Designing and installation of CCTV systems for large sites needs careful planning. Dermot Grace of the UK's Norbain SDL, a leader in CCTV systems, explains the various factors to be taken into consideration. PROTECTING a large site from potential vandals and criminal activity using a CCTV system can at first appear a tricky affair. How can the perimeter be covered? Won't it involve vast numbers of cameras? How do you get the images back to a central point? How can all the equipment be effectively controlled?

However, with a basic understanding and knowledge of how a CCTV system works and its fundamental components, ensuring an effective CCTV system on a large site should not be problem.

Defining a large site

A large site can be defined as one that has one or a combination of more than one building, more than 16 cameras and/or a camera that covers an area up to 150 m radius or more.

Design/site considerations

As with any CCTV system design, the first step is to determine what is required and whether a CCTV system will be able to do this. This should be followed by risk assessment, which clearly define which areas of the site require surveillance and in which order of importance. It may not be necessary to monitor the entire area and one may decide to use entry/exit cameras only. Look at what objects or targets are in each area that might obstruct the background view of the camera. This will assist in defining the number of cameras finally required.

At this stage, getting a clear understanding of any up and coming plans for site development could save a lot of hassle in the future. In other words, find out what, if any, aspects of the site are likely to change and over what time frame. This will enable one to build a degree of flexibility into the system design, if it is required.

In theory (and within reason), most CCTV equipment could be used in a large site. But certain types of equipment have been designed specifically with this in mind.

Cameras

A system camera, for example, is one that is best suited to working in a CCTV system where there are a number of other cameras. They come with a wider range of features than standard cameras, as a larger CCTV system requires a number of adjustable parameters if it is to be successful, such as the ability to cope with differing mains phases, light conditions and sources, and the ability to cope with a wider variations of lens options.

Cameras that work off mains voltage are ideal as they work at 50 fields per second, which synchronises to the mains voltage which also runs at 50 cycles per second; if all cameras' fields are started at the same time, that is, line locked, the image switching is steadier.

24v AC cameras can also be used as they offer the same line-lock facilities and are safer because of the lower operating wattage, although they will require the use of a power adapter and consideration needs to be given as to where this is to be installed. 24v AC is of particular advantage in multi-storey buildings, which have different mains phases on adjacent floors; a stage of cameras may be reliant on one PSU in this case.

Location is also important for two other aspects - the location of the camera itself (it needs to be accessible for maintenance) and the location of the back focus mechanism, which enables the imaging device to be mechanically aligned with the focal point of the lens.

Cameras that support a range of lens options are important when designing a system for a large site. Zoom lenses are normally chosen for pan/tilt system cameras because of the wide area they are able to cover. However, it should be noted that varifocal lenses are ideal for fixed cameras in a large site as they make installation of a large number of fixed cameras both simple and flexible.

Apart from the environmental factors which will determine housing selection, don't forget to look ahead - a relatively large housing may be required so that additional equipment can be put in at a later date.

Lighting

In a large site, consideration must be given to the lighting requirements. If R (infrared) systems are used, energy consumption can be high. Can existing lighting be used and is it adequate for the job at hand? If not, then what is the most cost-effective source of lighting and is it economical over a long period? Also, it might be worth considering if it is more economical to employ more expensive cameras that work in low light conditions in place of extra lighting.

Cabling

One unavoidable cost in a large site comes from cabling. It is important to work out the distance and route of the cabling from the cameras back to the monitoring point. It is also important to note that a direct route is not always possible. Power other than 240v AC may have unacceptable voltage drop over longer runs if not available locally. Ducts and segregation of cables can add costs, which are not immediately apparent.

Monitoring

Monitoring of the system should also be carefully considered. Will it be monitored continuously, over a 24-hour period? If not, there is no point in having pan and tilt units as there is no one there to operate them. In addition, look at what camera should go on what monitor. Be aware that to have a dedicated monitor for each camera is very expensive, so consideration needs to be given to using multiple images on fewer monitors, either by having consecutive sequenced images on a monitor, or by having a selection of them displayed in multiple format. Vigilance levels will suffer over time, so thought should be given to low level alarm responses to nudge the operator if quiet scenes become active as in Video Motion Detection (VMD).

Designing and specifying the right system is a highly skilled job which should be undertaken in conjunction with a reputable security installer. But don't think a large installer is necessary to undertake this and the installation work.

At the end of the day, it is not the size of the installation company, or even the experience in designing CCTV systems for large sites (although it certainly helps) that counts. The important aspects are an in-depth understanding of CCTV in general, plus the ability to be flexible in both the design and after-sales support.