I spent a fair bit of time putting up my lights this weekend, and had to do some troubleshooting
sets that weren't working.
This can be tough to do without a bit of help. While many bulbs have shunts that prevent
the string from going out, they don't always work, nor do they help when lamp wires
When that happens, I fall back on a non-contact voltage detector. Mine is a Fluke VoltAlert
1AC, which I bought at Home Depot for about $20. This is wonderful for general
electrical work, since you can detect hot wires without having to have access to the
conductor. Great if you're replacing an outlet and you want to make sure that the
power is really off.
For holiday lights, here's the way you find the bad lights.
Find which section of lights is off (duh). This may be only half of a 100-light string.
Start at one end of the string, testing each light, working your way along the until
you find a difference in state. Test the light by holding the tester near the bulb
part; if you get too close to the cable part, you'll get a positive from the traveler
wire that's in the cable. I generally like to start at the end where there is power.
The bad bulb will show power to it, but because it's bad, it won't be passing on power
to the next bulb in line.
One little note. Most plugs on US devices are polarized - the two plugs are different
in size - for safety purposes. This ensures (for example) that a lamp socket has the
hot lead for the tip, so that if you change a bulb without turning the light off,
the screw part will not be hot. Most holiday lights, however, don't have this issue,
so they're generally not polarized. This means that you can turn the plug around 180
degrees, and switch the hot and dead sets of bulbs in your broken set, which can sometimes
make things more convenient.
One final note: It's a lot easier to do this before you put the bulbs up on the roof,
or 25' into the air.