6.3.10 Conclusions. Osram HQI-E 150 W/NDL CL
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Rectifying effect causes a high DC current component.
As a result, the choke goes into saturated state with a marked decrease in choke impedance. In extreme cases, the lamp current is only limited by the choke’s ohmic resistance.
Permanently excessive current causes a dramatic increase in the temperature of the choke windings until the insulation is destroyed and short circuits occur between the choke windings.
6.3.10 Conclusions
• Safe operation of metal halide lamps depends on the use of luminaire parts (lamp holder, leads etc.) which can withstand the high temperatures that can possibly occur in the case of outer bulb discharge.
• Apart from burst-protected lamps for operation in open luminaires, all lamps must be operated in closed luminaires.
These phenomena can occur with metal halide lamps
(see warning in IEC 61167) so that the standards have stipulated safety measures for luminaires (see IEC
60598-1 Paragraph 12.5.1). Similar regulations exist for high intensity sodium vapor lamps in the IEC 60662 standard.
• All metal halide lamps with small wattage must be equipped with a safeguard to protect the lamp from the effects of asymmetrical conductivity (e.g. chokes with thermal protection).
• It is advisable to use ignition units with time cut-out.
A safety measure in the circuit such as a thermal switch or a thermal fuse, integrated into the magnetic ballast, protects the circuit from such damage.
• The use of electronic ballasts is beneficial if the electronic ballast has a corresponding cut-out mechanism.
In accordance with a declaration issued by lamp manufacturers in reaction to standard EN 62035, published by the LIF (Lighting Industry Federation Ltd) in Technical Statement No. 30, and by the ZVEI in the “Lamp manufacturers statement regarding EN 62035”, certain lamps do not require any safety measures to prevent asymmetrical conductivity. As far as OSRAM’s metal halide lamps are concerned, this refers to lamps with wattage levels of 1000 W and more.
It is wise not to operate metal halide lamps right up to the end of their natural service life, but to replace them at the end of the economic life. This is appropriate because the luminous flux decreases noticeably on exceeding the economic life, and the probability of undesirable effects increases at the end of the service life.
Although asymmetrical conductivity is generally possible in lamps with wattages > 1000 W both during the start and in steady state, the dimensions of the arc tube and lamp components means that the tendency to asymmetrical conductivity is far less than with smaller wattage levels, and is weak enough in steady state that no safety measures against asymmetrical conductivity are required.
Lamps should be replaced before reaching the economic life if
• the light colour of the lamp changes noticeably,
• the luminous flux decreases considerably,
• the lamp will not ignite,
• the lamp goes on and off intermittently (“cycling”).
OSRAM PTi is not affected by asymmetrical conductivity, as current and voltage are monitored and controlled, which is why it is recommended for the operation of discharge lamps.
On complying with all safety measures, metal halide lamps are safe to operate and provide a brilliant, efficient light.
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