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protection can be obtained by covering the insulation with a glass-fibre
sleeve.
In order to keep the chances of heat damage to the insulation to a
minimum, the wiring run is so chosen as to avoid as far as possible any
‘hot spots’ in the luminaire, such as ballast or lampholders.
The cable fasteners used to hold the wiring in place should allow it
some slight freedom of movement, for the insulating covering of wire
that is under mechanical strain will have a lower heat resistance than
that specified by the manufacturer.
There is an internationally standardised colour coding for electrical
wiring, namely that specified by the IEC: brown for live, blue for neutral
and yellow/green for earth.
The only time when a departure from this colour coding is permissible,
is in the case where luminaires have internal wiring that is visible
when the unit is in use.A white insulation is then often used so as to
blend in with the white of the housing.The proviso here is that the
connection block is clearly labelled.
Hum
In general, lamps, ignitors, capacitors and even luminaires do not produce
any disturbing noise level when correctly used in their application.
Sometimes during the starting process some hum or rustling can be
noticed, especially with glow-switch starters. If hum is noticeable, it
almost always comes from the ballasts. Anyhow, when used in indoor
applications, e.g. shops, the hum level caused by control gear should
be as low as possible.
The electric current passing through the coil of a ballast causes
a magnetic field, which arranges the disorderly arranged elementary
magnetic particles of the ballast iron. So we find in the iron
magnetostriction and magnetic poles.
The ordering of the elementary magnets causes a certain deformation
of the iron (magnetostriction),resulting in the iron expanding in certain
directions.This process is repeated every half cycle if alternating
current is used and results in a noise of 100 Hz and higher harmonics.
The magnetic poles exert forces of attraction in the air gap of the
ballast core, also resulting in a noise of 100 Hz and higher harmonics.
The generation of these magnetic vibrations can be suppressed to a
high degree by means of a suitable design of the ballast. In particular,air
gap filling and ballast encapsulation can contribute to low noise levels.
But the magnetic field also spreads outside the magnetic core.All
magnetic metal parts in the immediate surroundings of the ballast,such
as the ballast case, the sheet-steel of the luminaire, etc., are subject to
forces in this magnetic field and can cause noise.
To avoid unpleasant ‘humming’ noise, constructions for the ballast
mounting, as well as the ballast mounting itself, must be as rigid as
possible.The hum will be more pronounced if the ballast is mounted
on a resonant surface.Avoid loose metal parts and create distances
between ballasts and metal parts.
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3.14 Electrical wiring