We’re getting into the holiday spirit here in Spartanburg. Our stockings are hung by the chimney with care and we have strings of lights decorating our homes.
While stringing up lights, you may have wanted to learn how exactly holiday lights work. Here’s what Daniel Wood and Sarah Gerrity from Energy.gov have discovered.
Note that the primer below applies to incandescent light strands, which have been traditionally used to decorate our homes for the holidays — and are a great way to learn about the flow of electric current. LED holiday light strands are becoming more popular, due to being sturdier, lasting longer and consuming 70 percent less energy than conventional incandescent light strands. It only costs $0.27 to light a 6-foot tree for 12 hours a day for 40 days with LEDs compared to $10 for incandescent lights. Learn more about LED lighting on Energy Saver.
But now, back to incandescents. To start off, we have to understand how an incandescent light bulb works. In a simple circuit, electricity travels through a closed circuit, passing over a filament, causing it to glow brightly. The more current passes over a filament, the hotter it will get, the brighter it will burn, and quicker it will burn out. If the circuit is broken, or open, no electricity will pass over the filament and it will not light. If the current is too great, the filament will melt, or blow out, causing the circuit to become open.
But we want more than one light to shine on our Christmas tree or along the roofs of our homes. If you want to connect multiple light bulbs to the same power source, there are two ways to do that: either attach the lights in series or in parallel.
Parallel vs. Series Lights
When lights are attached in series, the electricity passes from the power source to the first light, and then from light to light until it returns to the power source. In this setup, when a filament within any one bulb blows out, it creates an open circuit in the wiring. As we mentioned earlier, when a circuit is incomplete, or open, electricity fails to pass through any of the wire, causing all the lights to go out.
When lights are attached in parallel, each light is on its own circuit to the power source. If one filament burns out, it has no effect on the remaining lights, as they each continue to be in a closed circuit with the power source. Check out the difference.
With strings of holiday lights, engineers decided that the best option was to connect several series of lights together in parallel. In other words, holiday lights are both in series and in parallel. Let’s see what this looks like.
This way, when one series of bulbs becomes defective — say from a loose bulb — it should have no effect on any of the other series of bulbs, since they are in parallel to the defective series. This is why sometimes only one portion of your lights will become defective, while others will remain functional. When additional strings of lights are attached to the end of a string, these lights are added in parallel to the original strand.
But what about when a bulb goes out in series? It used to be that when one bulb went out the entire series would go out. If this were the case, you would have to check each bulb individually to see which one was blown out. If multiple bulbs were blown out, this would become exceedingly difficult.
Enter the “shunt.” What is a shunt, you ask? I will tell you!
Shunts and Fuses Save the Day
A shunt is any device that allows current to continue flowing through a circuit by creating a path of lower resistance than the original path. In incandescent holiday lights, shunts are small wires wrapped beneath the filament. Initially, they are coated with a substance that makes them an insulator. In other words, electricity cannot pass across the shunt as long as the filament exists, because the coating gives the shunt a higher resistance initially than the filament, and the electrical current avoids the shunt in order to find the path of least resistance, through the filament.
If the filament burns out, however, the high temperature from the burnout will cause the substance coating the shunt to melt off, revealing the lower resistance wire beneath. Now the shunt has gone from an insulator to a conductor, and current passes along shunt, keeping the circuit closed, and the remaining lights burning. Check it out below.
The last thing to understand about how holiday lights work is the role of the fuse. Recently, as I was putting up lights, I tripped on a wire and the lights went out. The string of lights remained plugged in so I was stymied as to what happened. I unplugged, replugged … unplugged, replugged the string. Nothing. Then it came to me. “The fuse!” I said under my breath. I must have caused some sort of short circuit when I jolted the wire.
Fuses are important safety features for many electrical appliances, but most of us don’t even know that they’re there. In holiday lights, the fuse can be found near the part of the strand that plugs into the wall, often called — and I’m not making this up — the male end of the wire. Normally, the fuse is accessible through a small plastic door in the plug that can be opened and closed for replacing the fuse.
As the electrical current within a wire increases, the wire can heat up, at times causing melting or even fires. To prevent this, fuses were introduced as so-called “sacrificial devices” (so very selfless of them!). When the current increases past a safe level, instead of the wire melting or your Christmas tree catching fire, the fuse safely breaks the circuit, averting many disastrous scenarios.
Fuses are typically small sections of replaceable wire, rated to a maximum current level. Because they are more delicate than the rest of the wiring, a fuse will burn out before overcurrent has an opportunity to overheat other portions of the light strand. When a fuse breaks, the circuit becomes incomplete and current cannot flow through the remainder of the circuit.
What’s Wrong With My Lights?
So now that we understand the mechanics of holiday lights, let’s address some common problems we run into and how to remedy them.
If only one section of your lights become extinguished, this means that the circuit is open on that particular series. If this happens, there are a few things that could be going on. First off, you could simply have a loose bulb. This is a common problem, and the most easy to identify and fix, by simply screwing back in the bulb. Second, it could mean that one or more lights have blown out but that the shunt is defective, possibly from the insulating substance remaining intact. Finally, there could be something defective in the wiring, causing the series to become an open circuit. Unfortunately there’s not a lot you can do if that’s the case.
If only one light goes out, it almost certainly means that individual light is defective and needs to be replaced. This is the most common problem and the easiest to identify and fix. If this happens it is important to replace the bulbs quickly. When the filament of a bulb burns out and the shunt takes over, it has a lower resistance than the filament. As a result, each remaining bulb gets slightly brighter, burns a little hotter and burns out more quickly. As each bulb burns out, and the 120 volts from the power source is distributed to fewer and fewer bulbs, this process happens faster and faster. The last few bulbs will burn out very quickly. So it is best to replace burnt-out bulbs quickly when they go out.
If all of your lights go out, it is probably because you have too great a current coming from the power source and have blown a fuse. If, after replacing the fuse, fuses continue to blow, it could be because you have too many bulbs burned out or too many strands of lights connected to the same socket. This is called overloading a circuit. You could continue to replace fuses — but a blown fuse is typically a sign that some issue is causing too great of a current to pass through the fuse. Disconnecting one of the strands of lights could solve the problem. It is a good idea to have no more than three strands of lights connected together.
By now, you should be an expert at knowing when (and how) to fix your incandescent holiday lights and when to just chuck them out. It may be an arduous process — but hopefully this article will help you rescue some light strands that you were about to give up on!
Happy decorating and happy holidays!
Written by Daniel Wood, Energy.gov.