Outdoor CCTV security cameras can be prime targets for lightning. A lightning strike can destroy the camera and can damage the control console with energy flowing back through the coax and camera power wiring.
When lightning strikes a tower or other large structure, there is a high peak voltage at the strike point with current flowing downward and outward through any path to earth ground. A support pole develops a high L di/dt peak voltage drop along its length to earth ground. A large steel reinforced structure can conduct the energy to earth ground through its steel reinforced concrete footers and electrical ground system. A camera mounted and grounded to a building with steel reinforced construction will usually have less inductance to ground than a camera mounted on a self-supported tower or pole. Less inductance to earth ground means less peak voltage at the camera.
When lightning strikes a wooden or other insulating support, whatever voltage is necessary to continue the arc is developed at the strike point to overcome the resistance of the non-conducting structure. This usually has catastrophic results to the equipment on top.
Although very different, identical conditions exist for both examples. A high peak voltage occurs at the strike point with reference to earth ground. The video and power wiring to the camera are insulated from the strike point by electrical circuitry in the camera and the external insulation around the wire. Energy will flow through the camera circuitry in an attempt to equalise the wiring with the instantaneous peak voltage occurring at the strike point.
To protect equipment, there must be a low inductance path to earth for lightning energy and properly rated protectors on all interconnected wiring from the camera to the operating console. A properly rated protector at the camera allows the wiring to be equalised to the peak voltage at the strike point without allowing damaging currents flowing through the camera circuitry. An appropriate protector at the console blocks damaging incoming voltages to the control/monitor console.
A camera mounted on a building should be grounded to the building's structural steel as near to the camera as possible. Use 38 mm copper strap for grounding. If the camera is mounted on a metal pole, it should be grounded to the pole and a proper ground system installed at the base. When mounted on a wooden or other insulating support, the camera should be grounded to a minimum 76 mm copper strap running from the camera mount to the ground system installed at the base. An additional 3-inch copper strap would run from a lightning rod or diverter to the ground system at the base. Separate the two straps on opposite sides of the pole and connect together only below grade. Side mounting the camera or providing a diverter above the camera provides some additional protection from a direct strike.
A lightning ground system would be capable of dispersing large amounts of lightning energy (usually electrons) into the earth very quickly. The faster it disperses electrons, the less time there is for damaging surges to flow in the coax and power wiring back toward your operating console.
'Ground loops' can occur whenever long video coax runs are used. The symptoms include horizontal black bars (60 Hz hum bars) moving vertically through the picture. Ground loops are created when a potential difference exists between grounds and the coaxial cable is grounded at both ends. Current will flow through the coaxial cable shield and induce an opposing flow in the centre conductor. The induced current is usually 60 Hz a.c.
For more information contact Hardus Oelofse, Soft Control Integration, 011 315 7710, [email protected], www.softcontrol.co.za
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