For example, if you have 10 spy cameras distributed across the entire building, in that case, it needs 10 power supply units to provide power to them and that is not the main problem. The main reason to use PoE over the conventional power supply system is to reduce the cost of providing power supply to each peripheral that connects to the network. How Power over Ethernet works and why it is used
Since using this simple setup we delivered the power to a simple appliance through Ethernet cable we can say PoE is achieved. With source & load in the closed-loop, the circuit below is completed and the camera will show response. So one end should be connected to the power supply and other to the load, so to set an example we are using a battery as a power source and camera as a load. As said earlier this pair of brown and green wire is not used in low-speed Ethernet applications so stripping them of the jack will not cause any interruption to the communications. In this circuit, four wires are stripped away (Blue, Blue-White, Brown & Brown-White namely) from the jack on each end of the cable. Now let us look at the simple circuit for better understanding the PoE. This simple setup of using idle lines to deliver power to the load is called Power over Ethernet.
So the idle four lines can be used for power transfer. So the data exchange is possible even after we strip these idle lines at both ends, which is why in low-end Ethernet cable there are only four lines instead of eight lines. For example, consider the Ethernet cable shown in the figure below, it has 8 lines but among them, only four lines are commonly used for data exchange (Orange, Orange-white, Green &Green-white) while remaining four lines remain idle. We all know that Ethernet cables are used for data transfer and network connectivity but do you know that most of the time we do not use all the connecting lines present in the Ethernet cable.