Long CCTV Power Run Failing in the Afternoon
A campus has 16 fixed analog CCTV cameras, each rated 12 V DC, 400 mA. A single 12 V 10 A power supply is installed in the control room. The farthest camera is 180 m away and is wired with 0.5 mm² copper cable whose loop resistance is 0.068 Ω per metre. At commissioning, the farthest camera works in the morning but drops out in the afternoon; the measured voltage at its terminals is 7.0 V under load. The other 15 cameras are on runs shorter than 60 m and operate normally. The installer proposes replacing the 10 A supply with a 12 V 20 A unit. The site has frequent lightning, and each camera housing is earthed locally at its pole.
1. Calculate the voltage drop on the 180 m run and explain the afternoon failure.
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Loop resistance for 180 m = 0.068×180 = 12.24 Ω. At 0.4 A the drop is 12.24×0.4 = 4.90 V, leaving 12 − 4.90 = 7.1 V, consistent with the measured 7.0 V. Most 12 V cameras need about 10.5-12 V; at 7 V the camera's regulator drops out, especially when IR LEDs or a heater add load in the afternoon. The maximum run for a 1 V drop is R_loop = 1/0.4 = 2.5 Ω, so L = 2.5/0.068 = 36.8 m. For a 2 V drop it is 73.5 m. The 180 m run is far outside either limit.
2. Evaluate the installer's proposal and give two technically correct alternatives with trade-offs.
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The 20 A supply is not a fix. The camera draws 0.4 A regardless of supply capacity; the drop is I×R, set by the cable, not the supply's current rating. A bigger supply can deliver more current into a fault. Correct options: (a) install a locally earthed 12 V PSU at the far pole, with a surge protector and a mains feed; this is often cheapest for one distant camera but needs a power source and surge protection. (b) Distribute 24 V or 48 V and use a local DC-DC converter at each camera; current halves at 24 V, so the drop is about 2.45 V, but converters add cost and failure points. (c) Replace the cable with 2.5 mm² copper, whose loop resistance is about 0.0138 Ω/m, giving 2.48 Ω and about 0.99 V drop at 180 m; this is passive but costly in conduit.
3. What earthing and surge issue must be checked before adding a local supply or converter?
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Check the earth potential difference between the pole and the control room. If the camera is earthed at the pole and the control-room PSU is earthed at the building, lightning current or a nearby strike can create a large potential difference on the coax shield, damaging the camera and the DVR. Use a single-point earth for the video signal, add coax surge protectors at both ends, bond the pole earth to the building earth with a properly sized conductor, and prefer a local PSU or fibre/balanced video link to break the ground loop. Do not rely on the 12 V negative as the earth.