Video: If a mini light bulb burns out, will the rest stay lit?
One of the most common questions we hear about incandescent Christmas lights is: “If one bulb burns out, does the whole string go dark?”
The short answer is: it depends on how the light set is engineered. Cheaper incandescent light strings often lack the internal safeguards that allow electricity to continue flowing when a bulb fails — which is why one bad bulb can take out the entire string.
In this video, we explain how traditional incandescent light strings work and why better-designed sets continue to function even when a bulb burns out. It was filmed before manufacturers began adding hairline shunt wires at the base of each bulb — a tiny but important engineering detail that allows current to bypass a failed bulb instead of stopping altogether.
Important: This explanation applies to incandescent light strings only. LED Christmas lights are wired differently and behave differently when a bulb goes out.
Understanding how incandescent light strings are built makes it easier to choose reliable sets — and to avoid the frustration that comes with bargain lights that cut corners on engineering.
Video Transcript
Today, we’re answering a question from Charles: If one mini light burns out, will the rest stay lit?
That’s a great question. The answer is yes — but with some qualifications. In the early 1970s, mini lights were re-engineered with a tiny shunt wire at the base of each bulb. It’s about as thick as a human hair, and if you look closely with a magnifying glass, you might even be able to see it.
If a bulb burns out and the filament fails, the shunt wire continues to conduct electricity, allowing the rest of the light string to stay lit. However, if a bulb becomes completely unseated from the socket, that portion of the circuit will go out.
This particular light string contains two circuits of 50 lights. Removing one bulb from the socket takes down half of the string.
In fact, about 99% of the time when half a light string goes out, the issue is simply a bulb that has become loose. Instead of removing and reinserting every bulb — which is a great way to get blisters — do a visual check and gently snug bulbs back into place one at a time.
If your light string is older and you’re noticing repeated bulb failures along with wear on the wiring harness, it’s likely nearing the end of its lifespan and may be time to start looking for replacements.
— Shellie, Christmas Light Source
