Jun 102012
 

One of the main problems that need to be resolved in the design and construction of systems for video-surveillance is unconditionally the provision of power supply and connection lines. This broad topic directly affects not only the source of voltage, together with the power wires, but also the different lightning-protection modules and grounding elements. Practice shows that the correct design of the power module of video -surveillance systems can help avoiding a lot of trouble in the process of operation of these systems.
The main and most common mistake made when designing and installing video-surveillance systems is trying to implement the project in the cheapest possible way on account of using inappropriate power supply units and connection lines. This sometimes happens in spite of the bitter experience from mistakes already made by constructors of these systems, connected with reducing the reliability and the protection against accidental events impacting negatively the equipment as a whole.

The dalliance of construction electricians regarding power supply requirements creates prerequisites for errors, leading to malfunctions and failures of the operation of different components of video-surveillance systems, but also to serious damage of the whole equipment. Very often power supply failure leads to damages of expensive cameras (as the one, shown in the picture) and other electronics.

kamera za videonabludenie

In fact, in 99.9% of the cases of circuit- technical decisions, in the terminal step of the cameras there isn’t any contemplated galvanic, optical or other separation of the power supply line from the line of the video signal.

kamera

This means that these errors in such circuits provoke damages in the video signal of recording and control systems.
As a result, a cheap power supply unit unconformable with the technical assignment leads to failure of the whole video-surveillance system. At present three types of circuits for implementation of the video-surveillance power supply source are most common: linear, pulse and standby.
Considering the first constructive solution we need to specify that its practical realization has to follow a precise proof test of the whole equipment. Such caution is justified by the fact that linear stabilizers are most sensitive to inductive interference and generally to interference in reactive forming elements and the various fluctuations in the power supply. The principle of operation of the linear stabilizer is based on reducing and straightening the input voltage. If we have 220 VAC at the input, which is converted to 12 VDC at the output and we have alteration of the input voltage to 235-240 VAC, then this can instantly damage the video-cameras and the recorder, and the presence of a fuse in the circuit would save the system only from a fire and it is quite wrong to over-estimate its security functions. We should also mention one more considerable drawback of the linear stabilizer – its little efficiency.

All disadvantages of linear stabilizers mentioned so far are completely excluded in pulse converters. The inductivity of the power supply unit, its cable length, the external HF interferences, the arcing contacts and the difference in potentials of various locations in the system have little impact on the power supply unit. By using pulsed power supply units we can build a system not only with high reliability but also with high price/quality ratio.

A standby power supply source is most often used in specialized sites where the video-surveillance system is powered directly by a standby battery or by other sources of alternative energy in order to prevent failure in the operation of cameras in case of mains voltage drop, often intentionally switched off by thieves or criminals. In this case the problems in the mains line are minimized and the only condition for the absence of trouble is the fuse in the electrical circuit.

 

Connecting line
In order to build connecting lines in video-surveillance systems, depending on the installation conditions, there are three main types of cables: two-wire cable, twisted pair cable and combined cable. The most common of these is the two-wire cable.

fror
This cable is suitable for powering video-surveillance systems because of its temperature parameters and its protection from various influences, although its parallel conductors can accumulate difference in the potential if the cable is used for especially long distances. When building a power line it is necessary to take into account the presence or absence of other nearby power conductors. We need to pay special attention to the inacceptable parallel position of power and low-voltage lines, otherwise quite strong interference may occur in the line connecting the cameras in case of switching the power lines. You should know that such two-wire cables in most cases are unshielded and unprotected from HR interference. For this reason, very long cables begin to function as an antenna and receive all external interference.
The twisted pair cable, in turn, is not intended for use as a power cable, but despite that it is often used by various assembly groups.

ftp

The main mistake is the use of the twisted pair cable both as power supply cable for the cameras and as signal transmitting cable, no matter if the signal is analog or digital. This leads to two main problems: First, the proximity of the wires, twisted around each other in pairs, has a very strong negative effect on the quality of the image and second – the small cross section of the conductor significantly limits the length of the line. This type of cable must be used very carefully in video-surveillance systems, because while designing the track of the system we must take into account the voltage drop depending on the length of the line. The use of a twisted pair cable is inappropriate when connecting a video camera with a coaxial cable designed for a load of 75 Ohm.

kabel za videonablyudenie

When choosing a cable we must consider the complexity of the site, the proper evaluation of the interference from external sources and the safety requirements. According to high standards for picture quality and safety, the video signal cable must be mandatory shielded, the power cable must have a section calculated with 30% reserve and must be made of quality copper.

 

Commutation
As practice has shown the most reliable connections are made through a quality solder or a tight connector. The conductors must be properly cleaned and tinned because over time the copper base of the cable gets oxidized which causes interruptions during the operation of video-surveillance systems. The higher the oxidation level is, the greater the voltage drop is at this point of connection and at high currents this may lead to arcing, heating of the wire, short circuit, and consequently – a fire. Many manufacturers provide an M-F terminal clamp, which is suitable for indoor installation, but is completely inappropriate when installed outdoors. Finally, we will add that despite the wide variety of systems for video-surveillance on the market, we must be extremely careful because very often behind the beautiful packaging there are conductors with very low content of copper in the wire and the beautiful golden contacts get rapidly oxidized by the moisture in the air. You should trust only established manufacturers and retailers who have responsible attitude towards customers and a good reputation, as is the case with Vikiwat Ltd.

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