Q: How do you decide which type of cable to use when designing a CCTV system?
A: When designing a CCTV system each component of the design is of equal importance. It is no good installing high-resolution cameras and monitors within a digital CCTV surveillance system and then using inferior cable to connect it all together.
Here are the main factors that should be considered:
- Cable runs measured in metres (m)
- Location of cable run indoor or outdoor protected (run in ducting) or unprotected clipped to buildings or buried)
- Interference from other cables and equipment.
- The nature of its construction dictates that whatever cable you use will act as a resistance to the signal you need to transmit down it. The resistance encountered also increases as the frequency of the signal increases.
- Although a camera signal transmits over a wide frequency spectrum it is the high frequency end 5Mhz (highest loss) that is normally used to calculate which cable you use and how far you can run it.
- There are two types of cabling used, the Coax (unbalanced) cable and the TP -Twisted Pair (balanced) cable. Coax cable is used for shorter runs (eg RG59 – under 250m, CT125 -up to 500m) and TP on longer runs (up to 1.5 Km).
- 75 Ohm cable is the predominant standard in the CCTV industry. The coaxial cable is available in varying degrees of quality and cost.
- Obviously, the higher the quality (this is measured in loss of high frequency per 100 metres) the higher the cost. This will be covered in the next question.
Two additional considerations when buying coaxial cable are:
- The quality of the screen. The screen protects the signal from electrical interference, so before deciding which quality of screen is required you need to assess each cable run, e.g. is it travelling in a duct with other load bearing cables?
- The quality of the cable Jacket. The jacket protects the cable from the environment, for example RBS jacketed cable protects against moisture, if you are trenching and burying cable then a steel armour as well as a waterproof jacket should be considered.
- TP (balanced) cable has a different impedance to coaxial cable, 100 and 150 Ohms are the most commonly used. As opposed to having one centre core, the TP is made up of a pair of cables twisted at approximately one to two twists per metre. TP is widely used over longer cable runs, as the high frequency losses are a lot smaller than coaxial cable. The twisting construction of TP cable also provides better immunity to electrical interference.
CCTV equipment is usually designed to cater for 75-ohm coax cable termination, additional equipment is therefore normally needed in order to launch and recover video signals over TP cables. You will need to consider this when designing and budgeting your CCTV system.
Q: I often hear the term dB banded round the office – what does it mean and how does it relate to cables?
A: Cable performance is usually defined as having a certain loss of signal. This performance is measured in units called Decibels or (dB). The Decibel is a means of measuring the ratio of two signals in our case what we put into the cable at the camera end and what we get out of it at the monitor end.
Cable specifications quote signal loss as being a certain dB loss at 5MHz (High frequency end of our video signal) per 100 metres.
Ideally, it would be preferred if the video signal had no loss, however in practice an acceptable loss would be around 6dB.
If you equate this to RG59, which has a quoted loss of 2.25dB per 100 metres at 5 MHz, we can calculate the maximum distance we can run before exceeding 6dB:
=( 6 / 2.25) X 100 = 267 metres.
The table on this page lists typical cable types together with their dB loss and practical cabling distance.
When specifying the cable lengths it is always advisable to build in an additional ten per cent allowance to cater for practical variations that will be encountered when installing the cable, eg scale drawing tolerances, temperature variations and cable connections.