There are hundreds of different types and sizes of
wire and cable. Although any piece of wire may be used to join
point A to point B, some cables are much better at certain jobs
than others.
Generally, a wire will contain a single
conductor, usually insulated by a plastic sheath so that it
will not short out to nearby metal objects, other wires, or
allow anyone to be electrocuted. Cables usually contain more
than one conducting wire, insulated from each other and then
covered by an overall protective and insulating sheath. Broadly
speaking, wires and cables break down into four different areas:
This usually carries power from one place
to another, either inside equipment or between power sockets
and equipment.
These usually contain at least one core wire, covered
in an insulating sheath, which is then further covered by an
over-all grounded metal shielding layer that protects the inner
wire from acting as an aerial and picking up interference from
nearby power cables or other equipment. The protective shield
layer is, in turn, covered by at least one protective and insulating
plastic sheath.
RF cables are constructed in a very similar
way to single core audio cables, but are specially constructed
to 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 from the world around it, and sometime stransmitting
signals that may cause interference to nearby equipment. Use
of the correct RF cable will prevent this happening.
These are 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 the data signals from
being radiated to other sensitive equipment nearby.
The first thing you need to do is to work
out which of these basic types will be most suitable for the
job you want your wiring to do. From the descriptions above,
this should be fairly easy, but then the more difficult part
begins.
The answer to this is yes, you probably can make the
cable as long as you need to, provided you think carefully about
the job the cable will be doing, and where it is going to be
used.
Once you know what sort of cable you need,
you will probably find that there are many different wires or
cables that would seem to do the job. However, not all cables
will do the job as well as others. Let’s go through our main
four types again but look at the reasons you might choose one
cable or wire over another.
The main considerations here are the amount of current
to be carried by the cable, and whether it needs to be very
flexible, or is fixed in position once wired up.
Cables and wires have a maximum current rating and
this should never be exceeded. In fact you should choose a wire
or cable with a rating of about 30% higher than the maximum
current you expect to flow in the circuit.
If you don’t know the current that will
be drawn by your circuit, you can either measure it using a
multimeter or calculate it if the power consumed by the circuit
is known (rated in Watts). It is simple to work out the current
that will flow in a supply cable if you know the voltage supplied
and the power used. |
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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).
For mains electrical use, cables are supplied
in just a few standard current ratings so, once you know the
current drawn by the device to be wired, you simply need to
look at the appropriate range of cables and pick a cable with
a rating comfortably higher than the figure you have calculated.
Cables between mains sockets and equipment
generally need to be flexible, and multi stranded round cables
are usually the best choice here. The flat three core single
strand mains wiring (twin and earth) used within walls and conduits
to distribute mains around a building are inflexible and prone
to fatigue and break if used in a position where they are allowed
to flex.
The choice of mains distribution cables in a house
or other building is, in any case, covered by the IEE Wiring
Regulations, which set out the different current ratings of
cables to be used for the various types of ring main and spur
wiring.
Within equipment, the same considerations
exist. You need to work out how much current will flow through
internal wiring, then decide whether the wiring will be fixed
in place, or whether a multi-stranded, more flexible wire would
be more suitable, often the case if occasional movement of circuit
boards may be necessary for service or modification.
Power supply and switching contact wiring,
amplifier outputs and so on will be carrying the highest currents,
and you need to choose thicker wires to carry these without
heating. Thinner wires offer more resistance than thick ones,
meaning that power will be lost in the wiring if too thin a
wire is used. The longer you need a lead to be, the more resistance
it will present to the circuitry it is connected to. This means
that signal or power will be lost in the lead, but you can minimise
this by choosing a cable with a lower resistance, usually a
thicker cable.
This is particularly important where the
voltages involved are fairly small, and proportionally more
voltage drop occurs across a long lead. For example, the use
of thin wiring to supply an amplifier in the back of a car from
a fuse under the bonnet may result in a drop of several volts
along the length of the lead when the amplifier is being used.
This reduces the power supply available at the amplifier to
perhaps 9 Volts, resulting in a poor quality distorted sound
at anything above medium volume.
So, use a thicker wire for less loss and
better performance and efficiency, especially over long lengths.
Having decided that you need a screened
audio cable (except for loudspeaker outputs) you simply need
make a suitable choice from the many cables on offer.
The simplest audio cables a
re 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.
They are not suitable for longer runs or for high quality audio
connections.
Cables with braided screens are good for
general purpose audio use. They allow a high quality signal
transfer over short to medium runs and are available in single
and twin versions, for mono or stereo connections.
At the top of the range come various special
cables for the very best signal transfer and mimimum interference
and hum pick up. High grade cables include oxygen free copper,
special additional insulating sheaths, silver plated conductors
for minimum connection resistance and surface resistance.
Where capacitance per metre is quoted,
lower capacitance leads will have less of a loading effect on
high frequency signals and are suitable for longer runs.
If you need to make up long leads for line
level connection of, for example, a video recorder and a Dolby
surround amplifier, then 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 fine. |
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