Troubleshooting
Individual bulbs fail as either open or closed circuits. In a series string, when a bulb fails open, the whole string goes out, unless there is a special feature included to bridge the open bulb. Typical incandescent strings in the 1950s - 1960s were wired in series and frequently burnt out.
For a series string, fixing failures requires a tedious process: each bulb is replaced with a known good bulb, hunting for the broken one by trial-and-error. If more than one bulb is broken, then each bulb in the broken string needs to be tested in a known-good string. Sometimes a bulb will not actually break, but will come loose in its socket. Its connection can also become corroded. Those two problems can be fixed by resetting each bulb in turn.
In a parallel string, one shorted bulb makes the whole string fail, requiring a similar troubleshooting process. However, the shorted string must be protected by a current limiting device, which may require resetting in between tests.
Fiber optic sets are the simplest to troubleshoot, since they contain only a few light sources, sometimes only one or two, and the place of failure can be located very quickly. Units containing color wheels may also require maintenance of mechanical parts.
Read more about this topic: Christmas Lighting Technology