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Troubleshooting UTP CCTV Systems
Each system component deserves careful considerationThe Latest from SIW
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By Christine Baeta
Technical Services Manager, Network Video Technologies, Inc.
The use of Unshielded Twisted Pair (UTP) in CCTV systems is increasing, as is an interest in identifying common pitfalls or causes of poor video quality after installation. An ideal UTP CCTV system consists of several simple components, each deserving careful consideration in order to have a hassle-free installation. The key to troubleshooting is recognizing those ingredients. Compare the details of a problem installation to an ideal one and observe the video to identify possible culprits. Use these logical steps to reduce the time spent on the troubleshooting process.
When It is Not About The UTP
* Camera is Adequately Powered: Verify the camera's input voltage is within manufacturer's tolerances under load conditions (including heaters, blowers, etc.) With the camera connected and operating, measure the input voltage at the camera's power input terminals. Typically, it should be higher than 21 Volts AC for a 24VAC camera, and 11.5 VDC for a 12VDC camera.
* Camera is Properly Adjusted: Set focus, iris and shutter speed using a portable monitor. It is worth noting that many LCD monitors have internal Automatic Gain Control (AGC), making brightness adjustments difficult. The use of a Video Level Meter will result in more accurate levels, as demanded by today's sensitive DVRs. Alternately, use an oscilloscope or an older glass “jug” monitor to verify the signal.
* Avoid Ground Loops and Transient Damage: A ground loop is a system-grounding problem that occurs when one device in a system is connected to an electrical ground different from the ground of the other devices. At low voltage, the ground difference is detectable at voltages as low as 200mV, and can be identified as a “Hum Bar.” At high voltage, it is identified as a lightning hit or other transient. Large currents can travel through the wire to the other ground, damaging equipment along the way.
Leaving the camera floating is a far superior approach than local grounding. This may require rubber pads or plastic screws, but it is well worth the time in lightning-prone areas. Parking lot cameras or those mounted on metal-skinned buildings are particularly vulnerable.
If a transient protection device is used, place it right at the camera. Connect the protector's ground wire to the camera's chassis (or shield of the BNC connector), rather than the local ground, to prevent transients back-flushing into the system. This “single-point grounding” practice is highly effective for both high-voltage transients and low-voltage ground loops. These practices should always be subject to regulatory safety requirements such as Electrical Code.
Ground loops can also occur between cameras connected to a multi-output power supply if one or more of the cameras has an additional path to ground. Most multi-output power supplies do not isolate outputs from one another. Instead, consider powering cameras and peripheral equipment using supplies with isolated outputs.
