Failing Incandescent Lamps

In many investigations the question "where do you buy your replacements from?" reveals the answer "the local supermarket". The reason given is they are cheaper, as well as being convenient, etc. When scratching a little deeper it is often found that when the first lamp failed, the perpetual cycle of failing lamps started off. Until then, the household had been peaceful.

A little science lesson first. The piece of a lamp that actually emits light is known as a filament. This is made of an alloy called tungsten. The property of tungsten is it is usually flexible, to a point. The problem is, heat it up, and it becomes incredibly brittle (along the lines of forging steel). Back to the problem.

Buying at the supermarket is tempting, especially when faced with the price of the local electrical outlet on the corner! The problem is, in order for the supermarket to offer the low price, they in turn have to buy in massive bulk. The lights are then shipped from warehouse to warehouse, and because of their weight are rather roughly handled and often bounce a few times on the floor before landing (hard) on the shelf.

The local electrical shop buys directly from the manufacturer (or their warehouse) but, as everyone dealing with the packages knows what they are, treat them with the respect they deserve. The only likely chance of them being stressed is when the buyer throws them in the back of the car.

The difference in these two stories is simple. In the first the filaments were stressed, in the second not. This has a marked effect on the life of the lamp. Another bad practice though is electrical concerns will, at the request of the buyer, test the lamps and prove to the buyer they are in working order. This has just caused the filaments to become brittle. Unless the buyer treats them with the utmost of care between shop and home the failure rate will continue to be high.

There may be a dramatic price difference, but the saving in the long run is more than worthwhile. Trying to explain to a homeowner that the whole cycle was started when he bought the first lamp from the supermarket may prove a little too much, but do try to remain calm!

Till now, the underlying message is simply "cheap buying is not economic buying"; This is no truer than with incandescent lamps! Ok, so all lamps are being purchased from a reputable dealer and the lamps are still blowing!

Human Factor: Although this could convincingly be termed a cause, in reality it is a symptom of ignorance. In an attempt to keep the electricity consumption to a minimum, as it is perceived as an expensive commodity, members of the household are trained from an early age to turn the light off when leaving a room, regardless of the fact they are going back into the room a minute or two later.

What is not realized is the damage done to the filament during this action. Every time the lamp is turned on, the cold filament is strained with the excessive current that flows through it during the warm-up. The result is; Although the power is being used as economically as possible, this directly leads to a very uneconomical use of lamps which negates the saving in electricity by many times!

Severe Flicker: This is the primary reason power should be investigated. Severe voltage dips on the lighting circuit will cause premature lamp failure. This comes from the filaments cooling during a dip (the cooling also causes the resistance to decrease), and then having to ride through thermal shock when the voltage increases again (as well as a short burst of high current from the decreased resistance).

Overly Bright: This is simply over-voltage and causes the filament to disintegrate faster than designed and therefore leading to a much reduced lamp life. Also, the current taken by the lamp during switch-on with high voltage (owing to a cold filament and therefore low resistance) is far higher than with a normal or reduced supply voltage. During such switch-on the weaker portion of the filament is strained even further leading to failures being primarily noticed at switch-on.

So we've proved high voltage burns up a lamp a lot faster than when it is running within its operating range. But a simple cure exists. Buy decent industrial rated quality lamps. These are usually rated about 5-10% higher than normal domestic lamps (and cost about that much more too!). They are designed to be put in places where lamp failure could lead to dangerous situations developing. A prime example is aircraft warning lights as mounted on tall structures.

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