Get switched on to lamps
Though by all accounts they’re soon to be history, tungsten filament lamps are still in very widespread use. Resistive loads they most certainly are not. At the instant of switch on, a tungsten filament lamp draws an inrush current that can be anything up to 16 times its normal operating current. To put this into context, a motor started direct on line will normally only draw a peak current of around eight times its full load current. This makes it very clear that switching filament lamps is a particularly severe duty for contactors.
Other types of lamp have different but equally demanding requirements. Sodium vapour and mercury vapour lamps, for example can, at start up, draw a current of 2.2 times their operating current. That may not sound much, but the start up time during which this current flows can be as long as ten minutes. Metal-halide gas discharge lamps have similar characteristics to these metal vapour types, while mercury blended lamps – which are essentially metal-vapour lamps with an integral ballast – have a starting up behaviour similar to that of ordinary filament lamps.
We’ve left one of the most common types of lamp until last, simply because it deserves a more in-depth treatment. This is the fluorescent lamp, which is essentially a particular type of gas-discharge lamp that incorporates heated filaments to aid starting. While lamps of this type have been in widespread use for many decades, their importance is, at the present time, increasing because almost all of the high efficiency lamps currently in use are, in fact, compact fluorescent lamps (CFLs).
These are really no more than small diameter fluorescent tubes that are curled up so that they occupy a similar space to an ordinary filament lamp. They are also almost always supplied with integral ballasts, unlike ordinary fluorescent luminaires where the ballast components are usually separate from the lamp itself.
The starting behaviour of fluorescent lamps is largely determined by the type of ballast. With conventional choke-and-starter switching, a pre-heating current of around 1.25 times the normal operating current flows for a few seconds before the lamp strikes.
Note, however, that power-factor correction capacitors are frequently used to compensate for the reactive current produced by the choke. These capacitors draw a very large current spike as they charge up at switch on. This must be taken into account when selecting contactors for switching fluorescent luminaires.
When electronic ballasts are used, as they are in many modern luminaires for conventional fluorescent tubes as well as in all CFLs, short but large current peaks are generated at switch on, caused by the charging of capacitors in the electronic ballast circuitry.
Because, as we have seen, lighting loads are far from simple, contactor manufacturers like Moeller Electric have introduced products that are specifically optimised for switching them. Key features include extra-large terminal clamps to accommodate the oversize cables needed to avoid voltage drops on the long wiring runs found in many lighting installations and contacts that are specially designed to handle reliably the large inrush currents associated with many types of lamp.
Even when lighting contactors are used, however, it’s still essential to consider the type of lamp that’s being switched, and to choose a contactor with an adequate rating for that specific lamp type. It’s easy to see why if we use the ratings of Moeller Electric’s DILL12 contactor as an example. This has a thermal rating of 27A, which falls to 14A for a tungsten filament lamp load. For fluorescent tubes that have conventional ballasts, the rating is 20A, but for fluorescent lamps with electronic ballasts the rating is 12A.
Herein lays a very significant and none-too-obvious pitfall for the unwary. Low-energy bulbs are, as we have already seen, compact fluorescent lamps (CFLs) with electronic ballasts. When choosing contactors for switching loads of this type, therefore, the appropriate rating must be used – 12A in our example. That’s not, perhaps, a million miles away from a tungsten rating of 14A but, if the difference is not taken into account, it could still lead to shortened contactor life.
The situation is much worse if the contactor rating for conventional fluorescents (20A) is used in error for an installation fitted with CFLs where, as we’ve seen, the correct rating is 12A. With such a large difference in the ratings, the contactor life will, without doubt, be reduced dramatically and reliability, even in the medium term, may well become an issue.
Problems of this sort are perhaps, most likely to occur in lighting installations that are updated by replacing conventional fluorescent luminaires with modern energy-efficient electronic ballast types.
It’s all too easy to neglect to check the switching capacity of the existing contactors in relation to the new type of load – yet it’s eminently possible that they will be too small for their new duty and will, therefore, need to be replaced. Fortunately, modern contactors are physically smaller than their predecessors for a given switching capacity, so accommodating the replacements won’t usually be a problem. Trying to soldier on with the old devices, however, is almost certain to lead to reliability problems and even outright failures.
There is another easily forgotten factor that needs to be taken into consideration when selecting a lighting contactor, and that’s power factor correction. Many types of luminaire – including the majority of fluorescent types – have poor power factors. We’ve already mentioned that luminaire manufacturers almost always, in these power-quality conscious days, compensate for this by building in power factor correction capacitors.
These capacitors, however, draw a big spike of charging current when the luminaire is turned on and, if an installation includes numerous luminaires, this spike can be sufficient to damage the contactor.
For this reason, contactor manufacturers specify a maximum capacitor load that can be switched by their lighting contactors, and this must be taken into account as well as the normal switching rating of the contactor when choosing the most appropriate product to use in a particular application.
There hasn’t been space in this short article to consider in detail the characteristics of every type of lamp currently in use, but whatever the application, the same rules of contactor selection always apply: first determine the type of lamp and, where appropriate the type of ballast. Then choose a contactor with an adequate current rating for this type of lamp load. Finally, check that the contactor’s maximum capacitor load is not exceeded.
The key to success, of course, is having access to accurate and comprehensive data on switching lighting loads from the contactor manufacturer. Companies like Moeller Electric will invariably make this data easily available, and will also be ready to provide additional advice and support in dealing with more complex and demanding applications, such as those involving mixtures of luminaire types.