Power over Ethernet for Devices

Power over Ethernet is fantastic if you need to “bury” a switch in a wall, attic or Crawlspace. (Note that devices should always be “accessible”: but that doesn’t mean we can’t “hide” them).

More devices are having PoE capability built into them all of the time. Even if they don’t come shipped with POE capabilities, you can use an adapter to use POE. although it occasionally doesn’t look as clean – it works.

Pins 1, 2, 3 & 6 on a network cable (the orange and green pairs) transfer data. 4, 5, 7 & 8 (the blue and brown pairs) are usually redundant. This means that you can inject power into these pairs.

Historically I used brown for positive and blue for negative (brown being live in high voltage electrical installations). However, the “standard” is now the opposite (Pins 4/5 being negative, and 7/8 positive). It’s therefore important to be sure you know the setup of “homebrew” POE implementations before working on them.

The truth is, however, that passive PoE has been bodged into network installations for quite some time – without a fixed specification. So the important thing is to document what your specification is for your installation, leave it near the devices, and label the cables. Just because you’ll remember, it doesn’t mean everyone else will.

Things to consider include cable length, voltage drops, and ensuring the input and output match.

Every installation is unique. Sometimes a simple screw on adapter will be sufficient (IE: a single CCTV camera where you don’t have an existing POE switch), but then if you have multiple devices requiring a POE source then it’s wise to invest in a dedicated POE switch, or at least a quality power injector.