All fluorescent lamps enter an end-of-life condition when the electrode emitter material is depleted, ie burnt away due to the normal ignition or burning process. A symptom of emitter loss is end-blackening. When all the emitter is gone, the tungsten electrode material starts to sputter off electrons, causing severe end-blackening and ultimately the failure of the electrode. The lamp has now reached the end of its lifetime and should be replaced.
Without effective protection, the ballast will continue to drive the lamp or will try to ignite the lamp repeatedly. The temperature in the region of the electrode increases rapidly and causes overheating. If the electrode breaks, at temperatures reaching 800ºC for a very short time, the wire can drop onto the glass and may melt a hole in the tube or may cause discharge and high temperatures for several minutes, or longer if the lamp ring makes contact with a broken electrode.
One solution to the problem is an effective stop circuit in the electronic ballast. This stop circuit prevents the lamp from igniting when it has reached its end of life condition. The IEC standard for fluorescent lamps (IEC 61195) has introduced requirements to avoid the overheating of the lamp-ends by incorporating ‘suitable measures in the electrical circuit’. These requirements have also been incorporated into the draft EN 60079-7 Standard for Increased Safety Equipment.
In order to comply with the regulations cited below, dutyholders should be aware of the risks and precautions required in respect of the Ex equipment on their installations, including the risks of overheating due to end of life tubes. This requires knowledge of what types of fittings are installed, and what the manufacturer’s recommendations for maintenance are and whether the ballasts fitted are likely to cause overheating in the event of failed tubes.
It is the responsibility of the user to ensure that existing luminaires are safe and that the lamps are replaced before the end of life conditions become critical. There are basically four options.
– Carry out frequent inspections to identify and replace lamps that are failing or have failed. However, this will not remove all the risks. Typical symptoms to be identified are:
yellowish/reddish discharge near the electrode;
– Group replacement with fluorescent lamps with a proven low failure rate. Unfortunately, this does not remove all the risks and periodic inspections are still required.
– Replace all ballasts of existing luminaires with a type having the new stop circuit as specified in draft standard EN 60079-7.
– Replace the existing luminaires with a different technology not affected by ‘end-of-life’, eg Ex luminaires using induction technology.
It was specially actual in early 2000. We did not know why many light fittings went broken. Then We started to realize the real reasons.
Keep up good work.