Why does an Entire String of Christmas Lights Fail When a Single Bulb Burns out?

Michael Pollick
Michael Pollick

A seemingly innocent string of Christmas lights actually has many tricks up its sleeve, most of which won't be discovered by the household tree decorator until it's too late. Christmas tree lights seem to multiply in the box, and they appear to spend most of their days braiding themselves together in Gordian knots. The final act of cruelty is the "all or nothing" lighting trick. Depending on the design, an entire string can indeed fail if only a single bulb burns out or comes out of its socket. The reason for this has to do with the nature of electrical circuits and the Christmas bulbs that depend on them.

If a string of Christmas lights has a single wire connection, it can cause the entire string to go out when one bulb goes bad.
If a string of Christmas lights has a single wire connection, it can cause the entire string to go out when one bulb goes bad.

There are two basic electrical wiring scheme used to form a string of Christmas lights. One scheme, called a series, uses a single wire connection between bulbs to light them all. The electricity from the household outlet flows down that single wire and through the first Christmas tree bulb. That bulb glows as a certain amount of electrons escape into it. The rest of the electricity continues to flow through the wire to the next bulb in the series and so on until the string is either connected to another string or the electricity flows back into the negative side of the outlet.

This series arrangement works well until one of the bulbs burns out or is removed from the socket. At that point, the electrical circuit cannot be completed, and all of the remaining lights will not glow. The only way to restore the circuit in a series lighting scheme is to replace the bad bulb with one that is known to work. Finding the bad bulb or even bulbs in a string of lights is often easier said than done, however. The known good bulb becomes a tester as each bulb is replaced individually until the string lights up again. If a new bulb is placed in an empty socket in a series, the string should light again as well.

There is a second wiring scheme which should keep an entire light string from failing because of a single bulb, but it could still happen under the right circumstances. In a parallel Christmas light scheme, two wires carry the electricity through each bulb. Theoretically at least, if one bulb's filament burns out, the remaining lights should remain lit. The second wire guarantees that the circuit will not be broken entirely. The problem with a parallel system is the complete removal of a Christmas bulb from its socket. Unless the parallel string has a special shunt installed in the socket to bridge the gap, all of the lights may still go out.

Even LED strings of Christmas lights can fail if one bulb in the circuit burns out, especially if the string is wired in series, not parallel. It is always a good idea to keep spare bulbs available for last-minute emergencies and to inspect every string of Christmas lights carefully before adding it to the tree or stapling it to the roof outside. Parallel lights can be more expensive than series lights, but avoiding the tedious task of tracking down a single bad bulb may make the investment worthwhile.

Michael Pollick
Michael Pollick

A regular EasyTechJunkie contributor, Michael enjoys doing research in order to satisfy his wide-ranging curiosity about a variety of arcane topics. Before becoming a professional writer, Michael worked as an English tutor, poet, voice-over artist, and DJ.

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Discussion Comments


Awesome site, thanks. I finally "get" Christmas lights.


We have an artificial tree with lights that don't go out, even if you remove a bulb. Each string has a small plastic box attached. I can't open it because it has screws with a unique head. Some of the strings are out entirely. Could this be a function of what's in the box? I've also checked the fuses, to no avail.


I'm trying to find a bad bulb on a supposedly parallel christmas tree. The manufacturer decided to put an empty wire fake a parallel connection. Yay. Be warned when buying a new tree.


Just wanted to say thanks so much, this info helped me answer a question on my science assignment which i was really stuck on. Thanks! From Emma


I have a problem with a string of lights. The lights are permanently tied to a Christmas tree. The string of non lit lights is in the middle of three sets. All sockets have 90 volts in the base. I have removed all the lights from their sockets. If you place one light in the first or third string it lights. In the second, frustration. I just don't know, do you?


@dazed and confused: Before you even consider cutting the string in half, the load will be increased, increasing the voltage and will blow some lights. You are better off purchasing a set of 50.


help me with this one. i need to take a 100 mini bulb light string with an end to end plug and cut it in half and eliminate the female plug. I am dealing with three wires: two hot (connect to 50 percent each of the bulbs) and one return. While the hot wires go from bulb to bulb (series) the return wire runs the full length from male to female plug. This tells me that inside the female plug the hot and return must connect to make the circuit when no second string is used?

When I cut the wires to I then connect both hot and return together? Signed dazed and confused?


I, too, am having the same problem. About half of one strand on the tree is burned out. All 50 bulbs are black near the filament. I replaced all the bulbs and fuses in the plug, to no avail. This is odd and I am frustrated. There must be a problem with the wire somewhere between the last bulb that lights and the first bulb that doesn't? Any ideas?


Voltage makes a difference. You must come from a plug that does not string over five strings at once or you will blow some lights. Electrical surges from your power company can also blow lights.

a surge protector will prevent this. As usual it is more trouble that you would think with lights that can blow for many reasons.

One thing is sure, there is no guarantee that lights won't blow no matter what the manufacturer says.


"electrons escaping into the bulb" causing it to glow?

Come on, wise GEEK, Electrons are electricity, "Photons" are light!


lights are so cheap nowadays they are designed to fail so you have to buy new ones each yea., I have one set from 1960 and have had no problems year after year, except the unraveling, of course.


To jdmagness-- Take The Tree Back To The Store .


good information. Wish you all an early happy new year


Christmas lights have a fuse in the plug head that will blow out also. Merry Christmas.


someone could make a million if this crap didn't happen!


thought that was a good explanation about "electrons escaping from the bulb producing light". never heard that one before.


Become an atheist and dispense with Christmas, trees, lights, and all that, and live happily!


I am having the same problem! I'm about to throw out the whole tree and get a new one! ~frustrated


Last year we purchased a pre wired christmas tree, for good or bad. The treee is prewired with serial parallel strings. Question, part of one strand lights up but not the other part of the same strand. More confusing is that it is the first part of the strand that does not light and the bulb holders are energized, the back part of the strand does light. Any suggestions?

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