A wire is a conductor of electricity. As such, all conductors have a resistance to a DC current flowing in them. The higher the resistance, the less current flows. If a cable is used to transmit power, the more power it carries the thicker the cable must be.
CABLE CONSTRUCTION
Cables can be multi-stranded or of a single solid conductor. Stranded cable has a slightly higher resistance than a solid conductor, and it will make a difference over greater distances. But stranded cable is more flexible.
Different metals have different resistance, with gold, copper and silver being the best conductors of electricity. Copper is the norm for cables, offering good strength and good conductivity. But cheaper cables are available, and often made of steel coated in tin or copper. When used externally, steel and copper will corrode in damp air, so it's best to use tinned copper or steel cable.
The insulation on a cable has the duty of protecting the wire from voltages of other cables, and keeping the weather out. The plastic wrap around the wire conductor is called the dielectric. Cables for use in low voltage of 12/24 Volts will have an insulation rating of 50/100 Volts, whereas a 240V mains cable will have a safety rating of 500 Volts.
IDEAL OHMS
The selection of a cable is the consideration of: Impedance, Attenuation and Frequency.
Impedance is the resistance to the current flowing. At DC currents the impedance becomes the resistance of the wire. As the frequency increases, the impedance becomes a factor of the cable's capacitance and its inductance. Cable capacitance is created by the adjacent wires and insulation, storing small charges of electrons which charge and discharge along the length of the cable, and modify the data as the signals travel along.
Different signal sources are designed to operate into different terminal impedance. Most audio operates into a terminal impedance of 47,000 ohms. Power amps operate into 8R, domestic TV into 50R, CCTV video into 75R, telephones operate into 600R, and RS 485 into 100R.
Incorrect termination will result in the signal being reflected back into the cable, causing ghosting images. The cable impedance must be suited to the signal output impedance.
Attenuation is the loss in a signal at high frequency. The higher the frequency and the further it travels, the greater the loss. The unit of attenuation is the decibel (dB), calculation of which is very simple. For each 3dB of loss, the signal loses 50 per cent. A further 3dB loss will cut the signal by 50 per cent of what was left, or by 75 per cent.
For example, in the video world a 3dB loss on the 1 Volt peak-to-peak video signal is more than enough to turn a colour signal into a poor mono signal. Therefore, if we have a 3dB loss of signal, and boost it with a 6dB amplifier, we end up with a 3dB gain or a 1.5 V pp signal.
Frequency Electricity travels down wires. At DC (direct current) potentials, the current passes through the entire conductor – therefore thick copper cables offer the best conductivity and the lowest losses. As the frequency increases, the conduction takes place on the surface of the conductors. At RF (radio frequencies), all the conduction is on the surface of the wire, and cheaper copper-covered steel cable is adequate. CCTV video signals, on the other hand, consist of a mix of frequency from 0 Hz to 5MHz. The best conductor for CCTV is solid copper.
ROUND THE TWIST
The type of cable used in most intruder alarm systems is not very demanding. The signal is normally a DC voltage of about 200mA at a nominal 12 Volts. The standard cable used has a rating of 1A, 10R resistance over 100 metres, and insulation of 60 Volts. Typical alarm cable consists of even strands with a common twist, the material used can be copper or tinned steel.
If high current devices are used, as for powering Mag locks or cameras, the expected current is 0.5 A per lock. If the power is fed down a single strand of cable, then only 8-9 volts will be available at the business end trying to hold the lock in. The higher the current load the greater the voltage drop, therefore thicker cable must be used, with a low impedance of 10R/100M. Optionally, twist several strands of cable together.
BALANCING ACT
The type of cable used starts to get critical as the frequency of the data sent increases. Noise immunity and reliability of transmitted data become paramount. Data cable impedance is normally 100R/100M.
A step forward is the use of balanced lines and twisted pairs, which most data formats utilise. In balanced lines, instead of the signal varying between 0 Volts and plus Volts, it alternates across the 0 Volt reference, often alternating between plus 12V and minus 12V. The advantage of not referencing to 0 volts is that the signal is not connected to 0 V or earth. It will not become modulated by mains frequency it picks up from earth.
In an unbalanced line, the signal is referenced to the 0 Volts. If mains frequency (50 Hz) is picked up, the data signal will sit on top of the small, mains-induced mains voltage, and the total signal will fluctuate. Signal corruption will increase as the distances increase, limiting the distance the information can be usefully sent.
Additional benefits can be gained from balanced lines by twisting the data paired cables together. As the cable twists one way it would pick up a positive going noise voltage then, on the next twist, the induced noise would be negative, and the resultant noise voltage cancels itself out. Cables with a tighter twist offer better noise immunity.
TO SCREEN OR NOT TO SCREEN?
Screens protect the conductor wire from noise and EMI (electro magnetic interference), and in noisy environments add extra protection. However, the shield also acts as a micro capacitor along its length, absorbing the data signal. The distance a signal can travel will be reduced if screened cable is used.
To work correctly, the screen must be connected to a good earth at only one end. If both ends are connected to earth a mains earth loop will be created, with electrons flowing along the screen into the earth at one end and back through the earth to flow into the screen at the other end.
There are further considerations if using screened cables externally – if the screen is damaged or nicked, then the integrity of the screen will be broken, and a mains loop will develop into the screen.
Telephone transmission, PSTN modems, and serial data (RS 232/485) are not very demanding and almost any twisted pair cable will suffice. RS232 does not transmit well as it is of low power, but RS485 (used in most security systems) is robust, and can be transmitted up to 1 kilometre if terminated correctly.
HIGH FREQUENCY DIGITAL SIGNALS
Once we move into computer network transmission cable we are dealing with high frequency digital signals. One standard which defines quality of cable and operating frequency is called Category.
- Category 1 is voice and modems up to 34000 bps
- Category 2 is ISDN, up to 2 Mbps
- Category 3 is Base10 Network up to 10 Mbps
- Category 5 is Base100 Network and ATM (Asynchronous Transfer Mode) up to 100 Mbps
Base100 network hardware has only been developed in the past few years, yet all new networks are being installed to the new standard. Base100 operates at 100 Mega bits per second, therefore the quality and cost of this cable is higher.
High quality cable is no longer a length of wire, it is an engineered product with precision drawn wire, a controlled density dielectric, and precision formation to maintain the paired cables at an exact distance apart.
Data cable is available in single or multiple pairs, with or without screen, individual screened pairs or a common screen.
COPPER IN THE MIDDLE
Unlike data transmission, which utilises balanced signals, audio and video use unbalanced signal, where the signal alternates between 0 Volts and plus Volts. To maintain a consistent signal voltage at the receiver, the transmitter must send out a known voltage signal. Therefore all receivers have a fixed input impedance, to maintain minimum signal loss and stop the signal bouncing back off the receiver and causing ghost images.
Because of the twists in cable, twisted pair cable is unable to maintain a consistent impedance along its length. Only coaxial cable can achieve this and maintain a constant impedance, which is why it is used for audio, video and most digital data.
As the video signal varies between 0Hz and 5MHz, the centre conductor needs to be in copper. The better the screen, the greater the noise rejection. The further the signal travels, the greater the signal loss. Standard URM70 and RG59 are suitable for distances up to 200 metres, while for distances up to 500 metres, a thicker coax cable such as RG11 or CT125 must be used.
WATER IN THE WORKS?
Another factor in signal loss is the insulation material or dialectic around the conductor.
The best insulator is air, and low loss cable is available with a honeycomb insulator consisting of air pockets. This is adequate for indoor usage, but not for external underground use – if the insulator is damaged, the honeycomb will absorb and retain water, which will convert a low loss cable into a high loss cable.
Have you ever wondered why the quality of the picture fails after rain, or in cold weather? It might well be that water is getting into the cable or video connectors. Low loss CT 125 is also available with a solid dialectic, for burial in the ground or in ducts.
A regrettable downside to coax cable is that both ends of the cable are connected to earth, guaranteed to induce a mains earth loop on to the video signal. If the picture is showing earth or hum loop bars, the problem is sometimes easily rectified by fitting an isolating transformer, or video isolator, to the cable. The transformer breaks the screen and conductor, and thus the earth loop.
FIRE AND SAFETY
When installing cables in an office environment, there will normally be a requirement to install fire-inhibited cabling. Most telephone and alarm cable is made from cheaper inflammable plastics, whereas most network data grade cable is made from some form of flame-retardant plastic, either Riser or Plenum (LSF: low smoke and fume). When fitting video cable into office buildings, LSF coax cable is often required.
Source
Security Installer
