Philips Electromagnetic Lamp manual 3 10Electromagnetic interference, aThird harmonic of a phase

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3.9 Harmonic distortion

Fig. 125. Fundamental and third harmonic in a three-phase mains. R, S and T are the fundamentals in the three conductors. Owing to the phase shift, this results in a zero current in the neutral lead.

a)Third harmonic of a phase,

b)Third harmonic of all three phases in the neutral lead. The individual currents

reinforce each other.

R S T

(b)

(a)

0 60 120 180 240 300 360

3 10 Electromagnetic interference

Discharge lamps do not only emit visible radiation, they also generate radio-frequency energy in the radio spectrum.This can cause disturbance of the operation of electronic equipment such as computer keyboards, television or radio receivers, hence the name radio interference.

As the luminaires in which the lamps are used should fulfil international requirements such as EN 55015 (CISPR 15), the radio interference in practice is sufficiently low to have no harmful effects on the surrounding.

Products with the mark conform to VDE 875 part 1.

The generation of radio-interference radiation is normally caused by lamp electrode oscillations. It has a broad-band character, usually with frequencies of up to 1500 kHz, so FM and television receivers are not affected.

The electromagnetic waves, which can have effects on the AM broadcast band, are propagated in two ways: either directly through the mains into the receiver, or via radiation picked up by the aerial.

The latter form of interference will seldom occur with discharge lamps, as the ballast will suppress the broad-band signals.The radiation produced by the lamp will nearly always remain below the threshold value at which interference takes place, especially where the lamp is at some distance from the aerial (more than, say, 1 metre).

The supply cables can emit interference radiation when they are not buried in the ground or laid in earthed steel piping, which is the best screening against interference. However, it sometimes happens that an interference signal reaches the receiver by way of its mains input.The interference signal can consist of high-frequency harmonics of the mains frequency or high amplitude pulses.The former are generally adequately suppressed in the ballast. Experience has shown that interference may be caused by fluorescent luminaires with external ballast where the radiation from the supply wires is picked up by telephone or other cables. If external ballasts are used, the supply cables between ballast and luminaire should be as short as possible. Ballast coils should be split into two adjacent parts (split-windings type of ballast). In case of Class I luminaires the supply wires should be shielded and this shielding should be properly connected to the earth connection.

In practically all other cases it will be necessary to connect a delta filter between the mains supply and the input to the lamp circuit.

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