6.3.6 Breakage or differing wear of the electrodes. Osram HQI-E 150 W/NDL CL
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6.3.6 Breakage or differing wear of the electrodes
Breakage of an electrode or differing wear in the electrodes with choke operation can cause a flow of asymmetric current with DC components, which can result in the choke overheating. This effect of asymmetrical conductivity is dealt with in greater detail below.
A broken electrode in a ceramic lamp can cause leaks in the arc tube as a result of overheating capillaries, with the effects described above. In rare cases, a discharge attachment near the arc tube wall in ceramic lamps can cause the arc tube to burst.
When the outer bulb is broken after the tube has burst, very hot fragments of the arc tube come into contact with the luminaire.
OSRAM therefore strictly differentiates between lamps for open and closed luminaires. Lamps for open luminaires have a mechanical safeguard around the arc tube to ensure that all fragments remain intact within the outer bulb should the arc tube burst. Compliance with this requirement is ensured through inhouse tests at OSRAM, which are much more stringent than in actual operation and in some published standards, for example, ANSI Standards.
A broken electrode in a lamp with quartz arc tube can, after a longer period of time, cause the arc tube wall to bulge and possibly leak or burst, if the discharge still persists.
This is the corresponding pictogram for lamps of this kind as per IEC 62035.
6.3.7 Scaling of the base / socket
Particularly in the case of old ignition units without automatic cut-out and aged lamps or soiled contacts, high transition resistances can cause oxidation and thus overheating of the contacts. When ignition pulses persist for a longer period of time, if lamps have gone out because the re-ignition peak is too high or if the lamp has not ignited, it is possible for arc-over to occur in the socket. If scaling has occurred, the socket must be replaced as well as the lamp.
As it is generally not possible to rule out the possibility of the lamp bursting for all other lamps, metal halide lamps must be operated in closed luminaires, which are designed to contain all hot fragments of the lamp in the case of it bursting. The corresponding pictogram for lamp and luminaire according to
IEC 62035 is shown on the right.
Silicate glass panes are recommended as a cover screen. When plastic screens are used, it is important to ensure that the hot parts of the lamp will not melt or set fire to the screen should the lamp burst.
The cover screen must be both resistant to temperature change and break-proof.
Vibrations can cause the lamp to become loose with the possibility of arc-overs and scaling caused by the resulting poor contact. In these cases the usage of a lamp holder with retention device is recommended, as described in the standard IEC 60238 “Edison screw holders”, section 2.23 “Lamp holder with retention device”. The test conditions are described in section 12.14.
6.3.9 Rectifying effect
High intensity discharge lamps can assume an asymmetrical mode (rectifying effect). There are various possible causes:
Suspending the luminaire on a chain attenuates vibrations compared to suspending it on a rope.
6.3.8 Bursting of the lamp
It is generally possible for the arc tubes of metal halide lamps to burst. This is very rare for ceramic metal halide lamps; the probability is greater in lamps with a very old quartz arc tubes. With progressing age, the quartz crystallization increases, making it brittle. However, the lamps normally fail by going out.
• Differently heated electrodes:
This is typical when the lamp starts, but is normally only short-lived. The DC component sends the choke into saturated state, the magnetic resistance decreases and current is limited to a lesser extent, shown as an example in Fig. 32.
This effect is described in the standards as „inrush current“ (IEC 61167).
During operation, the arc tube is under great pressure. When the arc tube bursts, fragments can fly at great speed, destroying the outer bulb when they hit it.
• Malfunction of one of the electrodes:
This can be caused by differently worn electrodes, or in rare cases by a broken electrode.
The result is longer asymmetrical lamp current, or if an electrode has broken off, a permanently asymmetrical lamp current.
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The effects are similar to rectifying effect at the start, but the longer persistence can cause overheating of the choke and ignition unit.
• Discharge in the outer bulb:
As the leads are not geometrically the same, the discharge generated between them can be asymmetrical, with the effects described above.
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Fig. 32: Asymmetrical conductivity with lamp current and lamp voltage during a normal lamp start. This is only a transient effect which causes no harm.
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