I need to disable the power over ethernet being output by a device to prevent any damage to routers and switches. After looking more into how PoE works, there appears to be two alternatives to how the power is sent over the wire.

Alternative A - Uses phantom power. This method involves sending power alongside the data signals since they are at such different frequencies.

Alternative B - Uses the unused wires of a 10/100 cable to send power.

I'm not certain as to whether the device is 10/100 or if it supports gigabit connections. If there is a way to discover the gigabit capabilities that would tell me that it cannot be alternative B. I don't have many details on the hardware, as it's part of a larger system.

  • \$\begingroup\$ Do you have physical access to the Ethernet plug of the unknown device? Is so then all it takes is to connect any laptop with a gigabit card and check the windows reported connection speed , if is drops to 100Mbit then it's not a gigabit device \$\endgroup\$
    – alexan_e
    Jan 8 '14 at 18:02
  • \$\begingroup\$ For the sake of the question and future users looking for the answer, let's pretend that it is a 10/100 device. In my case, I've actually discovered an off-the-shelf PoE injector inside of the device, so I'll look into its specifics. \$\endgroup\$
    – agweber
    Jan 8 '14 at 18:19
  • \$\begingroup\$ For excluding the unused pairs you can get really inexpensive injectors/splitters and simply not connect a power supply. Example: 3 units at the bottom andahammer.com/poe \$\endgroup\$ Jan 8 '14 at 18:35
  • 3
    \$\begingroup\$ PoE won't give you full power unless the PD passes the "handshake" with the PSE. PoE PSE will not damage a non-PoE device. \$\endgroup\$
    – dext0rb
    Jan 8 '14 at 18:55
  • 2
    \$\begingroup\$ Note that I am talking about PSEs and PDs that actually conform to the IEEE standards. If you're talking about some "DIY/hack" PoE then it's a crapshoot. \$\endgroup\$
    – dext0rb
    Jan 8 '14 at 19:42

I think this document helps Introduction Basic principles of PoE Power over Ethernet (Page 4)

Spare-pair power feed

Spare pair power feeds utilize wire pairs that are not otherwise used, the voltage being placed directly on the spare pairs 4/5 (+) and 7/8 (-). This method can be used only on networks with eight-wire cabling. This does not apply to Gigabit Ethernet, because here all eight wires are used for signal transmission and there are therefore no spares.

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Phantom power feed

A phantom power feed has the voltage coupled to wire pairs 1/2 ( ) and 3/6 (+). This method can be used for networks with four-wire or eight-wire cabling.

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So you can check with a voltmeter, if there is a voltage in pins 4/5 - 7/8 then the method used is Spare-pair power feed.


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