How many bulbs should you change every month?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

16 March 2008

“I manage a large residential estate and I am concerned about the number of security bulbs that I’ve been changing every month”, so reads an email from Charles M. He continues, “When we moved into the site in January this year, we changed over 100 bulbs, then 200 in February. Now we are getting worried that there might be a fault in the wiring.”

Charles continues: “The site has 40 blocks and each block has 10 security bulbs in the stair case. We switch on the lights from 6pm and turn them off at 6am. Considering the average lifetime of a bulb, how many should I expect to change every month?”

Bulbs are “funny”: when you one burns out, all the others follow suit in a few days! The reason for this, of course, is that they all have approximately equal lifetimes and they are all installed at the same time – when you move into a house. Further more, many bulbs remain on for approximately equal times – especially in the living room and the corridors.

Incidentally, why do we take away the bulbs when we move house? Wouldn’t it be nice if we left them for the next occupant? After all, if everybody did that, we would also find bulbs in the new house that we shift to. Well, that’s a discussion for a different forum…

For now; the average life of an ordinary filament bulb is 1,000 hours. If it is switched on for 12 hours every day (6pm to 6am), it should last for about 83 days – that is, about two and a half months.

Therefore if all the bulbs in Charles’s estate were initially fixed on the same day, and they all lasted exactly 1,000 hours, then he should expect to change all of them every two and a half months. But life is not that exact – the bulbs will last a few days longer or shorter than the expected 83. And being human, Charles (or his predecessors) is not able to change the burnt lights instantly – you never know, some times he has to wait a day before getting new supplies.

As a result of these effects, the points at which individual bulbs are changed must have spread out evenly over the years so that, today, a fairly constant average number burns out each day. But still, all the 400 lights will have been changed every 83 days. So, how many bulbs should Charles expect to change every day?

That is now easy to calculate: He is changing 400 bulbs every 83 days, therefore he should expect to replace about 5 daily. That is about 150 each month. That sound quite high and one wonders whether it would be advisable to replace them with the energy-saving, long-life type. However, as discussed in a previous article, the cost would be too high to justify the change.