Untangling the Mystery of Christmas Lights: Why They Fail and How to Fix Them

Oh Christmas lights. The quintessential signal that Christmas is here! I’m willing to bet that if you were to ask 10 people about the reliability of Christmas lights, that 10 people would tell you it is poor. They will likely include some words which I will not mention in this article. This article should help explain how to troubleshoot Christmas lights and highlight the difficulties in obtaining a reliable system with multiple components in series. So what’s the problem with Christmas lights? Why do they fail so often and why are they complicated to solve?

To answer this question we must look at the design of Christmas lights. Christmas lights are typically designed to include both parallel and series circuits. This is the reason that typically only a portion of the Christmas light strand fails. Below is a schematic of the traditional incandescent Christmas light strand.

Based on this diagram, if the filament in a bulb were to fail it would open the circuit and none of the bulbs in this circuit would work. So how is it possible for one bulb to fail and the remainder of the lights continue to work? This is possible because each bulb also contains a shunt at the base of the bulb. This shunt creates the path needed to close the circuit in the event of a bulb filament failure. Below is a picture of this.


In a non fault condition, the electrical path travels along the path of least resistance which is the filament. This is due to a coating on the shunt which increases the resistance. During the burnout phase of the filament, the temperature increases which burns off the shunt coating and shorts the leads to close the circuit for the remaining lights.

The issue here is that all lights in series share the same current. So with each bulb failure, the shunt is activated, decreasing the overall resistance of the circuit which then increases the current in the circuit leading to additional bulb filament failure. The key to maintaining reliable Christmas lights is to replace each bulb as it fails.

To troubleshoot Christmas lights, the best tool on the market is the Light Keeper Pro. It has a non contact voltage detector which makes it easy to trace the open circuit failure point. It also has the ability to generate a high voltage pulse to help close a failed shunt. This often also helps identify the open circuit point by generating a visible arc at this point. Below is a picture of the light keeper pro and it’s features.

The answer to the big question on why Christmas lights are not reliable is due to the high number of components in series. Based on the Christmas light schematic above, there are often 50 or more bulbs in series. If we assume a reliability of each individual bulb to be 99%, the reliability of each circuit is surprisingly low. Calculating this is .99^50 which results in 60% reliability of the circuit.

On top of this, nobody wants to install a strand of lights with an entire circuit out, so we must view the entire strand of lights as required to be considered functional. Calculating this is .99^150 which results in 22% reliability of the entire light strand! This should help explain why only 1 out of the 4 light strands you put away each year don’t work the following year. 

Hopefully this article helps shed some Christmas light on the impact which series dependencies affect the reliability of a system. I hope it also helps you regain some self dignity by realizing these little Christmas lights are a much more complicated system than most realize.