PoE — Power over the Network Cable: How Cameras, Access Points and IP Phones Run on a Single Wire, Which Standards Exist and Why Cheap "Passive" Adapters Fry Equipment

When you need to install a camera above the gate, a Wi-Fi access point on a corridor ceiling or an IP phone on the receptionist's desk, a simple question comes up: where does the power come from? Running a separate electrical line and a socket to every device is expensive, slow and ugly. The solution used in practically every modern CCTV system and office network is PoE, Power over Ethernet — power delivered over the same network cable that carries the data. One wire from the switch and the device works. But this convenient technology has its rules, and breaking them leads to cameras restarting, Wi-Fi dropping out and even burnt-out ports.

How PoE works

A network cable has four pairs of wires. A PoE switch or injector sends a DC voltage of roughly 48–57 volts down them along with the data. The receiving device — camera, access point, phone — separates the power from the data. Standard PoE is safe: before applying voltage, the source checks that the device at the other end really supports PoE and works out how much power it needs. So you can plug an ordinary laptop into such a port and it won't be damaged.

Standards and power

• 802.3af (PoE) — up to about 13 W per device. Enough for most simple cameras, IP phones and small access points.
• 802.3at (PoE+) — up to about 25 W. Needed for powerful Wi-Fi 6 access points, PTZ cameras, and cameras with infrared illumination and heaters.
• 802.3bt (PoE++) — up to 51–71 W. For high-speed PTZ cameras, powerful outdoor equipment, some light fittings and thin clients.

What matters isn't just what one port delivers but the switch's total power budget. An 8-port PoE switch might have a total budget of, say, 60 W — and if you connect eight cameras with night illumination, at night, when the infrared lights come on, there may not be enough power and the cameras start restarting.

Passive PoE — a cheap trap

Besides the standard, there's so-called passive PoE: voltage is always present on the cable, with no check of what's connected at the other end. Some budget cameras and wireless bridges use it, and cheap "PoE adapters" from online shops are often passive too. Connect a device without PoE support — a laptop, router or TV — to such a line, and you can burn out its network port. Passive PoE voltages also vary — 12, 24, 48 volts — and a mismatch makes equipment unstable or kills it. In professional systems we use only standard PoE, and passive only where a specific device requires it, with clear labelling.

The cable matters

Current now flows through the cable, so its quality becomes critical. Cheap copper-clad aluminium (CCA) cable has higher resistance: the voltage at the far end drops, the cable heats up, and a camera 80 metres from the switch starts restarting. PoE needs pure copper cable, and a run must not exceed 100 metres. Badly crimped connectors and twisted joints add resistance and heat. Outdoor runs need outdoor cable protected from UV and moisture, plus surge protection.

The advantages of PoE

• One cable instead of two — faster, cheaper installation and fewer wires on the facade and ceiling.
• Centralised power. One UPS at the switch keeps every camera and access point running during a power cut.
• Remote restart. A frozen camera or access point can be restarted by switching the power on its switch port off and on, without climbing a ladder.
• Safety. Low voltage is safer than 220 volts outdoors or on a ceiling.

Common problems

• Cameras restart at night — the switch's power budget runs out when the illumination comes on.
• A distant camera works on and off — poor-quality cable or a run that's too long.
• An access point works, but in a reduced mode — the port supplies PoE instead of the required PoE+.
• A burnt-out port after a passive injector was connected to an ordinary device.
• The whole system goes dark in a power cut — no UPS for the switch.

What our technician does

1. Lists the devices and calculates each one's consumption, with headroom for night illumination and heaters.
2. Selects a PoE switch with the right port standard and total power budget, or injectors.
3. Runs copper cable of the right category, crimps the connectors and fits surge protection on outdoor runs.
4. Connects a UPS sized for the whole PoE load.
5. Tests every device under load, including at night, and sets up remote port control.

A real example

At a warehouse in Lilo, eight cameras worked fine during the day, but every night the recordings broke off: the cameras were restarting one after another. The owner had already replaced the "faulty" cameras twice. Our technician checked the system: the budget switch had a total budget of 65 W, while the cameras with infrared illumination on drew almost 90 W; on top of that, two of the distant runs used aluminium cable. We installed a switch with a 150 W budget and a UPS, and replaced the two runs with copper outdoor cable with surge protection. The restarts stopped, and recording now continues through power cuts too.

Call a technician

If you're planning a CCTV or Wi-Fi system powered over cable, or your cameras and access points are behaving erratically, we'll come to any district of Tbilisi around the clock. Learn more on our CCTV, local network installation and Wi-Fi setup pages.