Cable – with a twist

Traditionally, analogue CCTV equipment has been connected using coaxial cable, low voltage remote power cable, and RS422 cable, for video, power and control respectively.

Category 5 twisted pair now supports video, low voltage power and RS422, provided vendor-specific cabling guidelines are followed. (See Figure 1.) This has opened up the possibility of powering and controlling analogue CCTV cameras via standard twisted pair cable.

The support for copper twisted pair for the above signals has led to the problem of how to manage these multiple signal pairs within the context of a structured cabling system.

The main issue is how to efficiently combine and break out the appropriate signal pairs to and from each camera without resorting to cable splicing techniques along the home run cable itself. Similarly, at the central monitoring side, the same signals must be merged onto the Cat 5 cabling system for distribution to each camera.

For example, each camera requires at least one twisted pair for video.

If the camera is powered from the central console, then one or two extra pairs is needed. If the camera supports PTZ control, then a fourth pair is needed. (See Figure 2.)

Since not all cameras require all three signals, it is necessary to combine and extract the desired signals depending on the type of camera connection.

Maximum distance

In regard to the issues involved in CCTV signal concentration, one of the main issues is distance performance. The critical signal is the remote power.

Video and control will normally surpass remote power in terms of distance. Remote PTZ control can usually be transmitted up to 4,000ft, andvideo up to 2,200ft and 1,500ft with analogue monitors and DVR, respectively.

Remote power distance depends on a number of factors: cable grade, cable gauge, input voltage level, camera power consumption, and permissible voltage drop at the camera.

The accompanying tables illustrate the distance performance based on two or three pair Cat5 transmission of remote power. Based on this information, it is possible to determine what is the maximum distance that can be supported under actual conditions. The tables – developed by MuxLab – specify the maximum distances that remote power may be transmitted via two or three twisted pairs.

In order to construct these tables, the following assumptions have been made: distances are specified for temperatures from 20 to 30 deg C; maximum distances allow for a 10% voltage drop at the camera; distances for 28v AC power are based on a camera that is rated for 24v AC; and if 22AWG gauge wire were used, there would be an increase in distance of approximately 55%.

Protecting the wiring

The second issue in regard to power is short circuit protection. Cat 5 twisted pairs are often terminated under RJ-45 plugs and jacks, where accidental short circuits can occur more easily.

In order to protect against this, power supplies with individually fused outputs are highly recommended.

Furthermore, since CCTV remote power may now reside under cable jackets that carry other voice and data services, there is a greater need to protect the cabling.

Companies such as Altronix carry Class II power supplies that meet this requirement. According to one installer, institutions such as schools and hospitals insist on fused or over current circuitry protected power supplies in a CCTV installation.

Methods of combining video, power and control

The integration of video, power and control over Cat 5 requires products that combine and break out the three signals into their respective services in order to route them to the appropriate equipment. In a hybrid system, it is necessary to specify products that will integrate easily into a structured cabling system. One method is to use ‘Siamese’ or round composite cables that combine two Cat 5 twisted pairs for video and control, together with two 16AWG wires for remote power within a single cable. (See Figure 3.)

Another option is the use of CCTV ‘pass-thru’ video baluns that consolidate video, remote power and control under one Cat 5 cable. The baluns allocate two or three twisted pairs for power, depending on the model.

Fixed cameras allow for three twisted pairs for power (See Figure 4A).

PTZ cameras allow for two pairs for power (See Figure 4B).

Figure 5 illustrates an application that combines video, power and control over Cat 5 using pass-thru baluns.

The advantage of Siamese cable is that remote power may be transmitted further via 16AWG power wire.

The advantage of using pass-thru baluns versus Siamese cable is the savings in cost of Cat5 cable compared to Siamese.

Furthermore, from the structured cabling perspective, terminating the cable via a RJ-45 plug is neater and more user friendly.

As the CCTV industry heats up, installers and contractors are looking for more cost efficient CCTV cabling solutions without compromising on quality and reliability.

Improved cable and video balun technology are bringing this goal closer to reality.

• Jeffrey Herman is a product manager at MuxLab, which designs and manufactures CCTV and audio-video connectivity solutions for copper twisted pair cable. For more information about CCTV via twisted pair, contact Jeffrey Herman at j.herman@muxlab.com. For more information about Siamese and round composite cables for CCTV, contact Belden/CDT at info@belden.com.