We hear this question from folks who are puzzled that their lights worked fine last season but are half out when they pull them out of storage.
Or their lights suddenly half go out during installation.
Incandescent Christmas tree lights (aka mini lights) are wired in series so if your lights go partially out you have one of three issues:
1. A light bulb has come out of its socket or is half out of its socket and it has brought down the circuit. For light strings that commonly have over 50 bulbs, they are constructed in 2 or more continuous circuits. If a bulb is missing in a circuit only the bulbs in series of it will go out. That’s how you can lose 1/2 or 1/3 or 1/4 of the string.
But don’t lights stay on if a bulb burns out?
Yes, as a general rule of thumb, a small wire at the base of each glass bulb will continue to conduct electricity if the filament “burns out” but if the bulb is actually out of its socket, the lights don’t actually stay on.
The solution: Visually inspect each bulb in the part of the Christmas light string that is out. Don’t take out and reinsert each bulb because that’s hard on fingers and will more than likely create larger problems. Check for any bulb that is missing or partially unseated from the socket and if you find a space or wonky bulb, replace the bulb with one of the two extra bulbs that hopefully were included with your light set or gently push the bulb back into its socket.
2. The rating of the light string was exceeded and the bulbs blew out the wire at the base of each bulb that is supposed to conduct electricity. This happens sometimes when the bulbs are “smoked”.
The solution: Check the rating of the light string and don’t plug in more lights than the manufacturer specifies. Visually check the part of the light string that is out and replace any smoked bulbs.
Note, if you’ve run way too many lights in series you may have permanently damaged your light sets. Purchasing new sets may be necessary if replacing bulbs don’t work for you.
3. There is an issue with the wiring harness and the copper wire is broken in the string. If all the bulbs are seated and none are smoked, more than likely there is an issue with the actually wiring of the Christmas light string.
Damage to the wiring can be caused by age, squirrels, pulling and tugging on the light strings during take-down at the end of Season.
If this is the problem with your light string, the best solution is to begin again with a new string of lights.
What if the entire string of lights goes out?
If the entire string of lights is out, the issue could be that it is on a single circuit and is due to one of the issues above or the fuse for the entire string might be out.
See our blog about the entire string doing down for more information and troubleshooting tips.