PoE camera gets link but no power: injector and class checks

A PoE camera that shows link LEDs but never boots on Ethernet power alone is not a network fault, and it is usually not a dead camera. Data and power share one cable but negotiate separately: the link coming up proves the wiring and the camera's Ethernet PHY, while the boot waits on the powered-device handshake finishing first. This guide gives you the three checks that separate the two paths — substitute a known-good PoE source, compare the camera's power class with the injector's standard, confirm which pairset the injector powers — plus the arithmetic that rules out cable loss at bench lengths. It applies to any PoE-equipped GigE area scan camera or smart camera fed through an injector.

What the symptom proves and what it doesn't

Link LEDs, a pingable address, even a self-assigned 169.254.x.x link-local address, all prove the data path. Link-local is what a device does with no DHCP server on the bench, and it is enough to open the camera and stream — see GigE camera on link local address: open it by IP from Linux. A camera that only appears on the network while its USB or auxiliary supply is attached is telling you plainly: Ethernet data works, Ethernet power does not.

Treat it as two independent circuits. The mirror symptom — the camera has power but is not detected — has its own walk in camera has power but not detected: USB link checks after a cable swap. Here the data half is healthy, so everything below is about the power half: the handshake between the power-sourcing equipment (the PSE, the injector or switch port) and the powered device (the PD, the camera).

The handshake a camera must pass before it boots

Standard PoE is not voltage simply put on a wire. Four steps, in order:

1. Detection. The PSE probes the cable for a valid powered device: a resistance of about 25 kΩ (the standard accepts roughly 23.75-26.25 kΩ). No valid signature means no power — which is why a non-PoE device on a PoE port stays cold.
2. Classification. The camera draws a class current, class 0 to 4, announcing its maximum power.
3. Negotiation. Under 802.3at the camera can also state its actual need over LLDP, up to the PoE+ ceiling.
4. Power-up. The PSE applies 44-57 V DC, nominal 48 V, and watches the current while the camera's DC-DC converter starts.

An 802.3af device never draws beyond its class; 802.3at added the LLDP step so a class 4 camera can claim what it needs. The Power over Ethernet article summarises the standards and the power table.

Test 1: substitution decides injector versus camera

Put the same camera on a PoE switch port, or a different injector, that is known to power another camera.

• Boots there and stays dead on your injector: the injector is the fault — its standard, its class or its pairset. Tests 2 and 3 name which.
• Dead on every standard source: the camera's powered-device front end. Before you send it back, run tests 2 and 3, because a class mismatch on an under-rated port looks exactly like a dead camera. If it still boots only on USB or auxiliary power, the PD controller needs service.

Test 2: compare the camera's class with the injector's standard

Read the camera manual's PoE line — its class or its watts — and the injector's label. A class 0 camera, 12.95 W maximum, runs on any 802.3af port. A PoE+ camera needs a 30 W 802.3at port; on an af-only port it is either denied at classification or starved at boot. Budget matters on shared hardware too: a multi-port injector's 30 W may be shared across ports, and a port already carrying 20 W has little left for a class 4 device. Checking draw against budget is the same discipline as a GMSL camera power budget against the capture card.

Test 3: which pairset does the injector power?

Two-pair PoE can deliver power two ways. Mode A puts it on the data pairs, 1-2 and 3-6, through the transformer centre taps. Mode B puts it on the spare pairs, 4-5 and 7-8 — the classic midspan approach at 10/100, where those pairs carry no data. A compliant camera accepts both; devices that accept only one exist, and a camera fed on the pairset its input does not use shows link LEDs with no boot — exactly the symptom you are chasing. The substitution test usually settles it: injector fails, switch port succeeds means check the injector's data sheet for its mode, then replace it with an endpoint injector adapter or a PoE switch port of the right standard.

Worked example: cable loss cannot starve a camera at bench length

Cat5e conductors run about 9.4 Ω per 100 m each. Power goes out on one pair and returns on another, with both conductors of each pair in parallel, so the loop is also about 9.4 Ω per 100 m. At the 802.3af maximum current of 0.32 A, a full 100 m run drops about 3 V, leaving roughly 45 V at the camera — comfortably inside the 37-57 V window a powered device must accept. A PoE+ port at 30 W draws 0.625 A and drops about 5.9 V, leaving about 42 V. Across the 5-15 m of a bench or a machine cell the drop is fractions of a volt.

The decision: a short healthy cable cannot starve a camera, so do not chase cable swaps for voltage reasons. Sag only becomes a suspect past the standard's 100 m, or on damaged, kinked or counterfeit cable. When you rebuild the run anyway, use factory-terminated camera data cable with locking RJ45s so the plugs cannot work loose:

The LS-CBL-0117 gives a 15 m locking run for a wall-mounted camera; the LS-CBL-0199 is the 3 m high-flex GigE cable for a drag-chain move.

What a 48 V reading at the camera proves

An injector's output sits near 48 V whenever it is connected; the standard allows 44-57 V. So 47-48 V measured across the PoE input points on the camera board proves that the injector, cable and connectors deliver voltage to the input bridge. It does not prove the handshake finished. The PSE holds power only after detection and classification pass, and a camera that classifies above the port's capability, or whose signature is pulled off-nominal by leakage or damage, is denied or dropped within a fraction of a second — the idle voltage reads the same either way. Proof of accepted power is current: a camera that passed the handshake draws its class current while it boots.

One more trap: measure with every other power source removed. With USB or an auxiliary supply attached, the board runs from that supply and the PoE stage sits unpowered or half-idle, so the reading tells you about the injector, not about the camera's boot path.

Fix by case

Four outcomes cover the bench:

1. Boots on a PoE switch, dead on the injector — injector mismatch. Replace it with an endpoint injector adapter or a switch port whose standard and mode match the camera. This is the most common resolution.
2. Never powers from a standard source, boots only on USB or auxiliary power — camera powered-device front end. Send it for service, and keep the station running on its auxiliary supply in the meantime.
3. Boots, then browns out when streaming or when an accessory switches on — budget. Move the camera to a PoE+ port; 25.5 W at the camera is the PoE+ ceiling, and a 30 W port with headroom covers boot inrush as well as steady draw.
4. Intermittent drops with no pattern — cabling. Rebuild with factory-terminated Cat5e or better, no couplers, and re-test; damaged pairs can pass link pulses yet corrupt detection.

Case 2's stopgap, on a camera with a separate power and I/O connector, is a cable from the camera's power pins to a bench or DIN-rail supply:

The LS-CBL-0300 is an M8 8-pin male-to-open camera power and I/O cable, 7 m, made for drag-chain use, so the PoE port can be left to carry the network only.

Prevention: label the standard, keep a reference PSE

• Buy the PoE class deliberately — injector standard, camera class and pairset chosen at ordering time, not at fault time.
• Keep one PoE-capable switch on the bench as the reference power source for every camera that comes in.
• Label each injector in the plant with its standard (af or at), its class ceiling and its pairset mode.
• Use factory-terminated patch cords on PoE runs; field-terminated and coupled runs are where detection signatures drift.
• After bench work involving boot-mode switches or jumpers, put them back at the factory default before the camera goes on the wall.

If the substitution test points at the injector side, the camera data cables category and the accessories finder list the locking and drag-chain runs, and the camera configurator pairs the camera with its cabling in one pass. If the camera never powers from any standard source, contact us with the model, the injector's label and your voltage reading. The neighbouring faults — data link up but no image, or a board that sags when a camera powers up — are covered in camera detected but no image data and MIPI camera works, the board freezes, both under the systems topic.