The
use of electronic ballasts for bi-pin fluorescents has increased
significantly since 1990. It has occurred not only in everyday
applications but also in potentially explosive atmospheres. The
benefits of increased performance levels i.e. higher light output,
lower energy consumption and extended lifespan, has seen a steady
decline of electromagnetic ballasts.
In 1994, a number of problems were reported in the USA and other
countries, concerning the use of electronic ballasts in
combination with T8, 26mm fluorescent lamps in ‘cold start’ mode.
These T8 lamps, using the ‘cold start’ method of operation, are
employed in potentially explosive atmospheres.
The problems related to increased temperatures at the electrodes of
the lamps which were at the end of their lifetime.
This caused damage to the lamp holders (see photo).
The problem is now known as the ‘End-of-Life’ phenomenon.
What is happening to a fluorescent tube at the end of its
lifetime?
To facilitate the lighting process, the electrodes of a fluorescent
lamp are coated with an emitter material. This emitter becomes
depleted during the lifetime of the lamp due to sputtering at
ignition and gradual emitter loss during burning. A lamp reaches its
end-of-life when all the emitter is consumed. A symptom of emitter
loss can be visual as it causes end-blackening by attaching
itself to the phosphor in the electrode area.
When all the emitter is gone, even the bare tungsten material starts
to sputter off electrons, possibly causing severe end-blackening
and 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 easily reaching 800ºC
for a very short time, the wire can drop on the glass and even melt
a hole in the tube. As soon as a leak occurs in the glass,
the process of ignition will stop. Statistically, less than 1% of
the lamps fail this way.
If the wire breaks but does not fall on the glass, however, a
discharge can result between the wire of the electrode and the glass
for a period of 60-180 seconds. The resulting temperature can be as
high as 300ºC. Subsequently the glass can weaken and the
ring of the electrode sags towards the glass. This could result in a
discharge and create high temperatures at the lamp-ends. If the
ring comes in contact with the broken wire of the electrode, the arc
discharge created could be maintained for hours and even days
with an ever increasing temperature rise. The process will stop
eventually when a leak in the lamp has developed.
Statistically, ALL fluorescent lamps can inherently have this
problem. However, by manufacturing the lamps with a so-called
weak spot’ (Philips patent), the lamps will start leaking and the
process will stop, but in a controlled manner.
It is clear that in potentially explosive atmospheres, this
phenomenon can have disastrous results that cannot be ignored.
The 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 the end of its lifetime. Philips, on their own
initiative, have already fitted their industrial electronic ballasts
with such circuits. In the meantime, the IEC standard for
fluorescent lamps (IEC 61195) has introduced requirements so as to
avoid the overheating of the lamp-ends by incorporating ‘suitable
measures in the electrical circuit’. It is, however, noticeable
that only a handful of Ex manufacturers have incorporated such a
circuit, despite the fact that this problem has been known
for a long time.
On the other hand, many users are totally oblivious to the problem
because they have not been informed.
The question is: Are the current fluorescent luminaires safe to use?
The Notified Bodies have finally come up with a proposal
to also include these stop circuits in fittings for potentially
explosives atmospheres, and resolve the problem once and for all.
New electronic ballasts have to comply with the latest EN60079-7
standard for Increased Safety equipment.
However, this question still remains. What to do with existing
installations in flammable atmospheres, where the safety of
personnel must come first?
After consultation, it appears that there are basically 4 options:
1. Regular inspections to identify and replace lamps which are
failing or have failed.
Symptoms to be identified are:
The
frequent inspections increase the maintenance cost significantly
without removing all the risks.
2. Group replacement with fluorescent lamps with a proven low
standard deviation; in other words one which is less likely to
fail e.g. Philips TL-D Xtra and Xtreme. The advantage of this option
is the preventative character. Unfortunately, it does not
remove all the risks and frequent inspections are still required.
3. Replace all ballasts of existing luminaires with a type including
the new stop circuit as specified in draft standard EN 60079-7.
Although effective, it is very expensive.
4. Replace the existing luminaires with a different technology not
affected by ‘End-of-Life’
e.g. the induction based maintenance free ‘sealed for life’ IQL
range, manufactured by
IMT, does not use electrodes to generate the light.
Conclusion
1. Through proper risk assessments, the user will have to determine
the corrective action required in order to safeguard its
personnel and installations.
2. Users will need to be educated concerning the extent of the
problem. The Health and Safety Executive, together with the
manufacturers of fluorescent luminaires for potentially explosive
atmospheres, will have to face their responsibilities in this
matter.
