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All lamps based on incandescent technology produce light immediately after switch-on. The delay is of the order of a few thousandths of a second for the standard sizes, whereas high wattage types may take up to a maximum of about one second to heat their filaments to the full operating temperature.
When normally alight, the lamp's electrical resistance in ohms is given by the voltage squared, divided by the watts. But before the lamp is switched on the cold filament has a very much lower electrical resistance. Due to the low cold resistance a very large current will flow at the instant of switching on, typically around 14 times the normal current. As the filament warms up, its resistance increases and the current falls until the two stabilise. Typical inrush current curves are illustrated in Figure I18 below. For a low wattage lamp such as the 230V 100W rating, it closely approaches the nominal current within about 0.05 seconds. For higher wattage lamps the process can take much longer. In the case of the 240V 500W lamp illustrated, even after 1 second its current has not fallen to the nominal level. Full stability is reached only once the filament as well as its lead-in wires have reached their normal operating temperature - a process which may take several minutes.
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Figure I18 - Inrush Current associated with Incandescent Filament Lamps
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The inrush current does not persist for sufficient time to cause most ordinary fuses or circuit breakers to operate, and it has generally been ignored by switch manufacturers since a switch is more likely to fail when breaking a circuit rather than making it. However, in practice it may be found that some short-break designs do not stand up to the current surges associated with repeated switching, especially in the case of high wattage lamps such as used for projection and studio applications.
It is a fallacy that it is more economical to leave an incandescent lamp continuously burning than to switch it off for just a short period, so as to minimise energy consumption during the inrush current period. The cost of electricity consumed during the very short time interval of the surge cannot possibly be as much as the cost of current saved by switching the lamp off even for a few seconds.
In addition it is found that the effect of repeated switching has very little effect on the life of a filament lamp. A good example is found in the flashing roadway Belisha Beacons of British pedestrian crossings. The filament is switched 'off' to black-heat in between flashes, and ordinary household GLS lamps still achieve the full rated life of 1000 hours in this application (in which they must endure more than 1.5 million flashes). |
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