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Cables...
There are hundreds of different types and sizes of wire and cable. Some cables are much better at certain jobs
than others.
An electrical wire will generally contain a single conductor, usually insulated by a plastic sheath
protecting against short circuit to nearby metal objects.
An overview of cables
Wires and cables break down into four different areas:
Electric wire or cable
Usually carry power from one place to another; either inside
equipment or between power sockets and equipment.
Audio cables
Contain at least one core wire, covered in an insulating sheath
and then by an overall grounded metal shielding layer. The
shielding layer protects the inner wire from acting as an aerial
and picking up interference from nearby power cables or
equipment. The protective shield layer is, in turn, covered by at
least one protective and insulating plastic sheath.
Radio frequency cables
Constructed in a very similar way to single core audio cables,
but allow high frequency signals to connect from one place to
another, as efficiently as possible, without picking up any other
radio or interference signals. Any piece of open wire connected
to RF sensitive equipment will act as an aerial, both receiving
unwanted signals and sometimes transmitting signals that may
cause interference to nearby equipment. Use of the correct RF
cable will prevent this happening.
Data cables
Made up of a number of insulated wires that allow many data
signals to be connected from one digital circuit to another. If
the data has to travel from one piece of equipment to another,
the wires are sometimes wrapped up in an overall shield or
screen layer which prevents data signals from being radiated
to other sensitive equipment nearby.
| Cables in detail Electrical wiring Cables and wires have a maximum current rating that should never be exceeded. Choose a wire or cable with a rating of about 30% higher than the maximum current expected to flow in the circuit. Either measure the current using a multimeter or calculate from the power consumed by the circuit (rated in watts). If you know the voltage supplied and the power used; simply divide the power rating of the equipment by the voltage supplied to it. For example, a 1kW bar fire supplied with 230 volts will draw 4.35 amps (1000/230 = 4.35). Mains electrical use Cables between mains sockets and equipment generally need to be flexible and multi-stranded round cables are usually the best choice. The flat three-core single strand mains wiring (twin and earth) is used within walls and conduits to distribute mains around a building. It is prone to fatigue and could break if used in a position where it is allowed to flex. The choice of mains distribution cables in a house or other building is covered by the IEE Wiring Regulations that set out the different current ratings of cables for the various types of ring main and spur wiring. Audio cables The simplest audio cables are the thin lapped core cables, suitable for connections between audio or other low level signal boards inside equipment, or very short external runs in non hi-fi applications. Cables with braided screens are good for general purpose audio use are available in single and twin versions, for mono or stereo connections. They allow a high-quality signal transfer over short to medium runs. There are various special cables for the very best signal transfer and minimum interference. High-grade cables include oxygen-free copper, special additional insulating sheaths, silver-plated and gold-plated conductors for minimum connection resistance and surface resistance. If you need to make up long leads, use the best cable you can afford for the job. If, on the other hand, you just need a point to point run in a piece of equipment, an inexpensive thin lapped cable will be sufficient. |
Speaker leads Loudspeaker wiring is very different and needs a little consideration. Little loudspeakers, such as intercoms, can be wired up using almost any wire. Hi-fi loudspeaker cabling and in-car entertainment speaker wiring is much more critical. For any amplifier delivering over a few watts, a thicker and more substantial cable is needed for good quality reproduction of sound. You should select heavy-duty and hi-fi loudspeaker cables. If the bulk is a problem, there are several special flat variants that go under the carpet easily. Silver-plated and oxygen-free copper are also available and provide a very clean transfer of power to your loudspeakers. Radio frequency cables The first thing to consider is impedance; a combination of DC resistance as well as capacitive and inductive effects in the cable. It may be considered as the effective resistance of a cable to an AC signal. Most RF equipment is designed to work with a cable of a specific impedance: TVs, DVD players and satellite equipment are designed to use 75Ω cables, whereas radio transmitters use 50Ω cable. Select a cable with the correct impedance for the job, otherwise the signal could be lost, resulting in poor reception or, in the case of transmitters, heat up or burn out. Attenuation and impedance Attenuation is a measure of how much signal you will lose over a given length of cable at a stated frequency, expressed in Decibels (dB). A cable of the correct impedance for the application, with the lowest attenuation figure at the required frequency range will give the best results. Data cables Networking connections that use the BNC connector are designed to use 50Ω RF cable. Newer connections use special multi-way twisted pair cables to reduce both radiation and interference. There are various standards for these data cables (Category 5, for example) and it is always best to check the instructions supplied with each unit before choosing a cable. |
What else do I need? |
How do I join the ends up?
If you have many different terminations, you should use coloured or numbered sleeving at each end of each wire to aid fault finding or rewiring. Heat shrink sleeving joins cables where no connectors are required and shrinks over the individual joins to provide insulation or an additional protective layer.
Cable glands and grommets allow neat entry to boxes or through panels, and prevent cable chafing.
The ends of your cable will need to be securely attached to the equipment, by screw or clamp terminals or via a suitable set of connectors.
Check before choosing connectors that their current and voltage rating is sufficient and safe for the job in hand.
RF cables must be properly connected and joined by suitable RF connectors of the same impedance.