Philips Electromagnetic Lamp manual Hum, 139

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3.14 Electrical wiring

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.

3 15 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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