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Is it safe to connect a 24 V PoE switch output to an ordinary PC Ethernet input?

Davidw
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Toni Homedes i Saun
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There are many different manufacturers of network devices, and you didn't say what you are using, but I work with 24V PoE devices from Ubiquiti, so I'll answer based on those. If you are using something else, update the question and I'll update the answer.

No, it is not safe to plug an ordinary PC into a passively-powered 24V PoE switch output. Unlike standard 48V "active" PoE - i.e. 802.3af/at/bt - which checks whether the connected device needs to be powered, the "passive" PoE provided by 24V switches is always on if the port is configured to send power.

It is possible that some devices have current protection built into their network jacks, but that is definitely not something you want to rely on.

In theory, modern network jacks are required to be magnetically coupled instead of directly connected, specifically to prevent voltage spikes on the line from damaging the computer, but in practice this does not always seem to help for 24V Passive PoE.

Moshe Katz
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  • Ethernet jacks *are* magnetically coupled. However on some non-poe ports the cable-side center taps for the four pairs are shorted together. Apply something like ubiquiti passive PoE to the active pairs on such a port (IIRC with the pairs ubiquiti uses only gigabit ports have active stuff on the relavent pairs) and you are very likely to fry the transformer. – Peter Green Jun 20 '19 at 02:16
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    @PeterGreen You are correct - Ubiquiti uses pins 4 & 5 for `+` and pins 7 & 8 for `-` so it might not matter for non-gigabit devices. I said "in theory" about the magnetics because I remembered [this story](https://www.raspberrypi.org/blog/manufacturing-hiccup/). – Moshe Katz Jun 20 '19 at 02:48
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Passive always on PoE is NEVER safe to connect to anything else than the intended PoE enabled device. So for Ubiquity 24VDC passive PoE this is an Ubiquity device that supports 24VDC passive PoE. Other brands might have 24VDC passive PoE implemented in a different way.

I only use active 802.3af PoE Type A (power over data pairs 12/36) because it is designed to be compatible with ANY Ethernet device and any cable configuration. Even old 10Base-T Ethernet devices can be safely connected. And if the device supports 802.3af PoE, it will deliver power to this device.

The only place I would use passive PoE is on internal networks behind closed doors where only I and other competent IT personal has access to. For example, I don't see any problem with passive PoE for fixed WiFi accesspoints as long as you use one brand (f.e. Ubiquity) and one standard/type (f.e. 24VDC passive PoE power over spare pairs (45/78) so even the IT people cannot make mistakes (f.e. connecting 48VDC to a 24VDC device)

I've seen people jamming correctly labelled live RJ11 phonelines into RJ45 Ethernet connectors so when it comes to having trust the competence of most people, I have none.

Robin
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Theoretically, yes.

All modern ethernet is required to accept this.

See https://electronics.stackexchange.com/questions/27756/why-are-ethernet-rj45-sockets-magnetically-coupled for more

Now, could some devices be wrongly designed and be damaged by (or cause damage to) PoE? Sure. Don't blame me if your stuff blows up, blame the manufacturer! :D

Cameron Tacklind
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