Lighting Tutorial [E-BOOK]

3.1.3 The starting process and the lamp lifetime expectancy

In the washroom of a single-family home new lamps were installed around 1970 (Fig. 3.15). The new luminaires were refurbished with electronic starters in their very early days. Since then, one of the lamps has been replaced once and the other one never ever. Neither has any of the starters, of course. You can see that the old 38 mm diameter T12 lamp is still in place and working (Fig. 3.16), while these were taken over by the 28 mm thick T8 lamps already in 1980. The washroom is at the same time the passage to another cellar room so that the lights are switched relatively often, at average about 5 times a day, while the average operation time is low, barely an hour a day. So this lamp has done around 10,000 service hours and 50,000 starts to date.

Fig. 3.15: Fluorescent lamps in a residential washroom with magnetic ballasts and electronic starters: Only one lamp replacement within well over 30 years

Fig. 3.16: Removal of the cover reveals: One of the two lamps is still of the old T12 type (38 mm diameter) and is still working well.

The lifetime of fluorescent lamps is usually identified by means of the switching cycle given in IEC 60081, according to which the systems are operated for 2:45 h and turned off for 15 min. It says there that the starters used in such a test have to comply with the IEC 60155. However, this standard refers to glow starters, and hence the lamp lifetime rating with magnetic ballasts refers to ignition by means of glow starters. For the practical lamp life, or for the impact from the number of starts upon same, respectively, optimal pre-heating is crucial. The operation of one and the same lamp on an electronic warm-start ballast is sometimes rated with a 30%, sometimes with a 50% longer lifetime. Ratings for immediate-start electronic ballasts are generally not given, albeit these represent the generic implementation of electronic ballasts. With magnetic ballasts, however, the inverse procedure is always preferred and the – unfortunately very low – standard of the glow starter chosen to be the normative benchmark.

Regardless of this, some experts actually do regard the electronic ballast’s »warm start« as rather »lukewarm«. For an optimal lamp life, they say,1 the cathodes would need to be heated for at least 2 s, and nor was the pre-heat current normally what it should be, but such a long pre-heat time seems to be an intolerable intrusion to the contemporary user.

Obviously the same applies to the contemporary manufacturer, since according to Philips a triphosphor fluorescent lamp lasts for about 60,000 h without any switching on and off, while in the standard test, say 8 switch-on and switch-off cycles a day, it does 15,000 h. A halophosphate lamp reaches about half of these values, while special long-life brands may be able to more than double this life span – with all the appropriate caution in the context of such ratings, for it takes several years to collect several years of operating experience. The open question always is whether a recently accomplished experience is still topical by then, or whether the respective version of the respective type of product is already off the shelves again.

The further question is how long a fluorescent lamp really does last when optimally pre-heated. Unquestionably, however, the commonplace glow starter falls far below this optimum, since, as explained earlier, it replaces one starting process with several starting attempts and thereby multiplies the number of ignitions. Even the one out of several attempts finally leading to success may have run in an optimal manner only in a few exceptional cases. Rather, in the majority of cases, the instantaneous current amplitude at the instance of ignition may have sufficed only sparsely rather than with a rampant reserve for successful ignition, which again places some additional wear impact upon the filaments. The inevitable radio interference filtering capacitor inside the glow starter attenuates the voltage peak, rounds it, lops it and hence reduces its efficacy. So how high is then the improvement potential of starters »other than glow starters« according to IEC 60927, such as electronic starters, which, dependent on ambient temperature and other parameters, always fire after the optimal pre-heat time and always at the peak of the current, hence always successfully at first attempt? Commensurate data from industry is lacking here, too. Not even interested parties such as the manufacturers of such starters have it because these companies are small and such measurements are quite extensive and accordingly expensive.

But as the most common lamp types last roughly 4 times as long when operated in continuous mode as they do in the standardized test cycle, this facilitates the conclusion that with the switching frequency of above mentioned standard of 1 on / off period per every 3 h only 1/4 of the lamp wear is due to the »normal« generation of light but 3/4 account to the switching. Beyond, there are at least a few hints and experiences pointing towards the far superior lamp preservation through electronic starters.

For instance, on the demonstration panel by one of the manufacturers, which is regularly exhibited at trade fairs, the lamp operated with a glow starter (Fig. 3.17 bottom right) has to be replaced after every second fair – along with the starter. These lamps are operated according to the extremely fast test cycle according to IEC 60155 for testing the starters, i. e. 40 s on / 20 s off. Hence, the respective lamp has done about 6,000 ignitions when done. By then either one of the filaments is interrupted, or the starter’s contacts are welded together, so that the fuse of the »fused« starter trips, or the lamp only more flashes and flickers instead of burning continuously, which will also trip the fuse and set the lamp dark.

The lamp with electronic starter, however, has survived 25 exhibitions in the meantime, totaling up to no less than 120,000 starts (with only some 1,300 h of operation, though), still being fully functional with hardly blackened ends (Fig. 3.17 top left). Deviating from this, the lamp test according to IEC 60081 with a net operating time of 2:45 h per every gross 3 h of test duration, executed with glow starters and hence leading to a lifetime of 15,000 h, totals to an overall test duration of 16,364 h. So this duration will include 5,455 starts, obviously being responsible for 3/4 of the overall ageing effect. Let’s add another 1/4 on top, and the fluorescent lamp would theoretically be »done« after 7,264 glow starts without having burnt at all. Of course it always needs to have burnt for a moment in the respective time intervals between ignitions – at least long enough to verify that it did fire successfully. This complies quite precisely with the observation on the demo panel. Assuming it was not »about 6,000 starts« but precisely 7,264 starts within 81 h of operation, then this point, together with the two points »5,455 starts with 15,000 h« and »60,000 h with just one start« comes to lie exactly on the black line given in Fig. 3.18.

Hence, despite all caution applicable for lack of data, the available observations facilitate the conclusion that, in the standardized cycle including one switch-on and one switch-off per every 3 h, the major part of the ageing impact upon generic fluorescent lamps is due to the respective number of firings. Using the same lamp and the same magnetic ballast, however, the switching frequency obviously has as much as no effect at all upon the lamp life as soon as the glow starter is replaced with an electronic starter! This even remains valid if the »unscientific« ac hoc values used here should be by a factor of, say, 4 in error – including the exemplary prices named here.

An information brochure by the lamp and lighting section of ZVEI includes such a benchmark plotting of light flux curves of T8 fluorescent lamps operated on magnetic and electronic ballasts. Strangely enough both of the charts do not only resemble each other but appear to be absolutely identical right down to the last pixel. One tends to assume a typographical error, a swap of pictures, but why has nobody realized this yet since 2005? The other charts at least exhibit some divergence from each other. Care has been taken with these ones, however, to use a mean value out of a lead-lag compensation with 50% inductive and 50% capacitive lamps. This appears awkward, though, because at exactly this very time this very ZVEI issued the recommendation not to use the lead-lag compensation any longer (rather than to adapt the capacitance ratings for serial compensation to contemporary conditions – such as to include the shift of the voltage rating from 220 V to 230 V, the lamp power rating e. g. from 65 W to 58 W, as well as much narrower tolerance margins with lamps, starters and magnetic ballasts – see Section 5). Without this, however, the new lamps are substantially overloaded with the old capacitors.

Had separate charts been given for the inductive and the capacitive circuits of a lead-lag configuration, as would have been the correct approach, it would have turned out straight away why it was intended to abolish the lead-lag configuration. However, simultaneously it would have become obvious that the inductive circuit, optionally equipped with parallel compensation as recommended by ZVEI ever since, provides approximately the same lifetime expectancy as the warm-start electronic ballast.

Fig. 3.17: Trade fair demo panel – at the bottom right the lamp as well as the starter have already been replaced some 12 to 14 times, the one top left not yet ever

In an old report by Philips from 1995, however, which on top of all was designated »for internal use only«, the systems still used to be recorded separately. It gives rather precisely 16,000 h of lamp life when operated on an electronic ballast – with a relatively low uncertainty margin of about ±10%. In the uncompensated magnetic ballasts group they achieved quite precisely 15,000 h. This is no more than a 7% advantage for the warm-start electronic ballast. In return, the group with the immediate-start electronic ballasts achieved only 13,000 h and hence even 15% less lifetime expectancy than could be identified with magnetic ballasts and the decrepit glow starter (Fig. 3.18)! The suboptimal magnetic ballast configuration with serial compensation and the oversized capacitance value achieved 12,000 h and thereby only insignificantly less. It is very likely that a better pre-heating than at present commonly used on the market would be possible to implement in contemporary electronic ballasts and would also be commercially reasonable to use with respect to life cycle costing for the whole installation. More information on when it is worthwhile to turn the light off or when to better leave it running will be given in section 8.1. An industry, however, supplying electronic ballasts as well as lamps may still calculate a positive balance selling one electronic ballast at 30 € per luminaire and in return lose 1/3 of its turnover with replacement lamps. On the other hand, replacing a throwaway starter at 30 cents with an electronic one coming at 3 € and with a similar lifetime expectancy as the magnetic ballast at 12 €, both lasting about as long as the whole building, and in return sacrificing the entire electronic ballasts market along with about 70% of the lamp replacements is quite obviously very economical for the user but not for the supplying industry.

Fig. 3.18: Lifetime expectancy of generic triphosphor fluorescent lamps plotted against the number of starts and the types of ballasts and starters used

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