why do lights blow regularly on one circuit & very seldom on another, both supplied f

[AndyD]

Not too sure mikilianis, usually the cap on a motor would discharge through the windings when the power is switched off. Also being a ceiling fan it should be on a 2-pole switch (assuming compliant installation) so both live + neutral would be isolated.

[Sparks]

Another possibility which has not been mentioned is cheap imported lamps. They do not have enough solder to make decent contact. I have found replacing cheap lamps solved the problem on a number of occasions.

[AndyD]

Good point, I assume you mean on the bottom tip of the lamp.

This problem could also be compounded by insufficient spring or travel on the center contact inside the lamp holder or previous over-tightening of a lamp in the fitting which is all too common.

[Martinco]

Another possibility which has not been mentioned is cheap imported lamps. They do not have enough solder to make decent contact. I have found replacing cheap lamps solved the problem on a number of occasions.

Sparks, you have a point there !
These imported jobs ( normally from China ) are really cheap and nasty and do not have a good life.
What about replacing one or two with CFL lamps and compare.

[Martinco]

While we are on the subject of lamps................has anybody ever found a "8000 hour" CFL to last even close to the 333 days (continuous) on nearly 3 years if only used 7-8 hours / night ?

[AndyD]

Sparks, you have a point there !
These imported jobs ( normally from China ) are really cheap and nasty and do not have a good life..........

All major lighting brands manufacture to varying degrees in China.

Osram/Sylvania - manufacture in China
Philips - manufacture in China
Lohuis - manufacture in China
GE - manufacture in China
Lumin8 - manufacture in China
Megaman - Manufacture in China

If you want to avoid Chinese products for some reason you would need to start buying Brands such as Lichttechnik which are manufactured in Germany.

Some Philips lamps available in SA are 'manufactured' in Holland but many of their components such as cathodes, ballasts etc are produced under license in China.

You can run but you can't hide :-)

[AndyD]

While we are on the subject of lamps................has anybody ever found a "8000 hour" CFL to last even close to the 333 days (continuous) on nearly 3 years if only used 7-8 hours / night ?

Mostly, lamp life is stated according to IEC60969 standards.

According to IEC60969;

• Tests are conducted in a draught proof cabinet at (25°C) and a humidity of less than 65% with a stabilized and optimized supply voltage.
  
• The measured life to 50% failures (average life) shall not be less than the rated life to 50% failures (rated average life)

In short the lamp life stated on the box refers to the length of time of operation at which point at least 50% of the lamps from a random sample batch of at least 20 lamps should still be working (assuming controlled humidity and temperature, assuming draught-free conditions, assuming perfect electrical supply and running conditions ie. no rapid on-off switching.)

The real-life chances of a lamp reaching it's stated run time without failure are pretty damn slim.

Nearly all wholesalers and agents will refuse point blank to warranty a lamp that has failed prematurely.

[Dave A]

While we are on the subject of lamps................has anybody ever found a "8000 hour" CFL to last even close to the 333 days (continuous) on nearly 3 years if only used 7-8 hours / night ?

Not even close

[berndj]

While we are on the subject of lamps................has anybody ever found a "8000 hour" CFL to last even close to the 333 days (continuous) on nearly 3 years if only used 7-8 hours / night ?

Back in 1993 my dear mother gave me an Osram for my birthday, back when CFL's were still an unusual sight. That lamp had been working for hours every night, for about 15 years, after which I moved and it fell into disuse. It's dead now, but only because my dog knocked over the lamp and it broke. It must've done about 25000 hours. I don't think it was any particularly overengineered model. I think I have its remains in a bag somewhere if you want an exact model number!

Once when it was just a few years old I thought it had broken as it didn't switch on properly, or it made an odd sound so I left it (memory's hazy), but then one day it made a heroic BZZZT, ignited properly and then shone brightly for the rest of its life.

YMMV!

[Dave A]

Rough duty light bulbs last pretty well too, particularly if you don't torture them in a lead light like they're designed for.

They used to be pretty hard to source, though. Don't know what it's like nowadays...