Transcript Electrical Works

ELECTRICAL WORKS
NGfL CYMRU GCaD
DISCLAIMER
Neither WJEC or NGfL accepts any
responsibility whatsoever for any injury,
however caused, relating to or resulting
from this document. By reading this, the
reader accepts and agrees that neither
WJEC or NGfL are providing training,
intentionally or unintentionally for carrying
out electrical works.
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CABLE SIZES
Correctly sized cables should be selected so that the cable
meets the power demands of the equipment it serves.
Cable sizes are stated in square millimetres, which is the
cross-sectional area of the wire inside. The larger this area is,
the higher the current, or load, it can carry.
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Fire can occur where a cable is too small for the amount
of current carried.
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Generally, electrical cables consist of three parts, namely: Conductors, which carry electricity.
 Insulators, which insulate the conductors.
 Protective jacket, which protects the insulation and
conductors.
This applies to cables supplying small electrical devices,
such as computers, monitors, domestic appliances etc.
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Large incoming electricity supplies are brought into buildings
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on three-phase supplies.
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‘Three-phase’ means that three separate circuit
conductors bring electricity into a building at the same
frequency (also known as a cycle), one-third of a cycle
apart. At any one time, a scenario could be:L1 Brown phase at peak value
L2 Black phase at 2/3rds value
L3 Grey phase at 1/3rd value
Neutral Phase – blue – allows the 3 phase supply to run
simultaneously as a single phase supply.
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Supply cables are sometimes
brought into a building via
underground ducts through the
foundations. The cables shown
left are over 50mm in diameter,
and originate from electrical
sub-stations.
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A main distribution board sends electricity to smaller
distribution board that are placed throughout a building. At
each stage of distribution, the voltage is reduced until it
reaches 240 volts drawing 13 amps of current.
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Distribution board before cables are connected.
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A local distribution
board. The blanks
(indicated by the red
arrow) provide spare
capacity, which have
not been used but
could supply
additional circuits in
the future.
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A list, or schedule
inside each
distribution board
shows which circuits
are served by the
distribution board.
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Cables may be left exposed, as in the above photograph.
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Alternatively, cables may be hidden. In the picture shown
above, the cables are concealed in the body of the lamp
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fitting, and through the spotlight rail.
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Different cables are used for different purposes: the above
image shows data cables supplying information via a file
server and an Information Technology network.
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Above – network cables before being pulled, or placed in
position. Cables are tied together to form looms to help cable
installation.
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Above: data cables (orange in colour) drawn into position.
Power cables run in the plastic dado trunking above the data
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cables.
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In order to prevent damage, cables
are often placed in steel mesh cable
trays. These can be fixed to walls or
soffits (ceilings) of a building.
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Cables can also be
contained in surfacemounted plastic ducts.
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This photo
shows the
correct
fitting and
fixing of
cables.
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The next two slides show how cables should not be
fixed.
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Incorrect installation
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Incorrect installation
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Cables awaiting
fixing inside a
distribution board.
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ALWAYS REMEMBER!
Electricity, in the wrong hands, can kill.
NEVER start work or touch electrical
equipment unless you have been trained
and items have been isolated.
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DISCLAIMER
Neither WJEC or NGfL accepts any
responsibility whatsoever for any injury,
however caused, relating to or resulting
from this document. By reading this, the
reader accepts and agrees that neither
WJEC or NGfL are providing training,
intentionally or unintentionally for carrying
out electrical works.
www.ngfl-cymru.org.uk