light bulbs

[Fillz]

why does a light bulb always blow when you turn them on, and never when they are already illuminated ? ( or does this only happen to me ?? )

[heathfield]

In use, the tungsten filament gradually erodes, and some parts of it will become thinner than the rest. When you switch the lamp on, there is a surge of current, and when this passes through the thinnest bit the temperature can become so high as to melt the filament at that point ,so the bulb 'blows'. This can also happen when the lamp is already lit, but less often, it seems.

[Big Al1st]

Hi Fillz, Its how Heathfield said. In fact alot of electrical/electronic devices fail at switch on due to a surge in current . This is because electrical resistance is lower at cold temperatures thereby allowing a serge to occur, albeit very quickly, until the circuits warm up. Rgds Al

[kwicky]

The above answers are interesting. If the current surge does damage why are they not fitted as standard to the incoming electrical supply. I notice that you can buy surge electrical sockets for your PC but wonder why are not fitted to the mains?

[heathfield]

Electricity companies are required by law to keep the supply within limits that won't harm anything connected to it. This includes 'surges'. The problem is more to do with 'spikes' that they can't control. For instance a lightning strike close to an electricity supply line can produce a very large spike on your house mains. Computers don't like these! But for things like lights, heaters, etc, this isn't a problem, since their reaction time to a spike is relatively very slow.
Since a spike will normally represent a single high frequency pulse, most 'surge protectors' for computers, etc simply act as filters, allowing the 50Hz mains to pass through, but stopping anything of higher frequency.
The only sure way to stop surges or spikes is to use a UPS. (Universal Power Supply).These devices take the mains supply, convert the a.c. to d.c., then convert the d.c back to a tightly regulated a.c. at constant voltage level. No surges or spikes get through. But one to install a UPS for the whole house would be prohibitively expensive, and isn't needed for most things in your home.

[markja]

Fillz, ignore the previous replies,it's because people like us are jinxed and unlucky!!
you know when you put the roast in the oven and ten minutes later you get a power cut? the program you want to video over runs by fifteen mins so you miss the end? a small short power cut / hiccup in the night resets your alarm clock so you over sleep? it's electricity--it hates us!!!
hope this has answered your question.
if your electric is getting to lairy just wave a gas appliance page from a catalogue at your electric meter and it will behave for a while.

[Eddiecurrent]

The first two answers hit the nail on the head, the surge in question is a current surge and not a voltage surge. When cold, metals have a lower resistance than when hot, therefore if a filament is thinning this increase in current may cause the filament to fail. In Heathfield's second answer, the power supply mentioned is also known as a Switch Mode or Switching Power Supply. UPS can also stande for Uninterruptable Power Supply, whereby batteries are charged, and in the event of an outage, the battery supply will be inverted to 240V ac.
A surge arrester will normally incorporate a voltage dependent resistor. These are resistors whose resistance is normally very high at nominal voltage (230 - 240V), if however a voltage higher than 240V appears the resistance of the device will suddenly drop thus clamping the voltage. High quality surge arrestors will include the filters mentioned low quality types usually omit the filters.