In the good old days, when your Christmas lights went dark, you had to test each bulb. Nowadays, Christmas light testers make the job much easier and faster. Although light bulbs are always going to burn out, you won't have to spend hours looking for the culprits, and you can get back to playing with your new toys in no time.
Christmas light strings are wired in series, which means that every light must be functioning in order for the string to be lit. When a bulb burns out, meaning that the filament that actually produces the light has evaporated, it breaks the connection, and the whole string of lights goes dark. Newer light sets have a shunt, a small piece of wire, inside each bulb that maintains the connection when the filament is gone and keeps the other bulbs lit. When the shunts function properly, it's easy to spot the defective bulb. However, these shunts often fail, and the result is the same as if they didn't exist: The entire string of lights goes off.
You can check each individual bulb by replacing it with a bulb that you know to be good. But this method takes a lot of time and isn't foolproof, since you can't always tell whether you've managed to position the bulb in the socket correctly. Even if you find the burned-out bulb, you might be putting in a replacement bulb that isn't seated, and the string still won't light. Light testers are designed to find the defective bulbs without you having to remove every bulb from its socket.
Using Christmas Light Testers
A hum-tracer tester works by detecting the electromagnetic vibrations generated by household electrical current. You start at the plug and follow the wires of the Christmas lights into and out of each socket. An LED light on the unit stays lit when it detects a hum. The LED light goes out when the hum lessens noticeably. At that point, you've found a burned-out bulb.
When using this type of tester, it's important to know that three strands of wire connect the bulbs. One strand is the live AC current, or "hot" wire; another strand is the grounded, or "cold," wire; and the third strand is the bulb wire. You need to test only the bulb wire, which you can do by pulling it away from the other two strands.
When there is only one bad bulb and shunt, the hum tracer might find no hum at the adjacent bulb wire rather than the defective bulb itself, but one of the two wires will have no hum. If you encounter a successive number of bulb wires with no hum, you're dealing with more than one burned-out bulb. In this case, a hum tracer won't be able to pinpoint the bad bulbs because multiple defective shunts have broken the circuit and no AC current is running through that particular section of wire to produce a hum.
Another type of Christmas light tester forces AC current into the defective shunts, which causes the good bulbs to light. You can quickly see which bulbs need to be replaced. In the event that the shunts still fail and the good bulbs don't light, this tester also has a hum-tracer that beeps when it finds a hum. It's more sensitive than older hum tracers, which makes it easier to find the defective bulbs.