I have an older (10+ years) pre-lit Christmas tree with incandescent lights. There's also an equally old lit topper that behaved in a similar way, so they may be related. The middle section of my tree had half the lights out when I plugged it in. I went through and checked all of the bulbs.

At first I tested each bulb in another strand, but I quickly grew tired of it. So then I just checked to see if the filament, or the backup wire below the filament were broken. I found one light I know to be dead and swapped it out, but the section was still out.

After 20 minutes or so with the tree on the section was magically lit up! Given the primitive nature of the lights I assume there is something to do with heat building up and causing contact to be made, similar to how twinkling lights are made. My concern is that if the circuit wasn't made then there's no way for it to heat/shift, so I feel like there's a short somewhere in there.

Is there a simple explanation for this behavior, and is it risky to use the lights as-is?

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    When lights don't work it's not a short but an open. If it was a short the breaker would trip. – Ed Beal Nov 23 '18 at 2:20

Fault-tolerant Christmas lights fail open first and then fail shorted. Replace any dark lamps, if a fuse lamp has failes the string will stay dark unti it is replaced. replace any bad fuse lamps with fuse lamps.

So long as there's a fuse lamp in each string you're protected against short circuits causing fire.

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