I am about to install an ethernet device which requires PoE (a wireless bridge). I have an 8-port hub installed but it does not offer PoE so I am considering simply buying a PoE adapter to plug in where the device will be installed.
If someone comes along and plugs a regular ethernet cable into this would there be any risk? Or do PoE devices only deliver power when a connected device asks for it?
What if I have a PoE hub and I plug another hub into one of the sockets, and then a device plugged into the secondary hub requests power?
1 Answer
It depends which category cable you have. CAT5E and higher will support PoE on all voltages. CAT5 will support lower voltages too. Anything lower is not recommended.
Given that CAT5E is a standard for a long time now, chances are high that you currently have CAT5E cables.
That said, PoE have 3 classes.
Class1 has 4 Watt available.
Class2 has 7 Watt available.
Class3 has 15 watt available.
There are devices that can output higher watts. I've seen up to 30 Watts. These are all supported on CAT5e though. For Cat5, I would not exceed Class1 to be on the safe side, even though I have a feeling Class 2 will probably work depending on the usecase and where the cable is placed. For example a Class2 CAT5 cable that runs close to a heat source is not recommended.
As for delivery of the power, before there is any power transmitted over the cable, a low (non-damaging) voltage is sent over the cable to initiate a handshake. If the handshake is successful, only then will a higher current being transferred. So you can use a CAT3 cable on a PoE port if you purely use that for data. In that case, the PoE is simply not used and it is being used as a normal Data port.
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