How Christmas Lights Work

Why are Christmas lights so unreliable? Image by mai05

Question: “Every year, I pull out my Christmas ornaments, and find that the majority of strands of lights don’t work anymore. Why do Christmas lights only last one year?”

To answer this question, we describe the most common types of Christmas lights, and what can cause their failure.

Lights Made with Incandescent Bulbs

The most common Christmas lights are the traditional strings of lights made with incandescent bulbs.

In the incandescent bulb there is a metallic filament, usually made of tungsten (W) connected to the electric circuit. When there is an electric current going through the filament, this becomes hotter and hotter; consequently it starts glowing and generates light.

There are several other parts in the bulbs, the most important being:

  • The contact wires, which connect the tungsten filament to the electric circuit;
  • A case or envelope, to protect the filament.  As the bulb can get very hot (more than 200oC), the case has to be made of a material resistant to high temperature, such as glass. The case is filled with an inert gas, such as argon.

Why Incandescent Light Bulbs Can Fail

If the filament burns out, the bulb will fail. Image by juliaf

Generally if a light bulb fails, it is because the filament becomes too hot, and burns. This can happen for several reasons; the power supply, for instance, may not be stable, but may be higher for a short period of time (a spike). The higher electricity can cause an overheating of the filament, which may burn.

Due to the high temperatures reached by the filament, a slow process of evaporation can also take place; this will gradually consume the filament and cause the bulb to fail. The evaporation rate is reduced remarkably by the inert gas present into the bulb case, but it can never be completely eliminated.

The filament can also be consumed if air gets into the bulb case. The case should be properly sealed to avoid this, but there may be leaks. When the tungsten is in contact with oxygen, other gases or the moisture present in the air, some chemical reactions can happen, which consume the filament.

What Happens with Christmas Lights

In Christmas lights made with incandescent bulbs, it is the glass case, and not the filament, which gives them the different colors.

The lights can have all the problems mentioned above. Considering their particular use, normally they are switched on for many consecutive hours; in some cases this can cause overheating or accelerate the consumption by evaporation. If they are used outside, the level of humidity in the air can be much higher than in an inside environment; therefore, if there is an air leak, the effect on the filament could be much more severe.

Christmas lights are pretty, but leaving them on for long periods of time increases the chance of bulbs burning out. Image by kris69

Additional Problems Arising from Christmas Lights

Further to these problems, some extra issues may arise, due to the specific way Christmas lights are made.

A string of Christmas lights is generally made by several individual bulbs joined together; the connection is made by wires made from a conducting metal, such as copper.

In cheap strings, the wires may be not put in place properly and/or be loose. Consequently, the connection between the lights can be lost, and the string will not work anymore, even if the bulbs are not burnt. This can happen, for instance, when the lights are stored at the end of a Christmas season: folding the string can cause a small “displacement” in the wires and can results in the loss of the connection.

The connection between the different lights is made in series; this means that the electrical current goes through one light, then to the following one, etc. Consequently, even if only one light burns out, all the lights will stop working, as the circuit is broken and the current cannot continue through the string of lights.

Most strings of lights also contain a pair of fuses in the electric plug, which will melt, or blow, at a lower current than the copper wire. This is to prevent the copper wires from overloading and catching fire in the event of too much current. Obviously, if the fuses have blown, the string of lights will not work, even though the bulbs still function.

LED Christmas lights consume less power and last longer. Photo by Carbon Arc

LED Christmas Lights: Better Option?

In recent years a new type of Christmas light has become more popular; that is, strings based on LED bulbs.

LED is the acronym for Light Emitting Diode. The principle they are based on is the energy emission by a semiconductor; the energy, emitted in the form of light, will depend on the value of the bandgap of the semiconductor.

The main advantage of LED lights is that they do not become as hot as the incandescent lights; for this reason they do not burn out as easily, and tend to last much longer. For the same reason, the case does not have to necessaríly be glass, but it can also be made of other lighter materials, such as plastic. Furthermore, their energy consumption is much lower.

In principle, a LED-based string should be easier to maintain, and surely usable for more than one season. Also in this case, however, it is the quality that makes the difference. For instance, a bad wire connection between the different bulbs can cause the same problems described above for the strings made by incandescent bulbs.

Quality Is Important

What is apparent from above is that, whatever the type of Christmas lights used, if you want them to last more than one season is important to choose the best quality set possible.

© Copyright 2011 Clara Piccirillo, PhD, All rights Reserved. Written For: Decoded Science
Decoded Everything is a non-profit corporation, dependent on donations from readers like you. Donate now! Your support keeps the great information coming!

Donation Information

I would like to make a donation in the amount of:


I would like this donation to automatically repeat each month

Tribute Gift

Check here to donate in honor or memory of someone

Donor Information

First Name:
Last Name:
Please do not display my name publicly. I would like to remain anonymous


  1. Joe says

    I am an electronic technologist and I have extensive experience with them. Now the one comment about the socket rusting out after water gets in it that does happen and it did to me many times over the last 15 years. But that doesn’t explain why only the green and blue leds burn out on a string long before the other bulbs do. I check my leds , not bulbs as they are mistakenly called when I put them away for the season, and before I take them out for the next season. Now when I put my lights up I always have them pluged in so I know that they work. And no you will not get a shock for them as long as you don’t touch barewires. Plus an outdoor socket at a house should be a ( GFCI, Ground fault circuit interrupter, 210-8 Of the national electrical code requires it. These semi conductors are like everything else, Made not to last, and cheaply. By who else C H I N A. So they can make money and stay in business. Sorry for the long explanation. And if I new the answer, I wouldn’t be looking on here for one, sorry…

  2. Ted says

    Any reason why the blue lights always are the first ones to go out? It happens in all sizes of LED light strings. Solid blue strings, multi color strings of different size bulbs including miniature, number 6,7, and number 9 bulbs. All the other colors remain bright’

  3. Clara Piccirillo, PhDClara Piccirillo, PhD says

    To answer Ed’s question: probably what happened is what described above in the article, i.e. either one of the bulbs or the connecting wire went.
    I am not sure you can repair your lights. You can try to see if it is only light bulb; if this is the case, you could change it and the whole string may work again.
    If it is the connecting wire, you should see if/where it is out of place or if it got consumed in some part. This is not easy to see and/or to do, unless you have the knowledge of an electrician.

  4. Ed says

    I have a grapevine reindeer and all at once dozens of lights went out and blackened. What is the cause of this and how to repair.

    • Dr. Schrodinger says

      You must be diligent about replacing any failed bulbs as soon as possible. Each time a bulb fails, the shunt inside shorts out to allow the rest of the lights to continue to work. But this applies a higher voltage to each remaining bulb, and they will burn out faster. As more and more bulbs fail, this causes a cascade effect that will rapidly blow out the remaining bulbs. The fewer the number of bulbs in the string, the more important this is.

      Check your lights often for failed bulbs, and replace them with properly rated replacement bulbs to keep this from happening.

  5. Josh says

    I had the same issue so I dumped all my old lights and got new L.E.D. lights. That was a big mistake! The new L.E.D. may save electricity but that’s because they keep going out (LOL). Water keeps getting into the bulbs causing them to rust and the entire set to stop working. I have been trying different brands to store grade L.E.D. lights and will say the only brad L.E.D. lights that work with out a lot if problems are the Philips L.E.D. lights from Target. They don’t give me any issues unlike the others I have tried. I also started to buy the old incandescent lights aging, just not the cheap incandescent lights. I have been getting the commercial grade lights as they last longer outdoors.

Leave a Reply

Your email address will not be published. Required fields are marked *