropriate resistance range to activate PoE. In other words, 24v or even hundreds of volts for a few seconds is perfectly safe. 24v for minutes is likely to cause damage to devices without better protection than the spec requires. Old Ethernet equipment used to make the non-isolated components relatively easy to replace so that repairs after a problem like this were easier but now the isolation is a tiny surface mount part and replacing it will require tools and skill.
    - sacnoradhq 3y ago
      Most people don't realize the huge transient voltages in wires of any significant length that can be induced by nearby electrical storms. Granted, CAT 5-8 cables are supposed to be twisted pairs and sometimes shielded, but it's imperfect. PS TIL: https://en.wikipedia.org/wiki/Litz_wire https://en.wikipedia.org/wiki/Litz_wire
    - numpad0 3y ago
      Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE. On ISDN, pinout could vary but one I could find said 1-2, 4-5 for V+ and 3-6, 7-8 for V-. In either cases, both sides of isolation magnetics are connected to pins of same potential and current is only proportionate to voltage imbalance within each pair which should be minuscule. I wonder if the problem is that 1:1 signal transformers for decoupling are being replaced with simple DC blocking circuit. That can be most simply done with a spare 0.1uF and an R10k per pin, which by the way generate load of from V/R=I 48V/10kohm = 5mA and 48V * 5mA = 230mW > 1/6W. That could cause resistor to burn off if phone-sized components would be used. Or if the cap might only be rated for 10V, it could burn open. I have nothing to support these hypothesis though. I could be entirely off. Also I suspect the "passive PoE" mentioned above could be cheap injectors with very rounded corners and always active 48V. Those are widespread for surveillance cameras and other neckbeardy applications.
      - toast0 3y ago
        > Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE. There's three common pin outs. You've described 'alternate B' -- the unused pairs of 10/100; you can also use 'alternate A', using the pairs used for 10/100. Or 4-pair would have both.
somat 3y ago
Note that there are real 48v 802.3af/at power over ethernet devices where the power source has to assert that the power drain wants the power before it powers up and fake "passive" 24v systems where the line is always energized. probably where the GP got the injector from. My personal involvement on this was several years ago when I was going to buy several unifi access points. It turns out you have to be careful because many of the models advertise as being POE but in reality are jank 24v passive systems. I have not kept up with the current unifi lineup but at the time you had to make sure to get the "AC Pro" to have real 802.3af compatibility.
- mmastrac 3y ago
  I have a load of Unifi stuff here and it all supports active PoE, but works just fine with passive. The PoE outputs all autonegotiate.
  - somat 3y ago
    Eh, it is not like they are outright lying. when you dig into the specs they do say 24v passive. but when someone says poe I expect 48v 802.3af compatible poe. I don't know how true it is but I heard horror stories where ubiquiti 24v gear would activate, then fry when hooked to a real poe source. personally I suspect it was a 48v passive source. I did a quick informal survey of the ui store and it looks like the newest generation(U6) all handles 48v while the prior generation(AC) the pro models were 48v and the lite and long range models were 24v passive.
  - youngtaff 3y ago
    Some of the older Unifi kit was non standard By older I mean you could still buy it three / four (?) years ago
monocasa 3y ago
It should be that all devices have isolation. Ethernet is capacitively coupled which keeps you from having to coordinate your power/ground between devices. PoE is the recognition that if the DC levels don't matter because of that required isolation, we might as well not isolate in a power stage before the magnetics and deliver some DC power.
- NavinF 3y ago
  > Ethernet is capacitively coupled It's usually magnetically coupled. I've only seen capacitive coupling when both devices are on the same PCB