7.2 Susceptibility to disturbances

The same goes for the vulnerability of electronic ballasts. It is frequently reported that under certain conditions they keep on failing (Fig. 7.7), while nobody is able to identify exactly which these conditions are. And again, there is an implicit vow of silence spelt over the affair. In one case, for instance, a major electrical contractor received a complaint from a customer where among a large number of newly installed electronic ballasts a substantial share malfunctioned right from the instance of installation. The contractor replaced the failed devices and passed the complaint on to the supplier, one of the European market leaders in lighting equipment. He got a letter back saying, in polite wording, an initial failure rate of 17% was absolutely normal for electronic ballasts. The electrician told this to his customer, who requested a copy of that letter but which was declined.

Only at Paderborn-Lippstadt airport, a small but rapidly growing regional airport in Germany, two cases could be documented:

• Out of ≈80 electronic ballasts no less than 30 had failed within 4 months in one part of the installation. The same luminaires with the same type of ballasts, same producer and even same batch, work without a single problem in an adjacent part of the installation being fed from a different subdistribution but from the same transformer. No indication of the reasons for these failures have been found so far, except that from the branch with the faults exclusively this lighting arrangement was fed, while the other one also fed some other loads. This would mean that the ballasts kill each other, unless other loads absorb their litter, and provides further scope for speculation about the causes, but still no evidence.
• About half a year later the same problem occurred in another location of said airport, but with different ballasts from a different producer.

Fig. 7.6: An advantage of electronic ballasts: Offset of voltage variances. Disadvantage of this: Current intake increases as voltage sags

Fig. 7.7: Electronic ballast failures at Swiss Federal Institute of Technology Zurich within one year

Since the failing ballasts are now being replaced by high-quality magnetic ones, the failures have come to a halt. This provides further scope for speculation about the causes, but still no evidence.

At Kaufbeuren hospital about 480 luminaires were integrated into the ceiling, each fitted with 2 fluorescent lamps, rated 2*13 W, with 1 common electronic ballast. By end of 2004, some 800 lamps had to be replaced. The filaments had blown. After long vain efforts to find out about the causes, the electrician in charge found a coherence with the relatively long lines in the installation: On account of some very fast voltage fluctuation the electronic ballasts switched over to pre-heat mode. In the lab it was possible to reproduce this effect with a 50 m long line and a drilling machine, whereas it did not have to be a drilling machine but any other electronic device with a filtering capacitor at the input side did the »job«. It need not even be set into operation, just connecting it was enough to produce an extremely short (few microseconds) but very steep current rise time edge with an according voltage dip. The ballast misinterpreted this dip as an instance of switch-off and switch-on again and started to heat the filaments, waiting for the lamp current to rise as a signal of successful start, to shut off the heating current. But the lamp current did not rise because the lamp was already in operation, so the pre-heat current remained on and overloaded the filaments.

Another case occurred so to say right in place with a fluorescent lamp manufacturer at the final test of the production line for T5 lamps rated 80 W. The lamps are tested individually, so the test rack tests 1 piece every 6 seconds. Now the electronic ballasts installed in the test equipment did not bear this frequent switching and kept on failing, this making production stall each and every time it happened, along with all the cost impacts this brings about. But unfortunately T5 lamps cannot be operated with magnetic ballasts. Why can they not? With the 80 W lamp it does not work because the required lamp operating voltage is too high. At least as long as the applied voltage equals 230 V it is not possible but in commercial areas there is always a second supply level of 400 V available. At present a 400 V magnetic ballast is being developed with one of the ballast manufacturers. A prototype was exhibited at the 2004 Frankfurt Light & Building fair and is now being used in the shipment test procedure of said lamp manufacturer. Note that this implicitly means this manufacturer specifies its T5 lamps as fit for 50 Hz operation, since final test is carried out exclusively in this manner! The required 400 V electronic starter has already been made available [7] and is now being used in the test line – under the tough conditions of permanent response requirement, but without failure!

From another site it was reported the cause for permanent electronic ballast failures in a large hall had been searched for approximately two years until it was found out that they were due to mechanical oscillations. Fork lifters ran into and out of the hall all day long, and each time an automatic swinging door caused an air pressure wave that made the ceiling swing. Certain electronic components on the PCBs in the ballasts could not bear this and came loose.

Strangely enough, none of such failures have been reported so far about CFLs, although they employ the same working principle except for the electronic PFC. This may be because they are not used in such large quantities within a constrained area. It is more likely, however, that the PFC electronics is the main source of failures in electronic ballasts, since it needs to be located right at the input side of the inverter, where it is exposed to all surges and other disturbances coming in from the network.

Of course there is no alternative to the use of electronic ballasts wherever one and the same lamp is to be used on various voltages and frequencies or on DC. On many railway vehicles, for instance, lighting can reasonably be fed on DC only, as the vehicle is fed on DC or 16.7 Hz. Since the DC feeding makes the active power factor correction in the ballast superfluous, no mass failures have been reported so far, which again confirms that the PFC is the weak point. The older German »InterRegio« railway carriages may be counted as an exception, where quite obviously the ceiling lamps, which can be switched individually by travellers, are operated with magnetic ballasts and conventional starters, as can be concluded from the well known flicker during start. This means that a dedicated power system is created inside the carriage, fed by an inverter converting either the 16.7 Hz power from the locomotive or the 24 V DC supply of the carriage into 50 Hz, since using the 16.7 Hz would end up not only with a ballast of triple volume and weight, which would be a serious issue on a vehicle, but also with a stroboscope light. It is reported that this was done because typical disturbances on a train, such as pantograph sparking, had caused failures of electronic ballasts, but obviously this problem has been overcome, and today’s trains use electronic ballasts (but those without the dispensable electronic PFC) without causing any major trouble.

As for the voltage dependency or independency of the light output, one company in Germany carried out a test among various electronic ballasts and CFLs, an incandescent lamp (for comparison) and halogen lamps with electronic and conventional transformers. Surprisingly enough, just one type of electronic ballast from each of the three leading producers performed a complete compensation of input voltage variance (constant light output). It may be speculated that these three were the top models of the three brands. Some of the CFLs at least managed to come from a square relationship between voltage and power, as for resistive loads, down to a linear behaviour.