EoL (End Of Life) phenomenon vs Ex

Back in early 1999/2000 many flurescent tube Ex type light fitting manufacturers suffered from the effect of EOL. The more and more ballasts went broken with no real reason behind. Big players like P, O, etc. started to investigate and came to effect of EoL.
What is EoL?
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During the operation of fluorescent light fixtures, the electron-emitting material on the electrodes is consumed. This has the consequence that the energy to release the electrons is increased, and this, in turn, can lead to a higher voltage drop at the electrodes of the fluorescent lamps.
Since the electronic ballast functions as a constant current source, a higher voltage drop results in a power consumption increase. This produces a rise in the surface temperature at the lamp tube ends. The temperatures may exceed the limit values specified for explosion proof apparatus and thereby effect the overall explosion protection.
EoL 1 – Asymmetric pulse test
If an increased voltage drop is detected (as in the case of an old fluorescent lamp), the effected lamp power circuit is switched off.
EoL 2 – Asymmetric power test
In this testing circuit, the increased power consumption loss of the used fluorescent lighting element is measured. The lamp power circuit is switched off before the limit value for power loss (< 10 W) is reached.
The following applies to both of the test methods:
If the old lamp is replaced with a new one after electronic ballast switch-off, then the ballast functions properly once again. All safety circuits function selectively: only the power circuit for the defective lighting element is deactivated.
Currently all Ex type light fittings with flourescent tubes in possible hazardous environment shall be fitted with EOL integrated.

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