PoE Camera Reboots on a 78 m Cat 6 Run
A 78 m Cat 6 run feeds an 802.3at Type 2 outdoor IP camera from a 24-port PoE switch. The camera draws 12.5 W normally but reboots when the IR illuminator and heater turn on, about 25.5 W. The permanent link passes wire map and length on a qualified tester; TDR shows no opens or shorts. The switch reports class 4 and delivers 53 V DC at the PSE. At the camera, a PoE tester reads 44 V under the 25.5 W load. The cable is 23 AWG, and the run is in a conduit with 11 other cables. The contractor replaced the camera once and wants to replace the switch next. The camera datasheet allows 42.5-57 V at the device, so the contractor says 44 V is fine.
1. Calculate the expected DC loop resistance and voltage drop for the 78 m, 23 AWG run at the 25.5 W load, and compare with the measured 9 V drop. What does the comparison suggest?
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23 AWG copper is about 0.0669 ohm/m per conductor. For a 78 m run, the loop (out and back) is 156 m, so R_loop = 156 x 0.0669 = 10.4 ohm. At the camera, P = 25.5 W and V = 44 V, so I = 25.5/44 = 0.58 A. Expected drop = I x R = 0.58 x 10.4 = 6.0 V. The measured drop is 53 - 44 = 9 V, about 3 V higher, which implies about 5 ohm of extra loop resistance (9/0.58 = 15.5 ohm total). That is too much for 23 AWG cable of this length. The cable length is not the main problem; a high-resistance connection or damaged conductor is more likely. 44 V is within the datasheet range, but the excessive drop is a symptom and leaves little margin during inrush or heater cycling.
2. Wire map and TDR pass. What is the most likely fault, and what tests/actions would you take before replacing the switch?
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Most likely fault: a high-resistance termination or connection, such as a bad punchdown, damaged plug or jack, corrosion, or a partially cut conductor, which a continuity/wire-map test will not catch. TDR may show length and opens or shorts but may not resolve a small series resistance. Actions: measure pair resistance and resistance unbalance with a cable analyzer or DMM at both ends; inspect and re-terminate the TR and camera ends; swap the patch cords; check for moisture or corrosion in the outdoor connector; test the camera on a short known-good patch cord at the switch; and measure voltage at the camera under load after each change. Replace the switch only after the cabling and connections are proven good and the PSE output is loaded correctly.
3. The contractor argues that 44 V is acceptable because the camera datasheet allows 42.5 V. How do you respond?
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The datasheet minimum is the absolute operating limit, not a design target. At 44 V the camera is only 1.5 V above cutoff while the heater and IR load are switching, and the measured 9 V drop shows energy is being lost in the cabling. Startup inrush or a cold heater can pull the voltage below 42.5 V momentarily and cause the reboot. Also, the high-drop connection may heat or degrade further. The correct response is to find and fix the excessive resistance, then confirm the camera has margin under worst-case load. If voltage still approaches the minimum after repair, use a higher PSE voltage within standards, a shorter run, larger conductor, or a local power source, not a datasheet waiver.