6.3.1 Leaking arc tube. Osram HQI-E 150 W/NDL CL
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Glow discharge Arc discharge Incandescent lamp mode
Fig. 29: Various states of outer bulb discharge
6.3.1 Leaking arc tube
High temperatures and pressures in the arc tube, the aggressive chemical substances in the tube and the thermal cycling of a lamp place extreme strains on the arc tube. This can cause the tube to leak, allowing starting gas and filling particles to enter the outer bulb.
Depending on the size of the leak, this effect is usually a gradual process. It is initially noticed by a considerable change in the light colour. Increasing leaks of starting gas into the outer bulb can result in the discharge process moving from the arc tube to outer bulb discharge.
– For lamps with evacuated outer bulb, various abnormal discharge states can occur, depending on tube filling pressure and outer bulb volume.
and lamp holder and 250 °C at the electrical contact of the lamp pin to the lamp holder. The electrical contact is also relevant for the Temperature Code of the socket (see also chapter 7.3 lamp holder).
If metallic coatings in the pinching area form through material deposition from the leads so that they form a continuous conductive layer between the leads, then the result in the so-called incandescent mode. The metal coating offers sufficient resistance that power is consumed and the coating begins to glow. It is hereby possible that electrical values similar to normal operation are reached, which would make it impossible for an electronic ballast for example to detect this abnormal situation. This also causes high temperatures in the pinching area.
– For lamps with gas-filled outer bulb, usually lamps > 400 W, glow discharge and incandescent mode do not occur. Particularly in lamps operating with ignition units, the described faults result in a direct arc discharge. In extreme causes, this can cause the lamp to burst.
Glow and arc discharges can be detected by current and voltage values deviating from the normal levels, so that an electronic ballast with a corresponding automatic cut-out feature can switch off such lamps. In addition, the luminaire design must use components resilient to high thermal loads so that the possibly high temperatures will not lead to harmful situations for the operator.
In the case of glow discharge, the voltage across the lamp is high but only very low current. Sputtering causes material to be deposited on the outer bulb. It is possible for glow discharge to precede arc discharge.
The temperatures in the pinched area are lower than in normal operation.
In the case of arc discharge, the voltage across the lamp is low and the current is limited by the choke.
The attachment of the arc onto the leads in the outer bulb can cause these to melt. The high temperatures cause the material of the leads to evaporate and then settle on the outer bulb. The hot arc near the pinching area can result in high temperatures (in extreme cases they can exceed 800 °C). At the contact between socket and lamp holder and at the electrical contact, the temperatures are naturally much lower. Here in extreme cases 300 °C were measured at the contact between lamp
6.3.2 Increase in re-ignition peak
The re-ignition peak is a peak in the lamp voltage after the zero crossing of current and voltage. For sinusoidal lamp current, the current decreases gradually before the zero crossing. As the plasma is heated by the current flow, a decrease in current causes the plasma to cool down and reduces its conductivity. After the zero crossing, the cooled plasma can initially no longer conduct the current through the lamp. As the current does not rise through the lamp, an increasing amount of supply voltage falls across the lamp. The rise in voltage causes the ionization of the plasma and therefore the current to increase again, meaning the plasma is reignited, hence the name “re-ignition peak”. If the re-ignition peak exceeds the level that can be provided by the supply voltage, the lamp goes out.
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