Usually all cameras, switchers, recorders, monitors etc are powered at a central point. However, the major drawback to a single power source for all cameras is distance.
For example, cameras that are fitted to a building with more than one floor, it would be practical to install separate power supplies on each floor as opposed to a single power source. With DC-operated cameras it is important to remember that the longer the distance the camera from the power source, the greater the drop in voltage due to wire resistance.
Wire size
Wire size is the major consideration in supplying cameras with operating power. If a wire is too small, the power will decrease quickly due to resistance in the line, causing degraded, noisy or possibly no video at all. If the wires are too large, it will be costly and cumbersome to work with. However, when dealing with power and wire, the common law is 'better too big than too small'. The American wire gauge (AWG) measures wire sizes. This is a means of specifying the wire diameter. The higher the AWG, the smaller the diameter.
For example, a 22 AWG wire has a smaller diameter than an 18 AWG wire.
Wire size is also related to resistance. Resistance is the stubbornness of the wire to pass electric current. Heat is the main by-product of resistance and can cause the wire to become warm or even hot if too small a wire is used to carry a large current. This would mean that the camera would not work properly. The larger the wire diameter, the more the current is able to carry for longer distances. This is most easily compared to a two and four-lane highway.
24 V a.c. type cameras
When working with 24 V a.c. cameras, specific wire gauges will be used to carry the power to the camera at maximum distances. Overshooting these distances can quickly deteriorate the camera's ability to produce a proper video signal.
Total power requirements
When designing a single power source system, determine the amount of current needed to properly supply the system. This only requires minimal math's to determine:
Actual power requirements can be found in the spec sheets for each model. For example, in order to determine how much power will be required for a system with 10 cameras, each camera requiring 0,75 A, do the following:
10 x 0,75 A = 7,5 A (round up to nearest full number) = 8 A power supply.
Also, bear in mind future requirements. It never hurts to add a few amps more to accomodate extra cameras.
12 V d.c.
The 12 V d.c. type cameras are the most commonly used. Again, when working with power requirements use the simple formula:
10 cameras each using 0,65 A per camera
10 x 0,65 A = 6,5 A d.c. (always specify at least 30% above for DC type power supplies) so use an 8 A power supply.
Furthermore, it is important to remember auto iris lenses also consume power. When doing calculations, add the current required to the camera current consumption.
For example: camera current required + auto iris lens current required = total current required. Only video type auto iris will add to extra power being used.
Lightning protection
Always add protection against transients and lightning to the power supplies used. In South Africa, more especially the Gauteng area, electrical storms in the summer can be devastating to electronic equipment and this includes CCTV equipment. The use of power surge protectors and MOVs incorporated into power supplies and mains supplies will prevent damage to expensive equipment.
For further details contact Abie Ali, Frank Street on tel: (011) 838 4515.
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