KMEC

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 color changing LED mini Christmas 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 color changing LED mini 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.